Security in E-Voting Report Full

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A revised version of this paper was published in Proc. 2007 USENIX/ACCURATE Electronic Voting Technology Workshop (EVT’07),

August 2007. For the workshop paper and videos of demonstration attacks, see http://citp.princeton.edu/voting.

Security Analysis of the Diebold AccuVote-TS Voting Machine

Ariel J. Feldman*, J. Alex Halderman*, and Edward W. Felten*,

*Center for Information Technology Policy and Dept. of Computer Science, Princeton University

Woodrow Wilson School of Public and International Affairs, Princeton University

{ajfeldma,jhalderm,felten}@cs.princeton.edu

September 13, 2006

Abstract

This paper presents a fully independent security study of a Diebold AccuVote-TS voting machine,

including its hardware and software. We obtained the machine from a private party. Analysis of the

machine, in light of real election procedures, shows that it is vulnerable to extremely serious attacks. For

example, an attacker who gets physical access to a machine or its removable memory card for as little

as one minute could install malicious code; malicious code on a machine could steal votes undetectably,

modifying all records, logs, and counters to be consistent with the fraudulent vote count it creates. An

attacker could also create malicious code that spreads automatically and silently from machine to machine

during normal election activities—a voting-machine virus. We have constructed working demonstrations

of these attacks in our lab. Mitigating these threats will require changes to the voting machine’s hardware

and software and the adoption of more rigorous election procedures.

1 Introduction

The Diebold AccuVote-TS and its newer relative the AccuVote-TSx are together the most widely deployed

electronic voting platform in the United States [8]. In the November 2006 general election,

The Diebold AccuVote-TS voting machine in our lab

these machines are scheduled to be used in 357

counties representing nearly 10% of registered

voters. Approximately half these counties—

including all of Maryland and Georgia—will

employ the AccuVote-TS model. More than

33,000 of the TS machines are in service nationwide.

This paper reports on our study of an

AccuVote-TS, which we obtained from a private

party. We analyzed the machine’s hardware

and software, performed experiments on

it, and considered whether real election practices

would leave it suitably secure. We found

that the machine is vulnerable to a number of

extremely serious attacks that undermine the

accuracy and credibility of the vote counts it

produces.

Computer scientists have generally been skeptical of voting systems of this type, Direct Recording

Electronic (DRE), which are essentially general-purpose computers running specialized election software.

Experience with computer systems of all kinds shows that it is exceedingly difficult to ensure the reliability

and security of complex software or to detect and diagnose problems when they do occur. Yet DREs rely

fundamentally on the correct and secure operation of complex software programs. Simply put, many computer

scientists doubt that paperless DREs can be made reliable and secure, and they expect that any failures of

such systems would likely go undetected.

Previous security studies of DREs affirm this skepticism (e.g., [4, 14, 17, 24, 30]), but to our knowledge

ours is the first public study encompassing the hardware and software of a widely used DRE. The famous

paper by Kohno, Stubblefield, Rubin, and Wallach [17] studied a leaked version of the source code for parts

of the Diebold AccuVote-TS software and found many design errors and vulnerabilities, which are generally

confirmed by our study. Our study extends theirs by including the machine’s hardware and operational details,

by finding and describing several new and serious vulnerabilities, and by building working demonstrations of

several security attacks.

Main Findings The main findings of our study are:

1. Malicious software running on a single voting machine can steal votes with little if any risk of detection.

The malicious software can modify all of the records, audit logs, and counters kept by the voting

machine, so that even careful forensic examination of these records will find nothing amiss. We have

constructed demonstration software that carries out this vote-stealing attack.

2. Anyone who has physical access to a voting machine, or to a memory card that will later be inserted

into a machine, can install said malicious software using a simple method that takes as little as one

minute. In practice, poll workers and others often have unsupervised access to the machines.

3. AccuVote-TS machines are susceptible to voting-machine viruses—computer viruses that can spread

malicious software automatically and invisibly from machine to machine during normal pre- and

post-election activity. We have constructed a demonstration virus that spreads in this way, installing

our demonstration vote-stealing program on every machine it infects.

4. While some of these problems can be eliminated by improving Diebold’s software, others cannot be

remedied without replacing the machines’ hardware. Changes to election procedures would also be

required to ensure security.

The details of our analysis appear below, in the main body of this paper.

Given our findings, we believe urgent action is needed to address these problems. We discuss potential

mitigation strategies in more detail below in Section 5.

The machine we obtained came loaded with version 4.3.15 of the Diebold BallotStation software that runs

the machine during an election.1 This version was deployed in 2002 and certified by the National Association

of State Election Directors (NASED) [11]. While some of the problems we identify in this report may have

been remedied in subsequent software releases (current versions are in the 4.6 series), others are architectural

in nature and cannot easily be repaired by software changes. In any case, subsequent versions of the software

should be assumed insecure until fully independent examination proves otherwise.

Though we studied a specific voting technology, we expect that a similar study of another DRE system,

whether from Diebold or another vendor, would raise similar concerns about malicious code injection

attacks and other problems. We studied the Diebold system because we had access to it, not because it is

1The behavior of our machine conformed almost exactly to the behavior specified by the source code to BallotStation version

4.3.1, which leaked to the public in 2003.

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necessarily less secure than competing DREs. All DREs face fundamental security challenges that are not

easily overcome.

Despite these problems, we believe that it is possible, at reasonable cost, to build a DRE-based voting

system—including hardware, software, and election procedures—that is suitably secure and reliable. Such a

system would require not only a voting machine designed with more care and attention to security, but also

an array of safeguards, including a well-designed voter-verifiable paper audit trail system, random audits and

forensic analyses, and truly independent security review.2

Structure of the Paper The remainder of this paper is structured as follows. Section 2 describes several

classes of attacks against the AccuVote-TS machine as well as routes for injecting malicious code. Section 3

discusses the machine’s design and its operation in a typical election, focusing on design mistakes that

make attacks possible. Section 4 details our implementation of demonstration attacks that illustrate the

security problems. Section 5 examines the feasibility of several strategies for mitigating all of these problems.

Section 6 outlines prior research on the AccuVote system and DREs more generally. Finally, Section 7 offers

concluding remarks.

2 Attack Scenarios

Elections that rely on Diebold DREs like the one we studied are vulnerable to several serious attacks. Many

of these vulnerabilities arise because the machine does not even attempt to verify the authenticity of the code

it executes. In this section we describe two classes of attacks—vote stealing and denial-of-service—that

involve injecting malicious code into the voting machine. We then outline several methods by which code

can be injected and discuss the difficulty of removing malicious code after a suspected attack.

2.1 Classes of Attacks

2.1.1 Vote-Stealing Attacks

The AccuVote-TS machine we studied is vulnerable to attacks that steal votes from one candidate and give

them to another. Such attacks can be carried out without leaving any evidence of fraud in the system’s logs.

We have implemented a demonstration attack to prove that this is possible; it is described in Section 4.2.

To avoid detection, a vote-stealing attack must transfer votes from one candidate to another, leaving the

total number of votes unchanged so that poll workers do not notice any discrepancy in the number of votes

reported. Attacks that only add votes or only subtract votes would be detected when poll workers compared

the total vote count to the number of voters who checked in at the front desk.3

The machine we studied maintains two records of each vote—one in its internal flash memory and one on

a removable memory card. These records are encrypted, but the encryption is not an effective barrier to a

vote-stealing attack. Malicious software running on the machine would modify both redundant copies of the

record for each vote it altered. Although the voting machine also keeps various logs and counters that record

a history of the machine’s use, a successful vote-stealing attack would modify these records so they were

consistent with the fraudulent history that the attacker was constructing. In the Diebold DRE we studied,

these records are stored in ordinary flash memory, so they are freely modifiable by malicious software.

2Current testing agencies are often referred to as “independent testing agencies” (ITAs), but “independent” is a misnomer, as they

are paid by and report to the voting machine vendor.

3It might be possible to subtract a few votes without detection (if poll workers interpret the missing votes as voters who did not

vote in that race) or to add a few votes to compensate for real voters who did not cast ballots; but in any case transferring votes from

one candidate to another is a more effective attack.

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Such malicious software can be grafted into the BallotStation election software (by modifying and

recompiling BallotStation if the attacker has the BallotStation source code, or by modifying the BallotStation

binary), it can be delivered as a separate program that runs at the same time as BallotStation, it can be

grafted into the operating system or bootloader, or it can occupy a virtualized layer below the bootloader

and operating system [16]. The machine contains no security mechanisms that would detect a well designed

attack using any of these methods. However it is packaged, the attack software can modify each vote as it is

cast, or it can wait and rewrite the machine’s records later, as long as the modifications are made before the

election is completed.

The attack code might be constructed to modify the machine’s state only when the machine is in election

mode and avoid modifying the state when the machine is performing other functions such as pre-election logic

and accuracy testing. The code could also be programmed to operate only on election days. (Elections are

often held according to a well-known schedule—for example, U.S. presidential and congressional elections

are held on the Tuesday following the first Monday of November, in even-numbered years.) Obviously, it

could be programmed to operate only on certain election days, or only at certain times of day.

By these methods, malicious code installed by an adversary could steal votes without being detected by

election officials.4 Vote counts would add up correctly, the total number of votes recorded on the machine

would be correct, and the machine’s logs and counters would be consistent with the results reported—but the

results would be fraudulent.

2.1.2 Denial-of-Service Attacks

Denial-of-service (DoS) attacks aim to make voting machines unavailable on election day or to deny officials

access to the vote tallies when the election ends. It is often known in advance that voters at certain precincts,

or at certain times, will vote disproportionately for one party or candidate. A targeted DoS attack can be

designed to distort election results or to spoil an election that appears to be favoring one party or candidate.

Several kinds of DoS attacks are practical because of the ease with which malicious code may be executed on

the voting machines.

One style of DoS attack would make voting machines unavailable on election day. For example, malicious

code could be programmed to make the machine crash or malfunction at a pre-programmed time, perhaps

only in certain polling places. In an extreme example, an attack could strike on election day, perhaps late in

the day, and completely wipe out the state of the machine by erasing its flash memory. This would destroy all

records of the election in progress, as well as the bootloader, operating system, and election software. The

machine would refuse to boot or otherwise function. The only way to restore such a machine to a working

state would be to have a service technician visit, install a special EPROM chip on the machine’s motherboard,

and reboot the machine from that EPROM. If many machines failed at once, available technicians would

be overwhelmed. The result would be a fresh install of the machine’s software, with all records of past and

current elections still lost. (We have created a demonstration version of this attack, which is described below

in Section 4.4.) A similar style of DoS attack would try to spoil an election by modifying the machine’s

vote counts or logs in a manner that would be easy to detect but impossible to correct, such as by injecting

malicious code that adds or removes so many votes that the results at the end of the day are obviously wrong.

A widespread DoS attack of either style could require the election to be redone.

2.2 Injecting Attack Code

To carry out these attacks, the attacker must somehow install his malicious software on one or more voting

machines. If he can get physical access to a machine for as little as one minute, he can install the software

4Officials might try to detect such an attack by parallel testing. As we describe in Section 5.3, an attacker has various

countermeasures to limit the effectiveness of such testing.

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manually. The attacker can also install a voting machine virus that spreads to other machines, allowing him

to commit widespread fraud even if he only has physical access to one machine or memory card.

2.2.1 Direct Installation

An attacker with physical access to a machine would have least three methods of installing malicious

software [14]. The first is to create an EPROM chip containing a program that will install the attack code into

the machine’s flash memory, and then to open the machine, install the chip on its motherboard, and reboot

from the EPROM.5

The second method is to exploit a back door feature in Diebold’s code to manually install the attack files

from a memory card. When the machine boots, it checks whether a file named explorer.glb exists on

the removable memory card. If such a file is present, the machine boots into Windows Explorer rather than

Diebold’s BallotStation election software. An attacker could insert a memory card containing this file, reboot

the machine, and then use Explorer to copy the attack files onto the machine or run them directly from the

card.

The third method exploits a service feature of the machine’s bootloader. On startup, the machine checks

the removable memory card for a file named fboot.nb0. If this file exists, the machine replaces the

bootloader code in its on-board flash memory with the file’s contents. An attacker could program a malicious

bootloader, store it on a memory card as fboot.nb0, and reboot the machine with this card inserted,

causing the Diebold bootloader to install the malicious software. (A similar method would create a malicious

operating system image.)

The first method requires the attacker to remove several screws and lift off the top of the machine to get

access to the motherboard and EPROM. The other methods only require access to the memory card slot and

power button, which are both behind a locked door on the side of the machine. The lock is easily picked—one

member of our group, who has modest locksmithing skills, can pick the lock consistently in less than 10

seconds. Alternatively, this slot can be reached by removing screws and opening the machine. Some attackers

will have access to keys that can open the lock—all AccuVote-TS machines in certain states use identical

keys [24], there are thousands of keys in existence, and these keys can be copied at a hardware or lock store.

A poll worker, election official, technician, or other person who had private access to a machine for as

little as one minute could use these methods without detection. Poll workers often do have such access; for

instance, in a widespread practice called “sleepovers,” machines are sent home with poll workers the night

before the election [27].

2.2.2 Voting Machine Viruses

Rather than injecting code into each machine directly, an attacker could create a computer virus that would

spread from one voting machine to another. Once installed on a single “seed” machine, the virus would

spread to other machines by methods described below, allowing an attacker with physical access to one

machine (or card) to infect a potentially large population of machines. The virus could be programmed to

install malicious software, such as a vote-stealing program or denial-of-service attack, on every machine it

infected.

To prove that this is possible, we constructed a demonstration virus that spreads itself automatically

from machine to machine, installing our demonstration vote-stealing software on each infected system. Our

demonstration virus, described in Section 4.3, can infect machines and memory cards. An infected machine

will infect any memory card that is inserted into it. An infected memory card will infect any machine that is

5When the machine is rebooted, it normally emits a musical chime that might be noticed during a stealth attack; but this sound

can be suppressed by plugging headphones (or just a headphone connector) into the machine’s headphone jack.

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powered up or rebooted with the memory card inserted. Because cards are transferred between machines

during vote counting and administrative activities, the infected population will grow over time.

Diebold delivers software upgrades to the machines via memory cards: a technician inserts a memory

card containing the updated code and then reboots the machine, causing the bootloader to install the new code

from the memory card. This upgrade method relies on the correct functioning of the machine’s bootloader,

which is supposed to copy the upgraded code from the memory card into the machine’s flash memory. But if

the bootloader were already infected by a virus, then the virus could make the bootloader behave differently.

For example, the bootloader could pretend to install the updates as expected but instead secretly propagate

the virus onto the memory card. If the technician later used the same memory card to “upgrade” other

machines, he would in fact be installing the virus on them. Our demonstration virus illustrates these spreading

techniques.

Memory cards are also transferred between machines in the process of transmitting election definition

files to voting machines before an election. According to Diebold,

Data is downloaded onto the [memory] cards using a few [AccuVote] units, and then the stacks

of [memory] cards are inserted into the thousands of [AccuVote] terminals to be sent to the

polling places.

([7], p. 13) If one of the few units that download the data is infected, it will transfer the infection via the

“stacks of [memory] cards” into many voting machines.

2.3 Difficulty of Recovery

If a voting machine has been infected with malicious code, or even if infection is suspected, it is necessary to

disinfect the machine. The only safe way to do this is to put the machine back into a known-safe state, by, for

example, overwriting all of its stable storage with a known configuration.

This is difficult to do reliably. We cannot depend on the normal method for installing firmware upgrades

from memory cards, because this method relies on the correct functioning of the bootloader, which might

have been tampered with by an attacker. There is no foolproof way to tell whether an update presented in this

way really has been installed safely.

The only assured way to revert the machine to a safe state is to boot from EPROM. This involves making

an EPROM chip containing an update tool, inserting the EPROM chip into the motherboard, setting the

machine to boot from the chip, and powering it on. On boot, the EPROM-based updater would overwrite the

on-board flash memory, restoring the machine to a known state. Since this procedure involves the insertion

(and later removal) of a chip, it would probably require a service technician to visit each machine.

If the disinfection process only reinstalled the software that was currently supposed to be running on

the machines, then the possibility of infection by malicious code would persist. Instead, the voting machine

software software should be modified to defend against installation and viral spreading of unauthorized code.

We discuss in Section 5 what software changes are possible and which attacks can be prevented.

3 Design and Operation of the AccuVote-TS

Before presenting the demonstration attacks we implemented, we will first describe the design and operation

of the AccuVote-TS machine and point out design choices that have led to vulnerabilities.

3.1 Hardware

The machine (pictured on page 1) interacts with the user via an integrated touchscreen LCD display. It

authenticates voters and election officials using a motorized smart card reader, which pulls in cards after they

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Figure 1: The machine’s motherboard incorporates a (A) HITACHI SUPERH SH7709A 133 MHZ RISC

MICROPROCESSOR, (B) HITACHI HD64465 WINDOWS CE INTELLIGENT PERIPHERAL CONTROLLER,

two (C) INTEL STRATAFLASH 28F640 8 MB FLASH MEMORY CHIPS, two (D) TOSHIBA TC59SM716FT

16 MB SDRAM CHIPS, and a socketed (E) M27C1001 128 KB ERASABLE PROGRAMMABLE READ-ONLY

MEMORY (EPROM). A (F) PRINTED TABLE lists jumper settings for selecting the boot device from among

the EPROM, on-board flash, or “ext flash,” presumably an external memory inserted in the (G) “FLASH EXT

SLOT.

Connectors on the motherboard attach to the (H) TOUCH SENSITIVE LCD PANEL, (I) THERMAL ROLL

PRINTER, and (J) SECURETECH ST-20F SMART CARD READER/WRITER, and receive power from the

(K) POWER SUPPLY and (L) BATTERY, which are managed by a (M) PIC MICROCONTROLLER. An (N) IRDA

TRANSMITTER AND RECEIVER, (O) SERIAL KEYPAD CONNECTOR, and (P) HEADPHONE JACK are readily

accessible through holes in the machine’s case. A (Q) POWER SWITCH, (R) PS/2 KEYBOARD PORT, and two

(S) PC CARD SLOTS are reached by opening a locked metal door, while a (T) RESET SWITCH and (U) PS/2

MOUSE PORT are not exposed at all. An (V) INTERNAL SPEAKER is audible through the case.

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are inserted and ejects them when commanded by software. On the right side of the machine is a headphone

jack and keypad port for use by people with disabilities, and a small metal door with a lightweight lock of a

variety commonly used in ordinary desk drawers and file cabinets. Behind this door is the machine’s power

switch, a keyboard port, and two PC Card slots, one containing a removable flash memory card and the other

containing a modem card used to transfer ballot definitions and election results. The machine is also equipped

with a small thermal roll printer for printing records of initial and final vote tallies.

Internally, the machine’s hardware strongly resembles that of a laptop PC or a Windows CE hand-held

device. The motherboard, described in detail in Figure 1, includes a 133 MHz SH-3 RISC processor, 32 MB

of RAM, and 16 MB of flash storage. The machine’s power supply can switch to a built-in rechargeable

battery in case power is interrupted.

In normal operation, when the machine is switched on, it loads a small bootloader program from its

on-board flash memory. The bootloader loads the operating system—Windows CE 3.0—from flash, and then

Windows starts the Diebold BallotStation application, which runs the election. Unfortunately, the design of

the machine allows an attacker with physical access to the inside of the machine’s case to force it to run code

of her choice [23].

A set of two switches and two jumpers on the motherboard controls the source of the bootloader code that

the machine runs when it starts. On reset, the processor begins executing code starting at address 0xA0000000.

The switches and jumpers control which of three storage devices—the on-board flash memory, an EPROM

chip in a socket on the board, or a proprietary flash memory module in the “ext flash” slot—is mapped into

that address range. A table printed on the motherboard lists the switch and jumper configurations for selecting

these devices. The capability to boot from a removable EPROM or flash module is useful for initializing the

on-board flash when the machine is new or for restoring the on-board flash’s state if it gets corrupted, but, as

we discussed in Section 2, it could also be used by an attacker to install malicious code.

When we received the machine, the EPROM socket was occupied by a 128 KB EPROM containing a

bootloader that was older than, but similar to, the bootloader located in the on-board flash. The bootloader

contained in the EPROM displays a build date of June 22, 2001 whereas the bootloader contained in the

on-board flash displays June 7, 2002. The machine came configured to boot using the on-board flash memory.

On our machine, the on-board flash memory is divided into three areas: a 128 KB bootloader, a 3.3 MB

GZIP-ed operating system image, and a 10 MB file system partition.

3.2 Boot Process

When the machine is booted, the bootloader copies itself to RAM and initializes the hardware. Then it looks

for a memory card in the first PC Card slot, and if one is present, it searches for files on the card with special

names. If it finds a file called fboot.nb0, it assumes that this file contains a replacement bootloader, and it

copies the contents of this file to the bootloader area of the on-board flash memory, overwriting the current

bootloader. If it finds a file called nk.bin, it assumes that this file contains a replacement operating system

image in Windows CE Binary Image Data Format [22], and it copies it to the OS area of the on-board flash,

overwriting the current OS image. Finally, if it finds a file called EraseFFX.bsq, it erases the entire file

system area of the flash. The bootloader does not verify the authenticity of any of these files in any way, nor

does it notify the user or ask the user to confirm any of the changes that it makes. As Hursti [14] suggests,

these mechanisms can be used to install malicious code.

If none of these files are present, the bootloader proceeds to uncompress the operating system image

stored in on-board flash and copy it to RAM, then it jumps to the entry point of the operating system kernel.

The operating system image is a kind of archive file that contains an entire Windows CE 3.0 installation,

including the kernel’s code, the contents of the Windows directory, the initial contents of the Windows

registry, and information about how to configure the machine’s file system.

When Windows starts, the kernel runs the process Filesys.exe, which in turn unpacks the registry

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and runs the programs listed in the HKEY LOCAL MACHINE\Init registry key [21]. On our machine, these

programs are the Debug Shell shell.exe, the Device Manager device.exe, the Graphics, Windowing,

and Events Subsystem gwes.exe, and the Task Manager taskman.exe. This appears to be a standard

registry configuration [20].

The Device Manager is responsible for mounting the file systems. The 10MB file system partition on

the on-board flash is mounted at \FFX. This partition appears to use the FlashFX file system, a proprietary

file system from Datalight, Inc [5]. The memory card, if it is present, is mounted at \Storage Card,

and may use the FAT or FAT32 file system. The root file system, mounted at \, is stored in RAM rather

than nonvolatile memory, which causes any files written to it to disappear when the machine is rebooted or

otherwise loses power. This design could be leveraged by an attacker who wished to use the file system for

temporarily storing data or malicious code without leaving evidence of these activities.

Figure 2: Windows Explorer running on the

AccuVote-TS

Diebold has customized taskman.exe so that it automatically

launches the BallotStation application, \FFX\ Bin\BallotStation.exe. Another customization

causes taskman.exe to behave differently depending

on the contents of any memory cards in the PC Card slots.

If a memory card containing a file called explorer.glb

is present at start-up, taskman.exe will invokeWindows

Explorer instead of BallotStation (Figure 2). Windows

Explorer would give an attacker access to the Windows

Start menu, control panels, and file system, as on an ordinary

Windows CE machine. The, taskman.exe process

also searches the memory card for files with names

ending in .ins [14]. These files are simple scripts in

a Diebold-proprietary binary format that automate the

process of updating and copying files. Like the special

files that the bootloader recognizes, taskman.exe accepts

explorer.glb without authentication of any kind.

While taskman.exe requests confirmation from the user

before running each .ins script, we found multiple stackbased

buffer overflows in its handling of these files. This

suggests that a malformed .ins file might be able to bypass

the confirmation and cause the machine to execute

malicious code.

3.3 BallotStation Software and Procedures

All of the machine’s voting-related functions are implemented by BallotStation, a user-space Windows CE

application. BallotStation operates in one of four modes: Pre-Download, Pre-Election Testing, Election, and

Post-Election. Each mode corresponds to a different phase of the election process and is intended to have

its own associated election procedures. Here we describe the software’s operation under typical election

procedures. Actual procedures may vary somewhat from place to place.

At any given time, the machine’s mode is determined by the contents of the currently-inserted memory

card. Specifically, the current election mode is stored in the header of the election results file, \Storage

Card\CurrentElection\election.brs. When one memory card is removed and another is inserted,

the machine immediately transitions to the mode specified by the newly inserted card. In addition, if the

machine is rebooted, when BallotStation restarts it will return to the mode specified by the currently inserted

memory card. As a result, if a machine is powered off while an election is taking place, it will return to

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Election mode when it is turned back on.

3.3.1 Election Setup

Typically, the voting machines are stored by the local government or the voting machine vendor in a facility

with some degree of access control. Before the election (sometimes the night before, or in other cases the same

morning) the machines are delivered to polling places where they are set up and prepared by poll workers.

Prior to the election, poll workers may configure BallotStation by inserting a memory card containing a ballot

description—essentially, a list of races and candidates for the current election. If, instead, a card containing

no recognizable election data is inserted into the machine, BallotStation enters Pre-Download mode (shown

in Figure 3A). In this mode, the machine can download a ballot definition by connecting to a Windows PC

running Diebold’s GEMS server software.

After election definitions have been installed, BallotStation enters Pre-Election Testing mode (shown

in Figure 3B). Among other functions, Pre-Election Testing mode allows poll workers to perform so-called

“logic and accuracy” (L&A). During L&A testing, poll workers put the machine into a simulation mode where

they can cast several test votes and then tally them, checking that the tally is correct. Because the voting

software is in L&A mode, these votes are not counted in the actual election.

After any L&A testing is complete, the poll workers put the machine into Election mode. The software

prints a “zero tape” which tallies the votes cast so far. Since no votes have been cast, all tallies should be zero.

Poll workers check that this is the case, and then sign the zero tape and save it.

3.3.2 Voting

When a voter arrives at the polling place, she checks in at a front desk where several poll workers are stationed.

The voter announces her identity (and provides whatever evidence of identity is required by elections law).

The identity is checked against a list of registered voters. Assuming the voter is registered and has not yet

voted, poll workers record that the voter has voted. At this point the poll workers give the voter a “voter card,”

a special smart card that signifies that the voter is entitled to cast a vote.6 The voter waits until the voting

machine is free and then approaches the machine to cast her vote.

To cast a vote, the voter first inserts her voter card. The machine validates the voter card and presents

the voter with a user interface (shown in Figure 3C) allowing her to express her vote by selecting candidates

and answering questions. After making and confirming her selections, the voter pushes a button on the user

interface to cast her vote. The machine modifies the voter card, marking it as invalid, and then ejects it. After

leaving the machine, the voter returns the now-invalid voter card to the poll workers, who may re-enable it

for use by another voter.

3.3.3 Post-Election Activities

At the end of the election, poll workers insert an “Ender Card” to tell the voting software to stop the election

and enter Post-Election Mode (shown in Figure 3D).7 Poll workers can then use the machine to print a “result

tape” showing the final vote tallies. The poll workers check that the total number of votes cast is consistent

with the number of voters who checked in at the front desk. Assuming no discrepancy, the poll workers sign

the result tape and save it. Members of the public are invited to watch this procedure and to see the contents

of the result tape, including the vote tallies.

6Kohno et al. found numerous vulnerabilities and design flaws in BallotStation’s smart card authentication scheme [17], which

remain uncorrected in the machine we studied.

7They can also use a “Supervisor Card” for this purpose. Supervisor cards enable access to extra setup and administrative

operations in pre- and post-election modes.

10

A B

C D

Figure 3: Screenshots depicting BallotStation’s four primary modes: (A) PRE-DOWNLOAD, (B) PREELECTION

TESTING, (C) ELECTION, and (D) POST-ELECTION.

11

After the result tape is printed, the election results are transferred to the central tabulator, a PC running

the GEMS software described above. Like the ballot definitions, the election results may be transferred over

a local area network, a phone line, or a serial cable. Once results from all machines have reached the central

tabulator, the tabulator can add up the votes and report a result for the election.

For convenience, it is also possible to “accumulate” the results from several machines into a single

AccuVote voting machine, which can then transmit the accumulated results to the central tabulator in a single

step. To accumulate results, one machine is put into accumulator mode, and then the memory cards from

all of the local machines are inserted (in sequence) into the accumulator machine, which reads the election

results and combines them into a single file, that will be transferred to the central tabulator or used as an input

to further accumulation steps.

If a recount is ordered, the result tapes are rechecked for consistency with voter check-in data, the result

tapes are checked for consistency with the results stored on the memory cards, and the tabulator is used

again to sum up the results on the memory cards. Further investigation may examine the state stored on

memory cards and a machine’s on-board file system, such as the machine’s logs, to look for problems or

inconsistencies.

Exact procedures vary from place to place, and many polling places add additional steps to deal with

multiple voter populations (e.g. different parties or electoral districts) and other complicating factors. We

omit these details in our description, but we have considered them in our analysis and except where noted

below they do not affect the results.

4 Implementing Demonstration Attacks

To confirm our understanding of the vulnerabilities in the Diebold AccuVote-TS system, and to demonstrate

the severity of the attacks that they allow, we constructed demonstration implementations of several of the

attacks described above and tested them on the machine. We are not releasing the software code for our

demonstration attacks to the public at present; however, a video showing some of our demonstration attacks

in operation is available online at http://itpolicy.princeton.edu/voting.

4.1 Backup and Restore

As a prerequisite to further testing, we developed a method for backing up and restoring the complete

contents of the machine’s on-board flash memory. This allowed us to perform experiments and develop other

demonstration attacks without worrying about rendering the machine inoperable, and it ensured that we could

later restore the machine to its initial state for further testing and demonstrations.

We began by extracting the EPROM chip from its socket on the motherboard and reading its 128 KB

contents with a universal EPROM programmer. We then disassembled the bootloader contained on the

chip using IDA Pro Advanced [6], which supports the SH-3 instruction set. Next, we created a patched

version of the EPROM bootloader that searches any memory card8 in the first PC Card slot for files named

backup.cmd and flash.img. If it finds a file named backup.cmd, it writes the contents of the onboard

flash to the first 16 MB of the memory card, and if it finds a file named flash.img, it replaces the

contents of the on-board flash with the contents of that file. We programmed our modified bootloader into a

new, standard, 128 KB EPROM chip and inserted it into the motherboard in place of the original chip. By

changing the position of switches and jumpers on the motherboard, as described in Section 3, we configured

the machine to boot using the code in the chip instead of the normal bootloader in its on-board flash memory.

8While Diebold sells special-purpose memory cards for use in the machine, we were able to substitute a CompactFlash card

(typically used in digital cameras) and a CompactFlash-to-PC Card adapter.

12

The EPROM bootloader already contains code for checking for the existence of files on a memory card

and for erasing and writing to the on-board flash in order to process the special files described in Section 3.

As a result, our modified bootloader is able to call that code when it needs to perform those operations.

Furthermore, since the on-board flash is mapped into the physical address space, reading from the on-board

flash is as straightforward as reading from RAM. However, since the original EPROM bootloader does not

contain code for writing to a memory card, we needed to add code that issued the proper write commands via

the memory card configuration registers [3].

4.2 Stealing Votes

Figure 4: Our demonstration vote-stealing control panel

Several of the demonstration attacks that

we have implemented involve installing

code onto AccuVote-TS machines that

changes votes so that, for a given race, a

favored candidate receives a specified percentage

of the votes cast on each affected

machine. Since any attacks that significantly

alter the total number of votes cast

can be detected by election officials, our

demonstration software steals votes at random

from other candidates in the same race

and giving them to the favored candidate.

The software switches enough votes to ensure

that the favored candidate receives at

least the desired percentage of the votes

cast on each compromised voting machine.

Election results (i.e., the record of

votes cast) are stored in files that can

be modified by any program running

on the voting machine. For the currently

running election, the primary copy

of the election results is stored on

the memory card at \Storage Card\ CurrentElection\election.brs

and a backup copy is stored in the machine’s

on-board flash memory at \FFX\ AccuVote-TS\BallotStation\Cur

rentElection\election.brs. Our

software works by directly modifying both

of these files.

Our demonstration vote-stealing software

is implemented as a user-space Windows CE application written in C++ that runs alongside Diebold’s

BallotStation application. Since our software runs invisibly in the background, ordinary users of BallotStation

would not notice its presence. It is pre-programmed with three parameters hard-coded into the binary: the

name of the race to rig, the name of the candidate who is supposed to win, and the minimum percentage of

the vote that that candidate is to receive.

Alternatively, an attacker could create a graphical user interface that allows more immediate, interactive

control over how votes would be stolen. We have also created a demonstration of this kind of attack, as shown

13

in Figure 4. In practice, a real attacker would more likely design a vote-stealing program that functioned

invisibly, without a user interface.

Our demonstration vote-stealing applications can easily be generalized to steal votes on behalf of a

particular party rather than a fixed candidate, to steal votes only in certain elections or only at certain dates or

times, to steal votes only or preferentially from certain parties or candidates, to steal a fixed fraction of votes

rather than trying to ensure a fixed percentage result, to randomize the percentage of votes stolen, and so on.

Even if the attacker knows nothing about the candidates or parties, he may know that he wants to reduce the

influence of voters in certain places. He can do this by creating malicious code that randomly switches a

percentage of the votes, and installing that code only in those places. Any desired algorithm can be used to

determine which votes to steal and to which candidate or candidates to transfer the stolen votes.

Every time a new memory card is inserted into the machine, our demonstration vote-stealing software

looks for an election definition file on the card located at \Storage Card\CurrentElection\ election.edb and, if one is present, determines whether the current election contains a race it is supposed

to rig. If no such race is found, the software continues to wait. If a target race is found, it searches that race

for the name of the favored candidate, and if it is found, records the favored candidate’s position in the race’s

“base rotation order.” The base rotation order of the candidates in a given race is the order that votes are

recorded in the election results file. This order may or may not differ from the order that the candidates are

displayed on the ballots that are shown to voters. The software must keep track of the favored candidate’s

position in the race’s base rotation order so that it can compute which value in each election result record

needs to be changed to give a vote to the favored candidate.

Once the demonstration vote-stealing software has loaded the election description, it polls the election

result files every 15 seconds to see if they have been changed. Since BallotStation tries to obtain an exclusive

lock on these files every time it accesses them, if BallotStation attempted to open the results files while the

demonstration vote-stealing software had them open, then BallotStation would receive an error that would

be displayed to the user of the voting machine possibly causing the attack to be discovered. To avoid this

situation, before attempting to open the result files, the vote-stealing software suspends the BallotStation

process. If the vote-stealing software succeeds in opening the files, it keeps BallotStation suspended until

it is finished accessing the result files. If it is unable to obtain a lock on the files, the software resumes

BallotStation and tries to open the files again later. These brief suspensions of the BallotStation process

would not be noticeable to voters because they occur infrequently and each last only a fraction of a second.

If the demonstration vote-stealing software successfully opens the result files during one of its polling

attempts, it first checks the result files’ headers to see whether the machine is currently in Election mode. If it

is not, the software does not change any votes. This feature ensures that the software would not be detected

during Logic and Accuracy testing, which occurs when the machine is in Pre-Election Testing mode. The

software could be further enhanced so that it would only change votes during a specified period on election

day, or so that it would only change votes in the presence or absence of a “secret knock.” A secret knock is a

distinctive sequence of actions, such as touching certain places on the screen, that an attacker executes in

order to signal malicious software to activate or deactivate itself.

If the machine is in election mode and the demonstration vote-stealing software successfully opens the

result files, then the software checks whether any new ballots have been cast since the last time it polled the

files. For each new ballot cast, the software determines whether the race being rigged is on that ballot, and if

so, determines whether the corresponding result record contains a vote for the favored candidate or for an

opponent. The software maintains a data structure that keeps track of the location of every result record that

contains a vote for an opponent of the favored candidate so that it can come back later and change some of

those records if necessary. Since each result record is only labeled with the ID number of the ballot to which

it corresponds, the software must look up each record’s ballot ID in the election definition file in order to

determine which candidates the votes in the record are for.

Once it has parsed any newly cast ballots, the software switches the minimum number of votes necessary

14

to ensure that the favored candidate gets at least the desired percentage of the vote. The vote-stealing software

chooses which votes to switch by selecting entries at random from its data structure that tracks votes for

the opponents of the favored candidate. After the necessary changes have been made to the result files, the

software closes the files, resumes the BallotStation process, and continues to wait in the background.

The steps described above are all that is necessary to alter every electronic record of the voters’ intent that

an AccuVote-TS machine produces. Several of the machine’s supposed security features do not impede this

attack. The so-called “protective counter,” supposedly an unalterable count of the total number of ballots ever

cast on the machine, is irrelevant to this attack because the vote-stealing software does not change the vote

count.9 The machine’s audit logs are equally irrelevant to this attack because the only record they contain

of each ballot cast is the log message “Ballot cast.” Furthermore, the fact that election results are stored

redundantly in two locations is not an impediment because the vote-stealing software can modify both copies.

Finally, as we discuss in Section 2, the fact that the election results are encrypted does not foil this attack.

4.3 Demonstration Voting Machine Virus

In addition to our demonstration vote-stealing attacks, we have developed a voting machine virus that

spreads the vote-stealing code automatically and silently from machine to machine. The virus propagates

via the removable memory cards that are used to store the election definition files and election results, and

for delivering firmware updates to the machines. It exploits the fact that, when the machine boots, the

Diebold bootloader will install any code found on the removable memory card in a file with the special name

fboot.nb0 [14]. As a result, an attacker could infect a large population of machines while only having

temporary physical access to a single machine or memory card.

Our demonstration virus takes the form of a malicious bootloader that infects a host voting machine by

replacing the existing bootloader in the machine’s on-board flash memory. Once installed, the virus deploys

our demonstration vote-stealing software and copies itself to every memory card that is inserted into the

infected machine. If those cards are inserted into other machines, those machines can become infected as

well.

The cycle of infection proceeds as follows. When the virus is carried on a memory card, it resides in

a 128 KB bootloader image file named fboot.nb0. This file contains both the malicious replacement

bootloader code and a Windows CE executable application that implements the demonstration vote-stealing

application. The vote-stealing executable is stored in a 50 KB region of the bootloader file that would

normally be unused and filled with zeroes.

When a card carrying the virus is inserted into a voting machine and the machine is switched on or

rebooted, the machine’s existing bootloader interprets the fboot.nb0 file as a bootloader update and copies

the contents of the file into its on-board flash memory, replacing the existing bootloader with the malicious

one. The original bootloader does not ask for confirmation before replacing itself. It does display a brief

status message, but this is interspersed with other normal messages displayed during boot. These messages

are visible for less than 20 seconds and are displayed in small print at a 90 degree angle to the viewer. After

the boot messages disappear, nothing out of the ordinary ever appears on the screen.

Once a newly infected host is rebooted, the virus bootloader is in control. Since the bootloader is the first

code that runs on the machine, a virus bootloader is in a position to affect all aspects of system operation.

While booting, the virus bootloader, like the ordinary bootloader, checks for the presence of a memory card

in the first PC Card slot. However, if it finds a bootloader software update on the card, it pretends to perform

a bootloader update by printing out the appropriate messages, but actually does nothing.10 Thus, once a

9In any event, the “protective counter” is simply an integer stored in an ordinary file, so an attack that needed to modify it could

do so easily [17].

10In order to avoid printing out fake update messages when the copy of fboot.nb0 on the card was put there by the virus

bootloader itself, whenever the virus bootloader copies itself to a card, it sets the hidden, system, and read-only FAT attributes of the

15

machine has been infected, the only way to remove the virus bootloader is to restore the machine’s state using

an EPROM-resident bootloader.

If a memory card is present, the virus bootloader copies itself to the card as a file named fboot.nb0

so that it can spread to other machines. If the card already contains a file with that name, the bootloader

replaces it. Consequently, if a service technician performing bootloader updates tries to update an infected

machine using a card containing an fboot.nb0 file, the infected machine will not be updated (although it

will pretend to be), and all subsequent machines that the technician tries to update using the same card will

receive the virus bootloader instead of the updated one. Similarly, updates to the BallotStation software or

operating system can also propagate the virus.

The malicious bootloader also copies the vote-stealing executable to the memory card as a file named

AV.EXE. Then, immediately before starting Windows, the virus bootloader scans the region of RAM

occupied by the operating system image (0x8C080000–0x8C67FFFF) for the hard-coded string in the

taskman.exe binary that points to the BallotStation executable \FFX\Bin\BallotStation.exe

and replaces it with \Storage Card\AV.EXE. Consequently, when Windows starts, taskman.exe will

launch the demonstration vote-stealing application instead of BallotStation.

When the demonstration vote-stealing application on the memory card starts, it first renames the legitimate

BallotStation executable to \FFX\Bin\AccuVote.exe, and then it copies itself to the machine’s on-board

flash memory with the name \FFX\Bin\BallotStation.exe. It adopts the name of the BallotStation

executable so it will still run at start-up even if the machine is booted without a memory card in the first PC

Card slot. Next, it copies the malicious boot loader image from the card to the on-board flash . Thereafter,

the software periodically checks whether an uninfected memory card is present in the machine, and, if so, it

copies the virus files onto the card so that other machines where the card is used will become infected. Finally,

the vote-stealing application runs in the background changing votes in the manner described in Section 4.2.

4.4 Demonstration Denial-of-Service Attack

To illustrate how malicious software running on an AccuVote-TS could launch a denial-of-service (DoS)

attack, we developed a demonstration attack program that, on command, erases the contents of both the

currently-inserted memory card and the machine’s on-board flash memory. This attack not only destroys

all records of the election currently in progress (both the primary and backup copies), but also renders the

machine inoperable until a service technician has the opportunity to dismantle it and restore its configuration.

The demonstration DoS program is comprised of a user-space Windows CE executable that triggers the

attack and a malicious bootloader that functions like an ordinary bootloader, except that upon receiving the

appropriate signal, it completely erases the currently-inserted memory card and the machine’s on-board flash

memory. The user-space trigger program works by first writing a special value to a part of the machine’s onboard

flash memory that is accessible from user-space programs and then crashing the machine by invoking

the PowerOffSystem() Windows CE API call. The PowerOffSystem() API is supposed to put the

system in a low-power ”sleep” mode from which it can later ”wake-up,” but when this API is invoked on an

AccuVote-TS, the machine simply crashes. When the machine is rebooted (which must be done manually),

the malicious bootloader notices that the special value has been written to the machine’s on-board flash

memory. On this signal, it completely erases the contents of both the currently-inserted memory card and the

machine’s on-board flash memory. In so doing, the malicious bootloader destroys all of the data, software,

and filesystem formating on both the memory card and the on-board flash memory.

In order to account for the possibility that the malicious bootloader never gets a chance to completely

erase both storage media or that the memory card is removed before the machine is rebooted, the user-space

resulting fboot.nb0 file. Then, when the virus bootloader checks for the presence of the fboot.nb0 file on the card, it only

prints out fake update messages if the file does not have those attributes. Alternatively, the virus bootloader could identify copies of

itself by examining the contents of the fboot.nb0 file for some characteristic bit string.

16

trigger program deletes as much as it can before crashing the machine. It deletes all of the files on the memory

card and on the machine’s on-board \FFX file system including both the primary and backup copies of the

election results (election.brs), the audit logs (election.adt), and the BallotStation executable.

When it deletes these files, it overwrites each of them with garbage data to make it less likely that the files’

data will ever be recovered.

While our demonstration DoS attack is triggered by a user’s command, a real attacker could create

malicious software that only triggers the above attack on a specific date and time, such as on election day. An

attacker could also design the attack to launch in response to a specific trend in voting during an election,

such as an apparent victory for a particular candidate. Like a vote-stealing attack, a DoS attack could be

spread by a virus.

5 Mitigation

The vulnerabilities that we describe can be mitigated, to some extent, by changing voting machine designs

and election procedures. In this section we discuss several mitigation strategies and their limitations.

5.1 Modifications to DRE Software and Hardware

The AccuVote-TS machine is vulnerable to computer viruses because it automatically loads and runs code

found on memory cards without authenticating it. Its software could be redesigned to prevent the spread

of viruses, however. One approach would be to digitally sign all software updates and have the machine’s

software verify the signature of each update before installing it. Such a change would ensure that only updates

signed by the manufacturer or another trusted certifying authority could be loaded. 11 It would also be helpful

to require the person using the machine to confirm any software updates. Confirmation of updates would

not prevent a malicious person with physical access to the mchine from loading an update, but at least it

would make the accidental spread of a virus less likely while the machine was being used by honest election

officials.

While redesigning the voting machine’s software can help mitigate some of the security problems that we

identify, there are other problems inherent in the AccuVote-TS hardware architecture that cannot be addressed

by software changes. For example, there is nothing to stop an adversary who has physical access to the

machine from booting and installing his own malicious software by replacing the socketed EPROM chip on

the motherboard. Furthermore, because all of the machine’s state is kept in rewritable storage (RAM, flash

memory, or a memory card), it is impossible to create tamper-proof logs, records, and counters. In addition,

as is the case with ordinary PCs, it is difficult to determine with certainty that the machine is actually running

the software that it is supposed to run. Rootkit techniques [12] and virtualization technologies [16], which

are often used to conceal malware in the PC setting, could be adapted for use on the voting machines.

Researchers have proposed various strategies for building specialized hardware capable of maintaining

tamper-proof and tamper-evident logs, records, and counters (e.g., [28]), as well as software strategies that

provide more limited protection (e.g., [25]). Alhough such methods could prevent attacks that aim to alter

votes after they have been recorded, they could not prevent malicious code from changing future votes by

altering data before it is sent to the storage device.

Assuring a computer’s software configuration is also a notoriously difficult problem, and research has

focused on mechanisms to ensure that only approved code can boot [1] or that a machine can prove to a

remote observer that it is running certain code [28]. For example, commercial systems such as Microsoft’s

11Adding signatures would not be effective if a machine has already been infected with malicious code; machines would need

to be booted from EPROM and completely restored to a known state before their software were updated to a version that checked

signatures.

17

Xbox game console have incorporated mechanisms to try to resist modification of the boot code or operating

system, but they have not been entirely successful [13]. Although mechanisms of this type are imperfect and

remain subjects of active research, they seem appropriate for voting machines because they offer some level

of assurance against malicious code injection. It is somewhat discouraging to see voting machine designers

spend much less effort on this issue than game console designers.

While changes to the hardware and software of voting machines can reduce the threats of malicious code

injection and log tampering, no purely technical solution can eliminate these problems.

5.2 Limiting Access to Machines and Memory Cards

Despite the best efforts of hardware and software designers, any physical access to a computer still raises

the possibility of malicious code installation, so election offcials should limit access to voting machines’

internals, their memory cards, and their memory card slots to the extent possible.

There is some benefit in sealing the machine’s case, memory card, and card bay door with individually

numbered tamper-evident seals, in the hope of detecting illicit accesses to these areas. While these measures

may expose some classes of attacks, they make denial-of-service attacks easier. Suppose, for example, that

a malicious voter cuts a seal while an election is in progress. If machines with broken seals are treated

as completely untrustworthy, then cutting the seal is itself an effective denial-of-service attack. If broken

seals are usually ignored when everything else seems to be in order, then an attacker has a good chance

of successfully inserting malicious code that cleans up after itself. There seems to be no fully satisfactory

compromise point between these two extremes.

Even leaving aside the possibility that voters will deliberately break seals, broken seals are an unfortunately

common occurrence. The most comprehensive study of AccuVote DRE election processes in practice

examined the May 2006 primary election in Cuyahoga County, Ohio, which used AccuVote-TSx machines.

The study found that more than 15% of polling places reported at least one problem with seals [9].

The available evidence is that machines and memory cards are not handled with anything approaching the

necessary level of care. For example, the Cuyahoga County study [9] reported many procedural weaknesses:

“A lack of inventory control and gaps in the chain of custody of mission critical assets (i.e. DRE

memory cards, [DREs], . . . )” (p. 103)

“the systems of seals, signatures and other security features of the. . . machine memory cards were not

implemented on a consistent basis” (p. 109)

“It appears that memory cards are regularly removed and re-inserted when a DRE becomes out-ofservice.

Security tabs are broken and no log of this remove and replace activity is maintained. . . There

is no indication that a record comparing memory card to DRE serial number is kept.” (p. 138)

“Security seals are not checked for integrity at the end of Election Day, nor are they matched with

a deployment list of Security seal serial numbers. There is no attempt to reconcile memory cards

intended for the precinct with memory cards removed from the DREs at the end of the day. . . Therefore,

it is unknown whether these memory cards were tampered with during Election Day.” (p. 139)

“There is no established chain of custody during the transfer of the memory cards. . . from the vote

center to the BOE [Board of Elections].” (p. 140)

“Security seals are collected upon return to the BOE, but these serial numbers are neither logged nor

checked against the original security seal serial numbers deployed with the memory cards. Therefore,

it is unknown whether these memory cards were tampered with during transport to the BOE from the

polling location.” (p. 140)

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These problems require immediate attention from election officials.

Security seals do some good, but it is not a solution simply to assume that seals will always be used,

always be checked, and never be broken. Inevitably, some seals will be missing or broken without an

explanation, providing potential cover for the insertion of malicious code or a voting machine virus.

5.3 Effective Parallel Testing

In parallel testing, election officials choose some voting machines at random and set them aside, casting

simulated votes on them throughout election day and verifying at the end of the election that the machines

counted the simulated votes correctly. The goal of parallel testing is to trigger and detect any vote-stealing

software that may be installed on the machines.

A challenge in parallel testing is how to make the simulated voting pattern realistic. If the pattern is

unrealistic in some respect—if, say, the distribution of votes throughout the day doesn’t match what a real

voting machine would see—then vote-stealing software may be able to tell the difference between a real

election and parallel testing, allowing the software to steal votes in the real election while leaving results

unchanged in parallel testing.

Parallel testing is also vulnerable to a “secret knock” attack by a testing insider. Generally, parallel

tests are carried out by representatives from all political parties to ensure impartiality. However, if one

representative has placed vote altering code on the machines, she could disable the code on the machine

being tested by issuing a surreptitious command. For example, the code might watch for a specific sequence

of touches in a normally unused area of the screen and deactivate its vote altering function in response.

Preventing this kind of attack requires carefully scripting the testing procedure.

Alternatively, a secret knock might be used to activate malicious code. In this scheme, malicious voters

would perform the secret knock on the machines being used to collect real votes, or a malicious election

worker would perform it surreptitiously when setting up the machines, and vote-stealing software would

wait for this secret knock before operating. Machines chosen for parallel testing would not see the secret

knock and so would count votes honestly. This approach has the drawback (for the attacker) of requiring a

significant number of malicious voters or a malicious poll worker to participate, though these participants

would not have to know all the details of the attack.

These possibilities reduce the usefulness of parallel testing in practice, but we think it can still be a

worthwhile precaution when conducted according to rigorously controlled procedures.

5.4 Effective Whole-System Certification

Despite their very serious security flaws, the Diebold DREs were certified according to federal and state

standards. This demonstrates that the certification processes are deficient. The Federal Election Commission’s

2002 Voting System Standards [10] say relatively little about security, seeming to focus instead on the

machine’s reliability if used non-maliciously.

The U.S. Election Assistance Commission issued voluntary voting system guidelines [29] in 2005. These

are considerably more detailed, especially in the area of security, than the FEC’s 2002 standards. Though

it would not be entirely fair to apply the 2005 guidelines to the pre-2005 version of the AccuVote software

we studied, we do note that the AccuVote-TS hardware architecture may make it impossible to comply with

the 2005 guidelines, in particular with the requirement to detect unauthorized modifications to the system

software (see [29], Volume I, Section 7.4.6). In practice, a technology can be deployed despite noncompliance

with certification requirements, if the testing agencies fail to notice the problem.

In general, the certification process seems to rely more on testing than analysis. Testing is appropriate for

some properties of interest, such as reliability in the face of heat, cold, and vibration, but testing is ill-suited

for finding security problems. As discussed frequently in the literature, testing can only show that a system

19

works under specific, predefined conditions; it generally cannot ensure that there is no way for an attacker

to achieve some goal by violating these conditions. Only a competent and thorough security analysis can

provide any confidence that the system can resist the full range of realistic attacks.

Weak certification would be less of a problem if information about the system’s design were more widely

available to the public. Researchers and other experts would be able to provide valuable feedback on voting

machine designs if they had the information to do so. Ideally, strong certification procedures would be

coupled with public scrutiny to provide the highest assurance.

5.5 Voter-Verifiable Paper Trail and Random Audits

The most important strategy for mitigating vote-stealing attacks is to use a voter-verifiable paper audit trail

(VVPAT) coupled with random audits. The VVPAT creates a paper record, verified visually by the voter, of

how each vote was cast. This record can be either a paper ballot that is deposited by the voter in a traditional

ballot box, or a ballot-under-glass system that keeps the paper record within the voting machine but lets the

voter see it [19]. A VVPAT makes our vote-stealing attack detectable. In an all-electronic system like the

Diebold DREs, malicious code can modify all of the logs and records in the machine, thereby covering up its

vote stealing, but the machine cannot modify already created paper records, and the accuracy of the paper

records is verified by voters.

Paper trails have their own failure modes, of course. If they are poorly implemented, or if voters do not

know how or do not bother to check them, they may have little value [2, 9]. The real advantage of a paper

trail is that its failure modes differ significantly from those of electronic systems, making the combination of

paper and electronic recordkeeping harder to defraud than either would be alone. Requiring a would-be vote

stealer to carry out both a code-injection attack on the voting machines and a physical ballot box stuffing

attack would significantly raise the difficulty of attacking the system.

Paper ballots are only an effective safeguard if they are actually used to check the accuracy of the

machines. This need not be done everywhere. It is enough to choose a small fraction of the polling places at

random and verify that the paper ballots match the electronic records there. If the polling places to recount

are chosen by a suitable random procedure, election officials can establish with high probability that a full

comparison of paper and electronic records would not change the election’s result.

Another limitation of VVPATs is that they cannot stop a denial-of-service attack from spoiling an election

by disabling a large number of voting machines on election day. Given this possiblity, if DREs are used, it is

worthwhile to have an alternative voting technology available. A decidedly low-tech system such as paper

ballots may be suitable for this purpose.

6 RelatedWork

Several previous studies have criticized the security of the Diebold AccuVote DRE systems. The first major

study of these machines was published in 2003 by Kohno et al. [17], who did not have access to a machine

but did have a leaked version of the source code for BallotStation. They found numerous security flaws and

concluded that the software’s design did not show evidence of any sophisticated security thinking.

Public concern in light of Kohno’s study led the state of Maryland to authorize two security studies. The

first study, by SAIC, reported that the system was “at high risk of compromise,” but much of the basis for this

conclusion was redacted in the public version of SAIC’s report [26]. RABA, a security consulting firm, was

hired to do another independent study of the Diebold machines. RABA had access to a number of machines

and some technical documentation. Their study [24] was generally consistent with Kohno’s findings, and

found some new vulnerabilities. It suggested design changes to the Diebold system, and some steps that

Maryland might take to reduce the risk of security problems. The state of Maryland responded by adopting

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many of RABA’s suggestions [18].

A further security assessment was commissioned by the Ohio Secretary of State and carried out by the

Compuware Corporation [4]. This study examined several DRE systems, including the AccuVote-TS running

the same version of BallotStation as our machine, and concluded that several high risk security problems

were present.

In 2006, in response to reports that Harri Hursti had found flaws in Diebold’s AccuBasic subsystem, the

state of California asked Wagner, Jefferson, and Bishop to perform a study of AccuBasic security issues.

Their report [30] found several vulnerabilities.

Later in 2006, Hursti released a report describing several security weaknesses in Diebold’s software

relating to the automatic installation of unauthenticated software [14].

Our work builds on these previous reports. Our findings generally confirm the behaviors and vulnerabilities

described by Kohno et al., RABA, Wagner et al., and Hursti, and demonstrate through proof-of-concept

attack implementations that the vulnerabilities can be exploited to change election results. To our knowledge,

our work is the first comprehensive, public description of these aspects of Diebold’s design.

Several studies discuss general issues in the design of software-based attacks on DRE voting machines.

Kelsey [15] catalogs the attacker’s design choices; our analysis confirms that all or nearly all of the attack

options Kelsey discusses can be carried out against the Diebold machine we studied. The Brennan Center

report [2] offers a broader but less technical discussion; its discussion of malicious software injection attacks

is based partially on Kelsey’s analysis.

Additionally, there is an extensive literature on electronic voting in general, which we will not attempt to

survey here.

7 Conclusion

From a computer security standpoint, DREs have much in common with desktop PCs. Both suffer from

many of the same security and reliability problems, including bugs, crashes, malicious software, and data

tampering. Despite years of research and enormous investment, PCs remain vulnerable to these problems, so

it is doubtful, unfortunately, that DRE vendors will be able to overcome them.

Nevertheless, the practical question facing public officials is whether DREs provide better security than

other election technologies, which have their own history of failure and fraud. DREs may resist small-scale

fraud as well as, or better than, older voting technologies; but DREs are much more vulnerable to large-scale

fraud. Attacks on DREs can spread virally, they can be injected far in advance and lurk passively until

election day, and they can alter logs to cover their tracks. Procedures designed to control small-scale fraud

are no longer sufficient—DREs demand new safeguards.

In 2003, Diebold claimed that the AccuVote-TS software provided strong security guarantees:

The correctness of the software has been proven.. . . The assertion that there are any exploitable

attack vectors is false. The implication that malicious code could be inserted into the system is

baseless.

([7], p. 25, emphasis in original) Our analysis shows conclusively that these statements by Diebold were

incorrect—there are several exploitable attack vectors and malicious code can be inserted into the system.

We expect Diebold to respond to this paper by offering similar assurances about other versions of their

software and about their closely related AccuVote-TSx product. In light of past experience, public officials

should remain skeptical until such claims are confirmed by independent investigators with full access to the

machines and software.

Public officials who had planned to rely on Diebold DREs for the November 2006 elections face a

dilemma. The changes needed to conduct secure elections with the AccuVote-TS cannot plausibly be

21

implemented by November. One option is to switch to a backup election technology such as precinct-count

paper ballots. Such a change is costly and carries some risk. The other option is to implement some partial

countermeasures against malicious code and viral attacks, and hope for the best.

Electronic voting machines have their advantages, but experience with the AccuVote-TS and other

paperless DREs shows that they are prone to very serious vulnerabilities. Making them safe, given the

limitations of today’s technology, will require safeguards beginning with a voter-verifiable paper audit trail

and truly independent security evaluation.

Acknowledgments

We thank Andrew Appel, Jeff Dwoskin, Laura Felten, Shirley Gaw, Brie Ilenda, Scott Karlin, Yoshi Kohno,

David Robinson, Avi Rubin, Adam Stubblefield, Dan Wallach, and Harlan Yu for technical help, information,

and feedback. We are especially grateful to the party who provided us the machine to study.

This material is based upon work supported under a National Science Foundation Graduate Research

Fellowship. Any opinions, findings, conclusions or recommendations expressed in this publication are those

of the authors and do not necessarily reflect the views of the National Science Foundation.

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