Copy/paste, some formatting, no tables. Extra carriage returns (sorry)... "Implementing the gadgets" section stripped off...
Abstract
A secure voting machine design must withstand new attacks
devised throughout its multi-decade service lifetime.
In this paper, we give a case study of the longterm
security of a voting machine, the Sequoia AVC
Advantage, whose design dates back to the early 80s.
The AVC Advantage was designed with promising security
features: its software is stored entirely in read-only
memory and the hardware refuses to execute instructions
fetched from RAM. Nevertheless, we demonstrate that an
attacker can induce the AVC Advantage to misbehave
in arbitrary ways--including changing the outcome of
an election--by means of a memory cartridge containing
a specially-formatted payload. Our attack makes essential
use of a recently-invented exploitation technique
called return-oriented programming, adapted here to the
Z80 processor. In return-oriented programming, short
snippets of benign code already present in the system
are combined to yield malicious behavior. Our results
demonstrate the relevance of recent ideas from systems
security to voting machine research, and vice versa. We
had no access either to source code or documentation beyond
that available on Sequoia's web site. We have created
a complete vote-stealing demonstration exploit and
verified that it works correctly on the actual hardware.
1 Introduction
A secure voting machine design must withstand not only
the attacks known when it is created but also those invented
through the design's service lifetime. Because
the development, certification, and procurement cycle for
voting machines is unusually slow, the service lifetime
can be twenty or thirty years. It is unrealistic to hope
that any design, however good, will remain secure for so
long.1
In this paper, we give a case study of the long-term
security of a voting machine, the Sequoia AVC Advantage.
The hardware design of the AVC Advantage dates
back to the early 80s; recent variants, whose hardware
differs mainly in featuring a daughterboard enabling audio
voting for the blind [3], are still used in New Jersey,
Louisiana, and elsewhere. We study the 5.00D version
The AVC Advantage voting machine we studied.
(which does not include the daughterboard) in machines
decommissioned by Buncombe County, North Carolina,
and purchased by Andrew Appel through a government
auction site [2].
The AVC Advantage appears, in some respects, to offer
better security features than many of the other directrecording
electronic (DRE) voting machines that have
been studied in recent years. The hardware and software
were custom-designed and are specialized for use in a
DRE. The entire machine firmware (for version 5.00D)
fits on three 64kB EPROMs. The interface to voters
lacks the touchscreen and memory card reader common
in more recent designs. The software appears to contain
fewer memory errors, such as buffer overflows, than
some competing systems. Most interestingly, the AVC
Advantage motherboard contains circuitry disallowing
instruction fetches from RAM, making the AVC Advantage
a true Harvard-architecture machine.2
Nevertheless, we demonstrate that the AVC Advantage
can be induced to undertake arbitrary, attackerchosen
behavior by means of a memory cartridge containing
a specially-formatted payload. An attacker who
has access to the machine the night before an election can
use our techniques to affect the outcome of an election by
replacing the election program with another whose visible
behavior is nearly indistinguishable from the legitimate
program but that adds, removes, or changes votes
as the attacker wishes. Unlike those attacks described
1
in the (contemporaneous, independent) study by Appel
et al. [3, 4] that allow arbitrary computation to be induced,
our attack does not require replacing the system
ROMs or processor and does not rely on the presence of
the daughterboard added in later revisions.
Our attack makes essential use of return-oriented programming
[24, 8], an exploitation technique that allows
an attacker who controls the stack to combine short instruction
sequences already present in the system ROM
into a Turing-complete set of combinators (called "gadgets"),
from which he can synthesize any desired behavior.
(Our exploit gains control of the stack by means of
a buffer overflow in the AVC Advantage's processing of
a type of auxiliary cartridge; see Section 5.) Defenses
that prevent code injection, such as the AVC Advantage's
instruction-fetch hardware, are ineffective against
return-oriented programming, since it allows an attacker
to induce malicious behavior using only preëxisting, benign
code. Return-oriented programming was introduced
by Shacham at CCS 2007 [24], a full two decades after
the AVC Advantage was designed. Originally believed
to apply only to the x86, return-oriented programming
was generalized to the SPARC, a RISC architecture, by
Buchanan et al. [8]. In Section 4 we show that returnoriented
programming is feasible on the Z80 as well,
which may be of independent interest. In addition, we
show that it is possible starting with a corpus of code an
order of magnitude smaller than previous work.
Using return-oriented programming, we have developed
a full demonstration exploit for the AVC Advantage,
by which an attacker can divert any desired fraction
of votes from one candidate to another. We have
tested that this exploit works on the actual hardware; but
in developing our exploit we used a simulator for the machine.
See Sections 5 and 6 for more on the exploit and
Section 2 for more on the simulator.
Our results demonstrate the relevance of recent ideas
from systems security to voting machine research, and
vice versa. Our attack on the AVC Advantage would
have been impossible without return-oriented programming.
Conversely, the AVC Advantage provides an ideal
test case for return-oriented programming. In contrast to
Linux, Windows, and other desktop operating systems,
in which the classification of a process' memory into
executable and nonexecutable regions can be changed
through system calls, the AVC Advantage is a true Harvard
architecture: ROM is executable, RAM is nonexecutable.
3 The corpus of benign instruction on which we
draw is just 16kB, an order of magnitude smaller than in
previous attacks.
In designing our attack, we had access neither to
source code nor to usage documentation; through reverse
engineering of the hardware and software, we have reconstructed
the functioning of the device. This is in contrast
to the Appel et al. report, whose authors did have
this access, as well as to most of the previous studies of
voting machines (discussed in Section 1.1 below). We
had access to an AVC Advantage legitimately purchased
from a government surplus site by Andrew Appel [2]
and a memory cartridge similarly obtained by Daniel Lopresti.
Since voting machines are frequently left unattended
(as Ed Felten has documented, e.g., at [12]), we
believe that ours represents a realistic attack scenario.
We hope that our results go some way towards answering
the objection, frequently raised by vendors, that voting
security researchers enjoy unrealistic access to the systems
they study.4
1.1 Related work
Much of the prior research on voting machine security
has relied on access to source code. The first such work
by Kohno et al. [18] analyzed the Diebold5 AccuVote-
TS voting machine and found numerous problems. The
authors had no access to the voting machine itself but the
source code had appeared on the Internet. Many of the
issues identified were independently confirmed with real
voting machines [9, 21, 22].
Follow up work by Hursti examined the AccuVote-
TS6 and AccuVote-TSx voting machines using "source
code excerpts" and by testing the actual machines. Backdoors
were found that allowed the system to be extensively
modified [17]. Hursti's attacks were confirmed
and additional security flaws were discovered by Wagner
et al. [27].
In 2006, building on the previous work, Feldman et al.
examined an AccuVote-TS they obtained. The authors
did not have the source code, but they note that "the behavior
of [the] machine conformed almost exactly to the
behavior specified by the source code to BallotStation
version 4.3.1" which was examined by Kohno et al. In
addition to confirming some of the security flaws found
in the previous works, they demonstrated vote stealing
software and a voting machine virus that spreads via the
memory cards used to load the ballot definition files and
collect election results [11].
In 2007, California Secretary of State Debra Bowen
decertified and then conditionally recertified the direct
recording electronic voting machines used in California
as part of a top-to-bottom review. As part of the recertification,
voting machine vendors were required to
make available to independent reviewers documentation,
source code, and several voting machines. In all cases,
significant problems were reported with the procedures,
code, and hardware reviewed [6].
Also in 2007, Ohio Secretary of State Jennifer Brunner
ordered project EVEREST--Evaluation and Validation
of Election Related Equipment, Standards and Testing--
as a comprehensive review of Ohio's electronic voting
2
machines. Similar to California's top-to-bottom review,
the reviewers had access to voting machines and source
code. Again, critical security flaws were discovered [7].
2 The road to exploitation
In 1997, Buncombe County, North Carolina, purchased
a number of AVC Advantage electronic voting machines
for $5200 each. In January 2007, they retired these machines
and auctioned them off through a governmentsurplus
web site. Andrew Appel purchased one lot of
five machines for $82 in total [2].
Reverse-engineering the voting machine. Two members
of our team immediately began reverse engineering
the hardware and software. The machine we examined
is an AVC Advantage revision D. It contains ten circuit
boards, including the motherboard shown in Figure
1, with an eleventh inside the removable memory
cartridge--see below. Each is an ordinary two-sided
epoxy-glass type. Since these are somewhat translucent,
with the use of a bright light, magnifying glass, lowvoltage
continuity tester, and data sheets for the components,
we were able to trace and reconstruct the circuit
schematic diagram, and from that deduce how the unit
worked. We filled in remaining details by partially disassembling
the machine's software using IDA Pro.
After approximately six man-weeks of labor, we produced
a functional specification [14] describing the operation
of the hardware from the perspective of software
running on the machine. We documented 47 I/O functions
that the processor can execute to control hardware
functions, such as mapping areas of ROM into the address
space, interfacing with the voter panel and operator controls,
and reading or writing to the memory cartridge.
Reverse-engineering the results cartridge. The AVC
results cartridge is a plastic box about the dimensions of
a paperback book with a common "ribbon-style" connector
on one end that mates to the voting machine. Inside,
there is an ordinary circuit board containing static RAM
chips--backed by two type AA batteries--and common
TTL 74-series integrated circuits. There is no microcontroller;
instead all control signals come directly from the
voting machine. Much of the internal circuitry appears
to have been designed to withstand hot-plugging and to
prevent accidental glitching of the memory contents.
There is an additional 8bit of nonmemory data that
can be read from the unit corresponding to the type and
revision of the memory cartridge. This data is set by etch
jumpers on the circuit board. We were able to change
the type and revision of the cartridge by cutting the associated
trace on the circuit card and wiring alternate
jumpers.
The contents of memory can be read or written by
powering the device and toggling the appropriate input
signals. We constructed a simple microcontroller circuit
to interface with the cartridge to perform reads and
writes. The microcontroller simply controls the appropriate
signals on the cartridge connector to perform the
operation indicated by a controlling program communicating
with the microcontroller via a serial port. No access
to the inside circuitry was necessary.
By disassembling the software and looking at the contents
of a valid results cartridge, we were able to understand
the format of the file system used on the memory
cartridges (and also the internal file system of the 128kB
SRAM described below) and many of the files used by
the voting machine.
Crafting the exploit. Joshua Herbach used the hardware
functional specifications to develop a simulator for
the machine [15], which another member of our team
subsequently improved.6 Our simulator now provides
cycle-accurate emulation of the Z80, and it executes the
AVC election software without any apparent flaws.
We developed our exploit almost entirely in the simulator,
only returning to the actual voting machine hardware
at the end to validate it. Remarkably, the exploit
worked the first time we tried it on the real hardware.
Total cost. Starting with no source code or schematics,
we reverse engineered the AVC Advantage and developed
a working vote-stealing attack with less than 16
man-months of labor. We estimate the cost of duplicating
our effort to be about $100,000, on the private market.
3 Description of the AVC Advantage
In this section, we give a description of the hardware and
software that makes up the AVC Advantage in some detail.
Readers not interested in such low-level details are
encouraged to skip ahead to Section 4, referring back to
this section for details as needed.
3.1 Software
The core of the version-5.00D AVC Advantage is a
Z80 CPU and three 64kB erasable, programmable ROMs
(EPROMs) which contain both code and data for the Advantage.
Each EPROM is divided into four 16kB segments:
BIOS, System Toolkit, Toolkit 2, Toolkit 3, Election
Program, Election Toolkit, Reports Program, Consolidation
Program, Ballot Verify Program, Define Ballot
Program, Maintenance Utilities, and Setup Diagnostics;
see Figure 2.
When the Advantage is powered on, execution begins
in the BIOS at address 0x0000. The BIOS contains a
mixture of hand-coded assembly and compiler generated
code for interrupt handling, remapping parts of the address
space (see Section 3.2), function call prologues and
epilogues, thunks for calling code in other segments, and
code for interacting with the peripherals.
3
Figure 1: We reverse engineered the AVC Advantage hardware. The motherboard, shown here, is composed mostly of
discrete logic and measures 14in14in. Election software is stored in removable ROM chips (white labels). The results and
auxiliary memory cartridges are plugged directly into the motherboard (upper right).
Apart from the BIOS, each EPROM segment contains a
16B header followed by a mixture of (mostly) compilergenerated
code and data. The segments with "Toolkit"
in their name7 in addition to the Reports Program consist
of the header followed immediately by a sequence of
jp addr instructions, each of which jumps to a global
function in the segment. For the entries in this sequence
corresponding to global functions, there is a corresponding
thunk in the BIOS which causes the segment to be
mapped into the address space before transferring control
to the function. Functions in one segment can call global
functions in another segment by way of the thunks.
Each of the remaining segments is a self-contained
program with just a single entry point immediately after
the segment header. When a program is run, much of
the state of the previous program--including the stack
and the heap--is reset. In particular, any data written to
the stack during one program's execution are lost during
a second program's execution.
A typical sequence of events for an election would
include the following. The machine is powered on and
begins executing in the BIOS. The BIOS performs some
initialization and tests before transitioning to a menu in
Maintenance Utilities awaiting operator input. The operator
selects the Setup Diagnostics choice and the corresponding
Setup Diagnostics program is run. This performs
various software and hardware tests before transitioning
to the Define Ballot Program. This program
checks the memory cartridge inserted into the machine
and upon finding a ballot definition transitions to the Ballot
Verify Program. The Ballot Verify Program checks
that the format of the ballot is correct and ensures that
the files which hold the vote counts are empty. After
this, it illuminates the races and candidates so that the
technician can verify that they are correct. Assuming everything
is correct, control transfers to the Election Program
for the pre-election logic and accuracy testing. The
voting machine is powered off at this point and shipped
to the polling places. After it has been powered back on,
control again passes to the Election Program, this time
for the official election.
The ZiLOG Z80 CPU is an 8bit accumulator machine.
All 8bit arithmetic and logical operations use the accumulator
register a as a source register and the destination
register. Apart from the accumulator register, there are
six general purpose 8bit registers b, c, d, e, h, and l
which can be paired to form three 16bit registers bc, de,
and hl. These registers along with an 8bit flags regis-
ter f and 16bit stack pointer sp and program counter pc
registers are compatible with the Intel 8080. In addition,
there are two 16bit index registers ix and iy, an interrupt
vector register i, a DRAM refresh counter register r,
and four shadow registers af', bc', de', and hl' which
can be swapped with the corresponding nonshadow registers.
The Advantage uses the shadow registers for the
interrupt handler which obviates the need to save and restore
state in the interrupt handler. See [28] for more
details.
Due to the limited ROM space for code and data,
compiler-generated functions which take arguments or
have local variables use additional functions to implement
the function prologue and epilogue. The prologue
pushes the iy and ix registers and decrements the stack
pointer to reserve room for local variables. It then sets
iy to point to the first argument and ix-80h to point
to the bottom of the local stack space. Finally, it pushes
the stack-address of the two saved index registers and the
address of the epilogue function before jumping back to
the function that called the prologue. See Figure 3. The
epilogue function pops the saved pointer to the index registers
and loads sp with it. Then ix and iy are popped
and the epilogue returns to the original saved pc. It is the
caller's responsibility to pop the arguments off the stack
once the callee has returned.
3.2 Address space layout
The AVC Advantage has a 16bit flat address space divided
into four distinct regions. The bottom 16kB
is mapped to the BIOS. The 16kB-32kB range can
be mapped to one of the 12 16kB aligned segments
on the three program EPROMs. This mapping is controlled
by the software using the Z80's out instruction.
The 32kB-63kB range addresses the bottom 31kB of a
32kB, battery-backed SRAM. Finally, the top 1kB of the
address space can be mapped to either the top 1kB of
the 32kB SRAM or it can be mapped to any 1kB aligned
region of a 128kB, battery-backed SRAM. This mapping
can be changed by the software using the Z80's out instruction.
For more detail, see [14].
The AVC Advantage's stack starts at address 0x8FFE
and grows down toward smaller addresses. The heap occupies
a region of memory starting from an address specified
by the currently active program to 0xEBFF. Scattered
throughout the rest of 32kB main memory, there
are various global variables and space for the string table
of the active program. In addition, starting at 0x934E
and growing down, there is space for a module call stack
which allows modules to make calls to functions in other
modules, such as printf or strcpy. See Figure 4.
As the lower 32kB of the address space corresponds
to EPROMs, data cannot be written to those addresses
and attempts to do so are silently ignored by the hard-
ware.8 Similarly, as the upper 32kB of the address space
is for writable memory, not program code, any attempt
to fetch an instruction from those addresses raises a nonmaskable
interrupt (NMI). The NMI causes the processor
to load a known constant into the pc register and execution
resumes in the BIOS where the processor will be
halted after displaying an error message on the operator
LCD. This design makes the AVC Advantage a Harvardarchitecture
computer.
4 Return-oriented programming
Since the AVC Advantage is a Harvard architecture computer,
traditional code injection attacks cannot succeed
because any attempt to read an instruction from data
memory causes an NMI which will halt the machine. In
practice, given a large enough corpus of code, this is
not a barrier to executing arbitrary code using returnoriented
programming--an extension of return-to-libc
attacks where the attacker supplies a malicious stack containing
pointers to short instruction sequences ending
with a ret [24, 8].
The Z80 instruction set is very dense. Every byte is
either a valid opcode or is a prefix byte. As there are no
invalid or privileged instructions, instruction decoding of
any sequence of data always succeeds. This density facilitates
return-oriented programming since we can exploit
unintended instruction sequences to build gadgets--a
sequence of pointers to instruction sequences ending
with a ret. For a concrete example, the BIOS contains
the code fragment ld bc,2; ret--a potentially useful
instruction sequence in its own right--which is 01
02 00 c9 in hex where the first three bytes are the
load and the last is the return. If we set the program
counter one byte into the load instruction, then we get the
instruction sequence 02 00 c9 corresponding to the
three instructions ld (bc),a; nop; ret which stores
the value of the accumulator into memory at the address
pointed to by the register bc.
Shacham [24] and later Buchanan et al. [8] had code
corpora on the order of a megabyte from which to construct
gadgets. In contrast, Francillon and Castelluccia
[13] had only 1978B of code with which to craft gadgets;
however, they did not construct a Turing-complete
set of gadgets. This prompts the question: What is the
minimal amount of code required to construct a Turingcomplete
set of gadgets? By constructing a Turingcomplete
set of gadgets using only the AVC Advantage's
BIOS--which consists of 16kB of code and data--we
make progress toward answering that question.
Following Shacham, we wrote a small program to
find sequences of instructions ending in ret. We ran
this program on the AVC Advantage's BIOS. We then
manually devised a Turing-complete set of gadgets from
the instruction sequences found by our program, including
gadgets to control the peripherals like the LCDs
and memory cartridges. We build a collection of gadgets
that implement a 16bit memory-to-memory pseudoassembly
language. See Table 1 for a description of the
pseudo-assembly language and Appendix A for the implementation
of many of the gadgets and a precise explanation
of the notation that will be used in the remainder
of the paper.
(We stress that demonstrating return-oriented programming
on the Z80 is a major contribution of this paper
and of independent interest; we have moved the details to
an appendix to improve the paper's flow.)
Some of the gadgets in Table 1 are straightforward to
construct; others require more finesse due to tricky interactions
among the registers used in the instruction sequences.
For ease of implementation, no state is presumed
to be preserved between gadgets. That is, all arguments
are loaded from memory into registers, operated
upon, and then stored back into memory.9 In this way,
each gadget can be reasoned about independently. The
operands to the gadgets are either global variables--declared
with the .var directive--or immediate values;
labels are resolved to offsets and thus are immediate values.
Some of the instruction sequences described in Appendix
A contain NUL bytes which make them unsuitable
for use in stack smashing attacks using a string copy. An
early implementation of the gadgets took great pains to
avoid all zero bytes. However, using the multi-stage exploit
described below, avoiding zero bytes was unnecessary
except for in the first stage of the exploit which did
not use the gadgets presented in this section. As such,
the simpler form of the gadgets is presented.
It has become traditional in papers on return-oriented
programming to show a sorting algorithm implemented
as a return-oriented program [8, 16]. In Appendix B, we
give the listing for a return-oriented Quicksort. We have
verified that this sorting algorithm works on the actual
AVC Advantage as part of a larger program that prints a
list of numbers on the printer, sorts them, and prints the
sorted list.
5 A multi-stage exploit for the AVC Advantage
Even though many parts of the code we reverse engineered
appear to handle data from memory cartridges
safely, we have been able to find a stack buffer overflow
vulnerability. In this section, we describe this vulnerability
and discuss how an attacker can exploit it to overwrite
the AVC Advantage's stack and reliably induce the execution
of a return-oriented payload of his choice.
We stress that the buffer overflow that we have identified
appears to be unrelated to the one identified by Appel
et al. in their report [3, Section II.26]. Our buffer overflow
occurs in cartridge processing whereas Appel et al.'s
occurs in interaction with the daughterboard (which the
machine we studied lacks); our overflow requires manual
action, whereas Appel et al.'s is triggered on boot; our
overflow is exploitable for diverting the machine's control
flow, whereas Appel et al.'s appears to allow only a
denial of service. We do not know whether the overflow
that we found persists in the more recent AVC Advantage
version that Appel et al. examined.
One of the programs not normally used in an election,
but accessible from the main menu, contains a buffer
overflow while reading from an auxiliary cartridge of a
certain type. (As described in Section 2, we physically
modified a results cartridge so that the AVC Advantage
would recognize it as a cartridge of the type for which the
appropriate menu item is enabled.) A maliciously crafted
field in one of the files allows roughly a dozen bytes to
be written at the location of the saved stack pointer. In
the first stage of the exploit, the hl register is set and
the stack pointer is modified using the sp hl instruction
sequence, inducing a return-oriented jump to
an attacker-controlled location in memory.
For stage two, a section of memory under attacker control
needs to contain gadgets. Fortunately (for the attacker),
a file of fixed size but with several dozen unused
bytes is read from the memory cartridge into a buffer allocated
by malloc. By the time of the overflow in stage
7
Figure 5: The machine has slots for two memory cartridges.
The first cartridge stores ballots and votes. An
attacker could install vote sealing code by inserting a prepared
cartridge into the second slot.
one, this buffer has been deallocated but most of its contents
remain in memory at a known location. This unused
space can be changed to contain gadgets that make up
the second stage of the exploit. The first thing that stage
two does is reallocate memory for the buffer so that additional
allocations will not overlap and thus write over
the gadgets. At the same time, enough memory is allocated
to hold the contents of an additional file from the
memory cartridge. The data from this file--stage three
of the exploit--is read into the allocated buffer. Control
then transfers to stage three which can perform arbitrary
code execution using the gadgets described in Section 4.
We have tested on an AVC Advantage that the exploit
procedure described in this section works, using it both to
run the sorting program described in the previous section
and the vote-stealing exploit described in the next.
6 Using the exploit to steal votes
We have designed and implemented a demonstration
vote-stealing exploit for the AVC Advantage, using the
vulnerability described in the previous section to take
over the machine's control flow. We have tested that our
exploit works on the actual AVC Advantage. (Although
it was designed and debugged exclusively in our simulator,
the exploit worked on the real hardware on the first
try.) In this section, we describe both the actions that an
attacker will undertake to introduce the exploit payload
to the machine and the behavior of the payload itself.
We also note several ways in which the exploit could be
made more resistant to detection by means of forensic
investigation.
Our attacker accesses the AVC Advantage when it is
left unattended the night before the election. Ed Felten
has described how such access is often possible (see, e.g.,
[12]). At this point, the machine has been loaded with
an election definition and has passed pre-LAT.10 The attacker
picks the locks for the back cabinet, the voter
panel, and (later) the open/close polls switch. Appel et al.
have shown that these locks are of a low-security kind
that is easily bypassed [3, Section I.9]. The attacker does
not need to remove any tamper-evident seals; in particular,
he does not need to remove the circuit-board cover.
Having gained access to the back cabinet of the AVC
Advantage, the attacker uses the normal functions to
open the polls, cast a single vote, and close the polls.
(The polls cannot be closed with no votes cast.11) Once
the polls are closed, the attacker unseats the results cartridge.
The cartridge cannot be removed completely because
of the tamper-evident seal; however, the seal is
small enough compared to the holes through which it
is inserted that the cartridge can be disconnected from
the machine. With the polls closed and the cartridge
removed, the attacker uses the two-key reset gesture
("print-more" and "test") to gain access to the machine's
post-election menu. From this menu, he can reset the machine;
after the reset, the machine's main menu is accessible.
(Were the results cartridge not removed, the data
on it would be erased by the reset. The attacker might
be able to recreate this data and rewrite it to the results
cartridge, but unseating the cartridge before the reset obviates
this.)
To this point, the attacker is simply following the same
procedures poll workers and election officials use in running
an election and resetting the AVC Advantage for
the next election. His goal is to gain access to the main
menu, from which he can direct the machine toward the
vulnerability described in Section 5.
The system reset appears to clear the audit logs on the
machine. Our demonstration vote-stealing exploit does
not undo this log-clearing, though a more stealthy attack
might wish to; otherwise, a post-election audit might discover
that log entries are missing. (Although, as Davtyan
et al. have found in their audit of the AccuVote AV-OS
system [10], discrepancies in logs are not uncommon and
may not be perceived as signs of an attack.) Even if
the attack is detected, the original voter intent will not
be recoverable. The attacker can use the post-election
menu to dump the contents of the logs either to a trans-
fer cartridge or to the printer and cause his exploit payload
to restore them once the system is compromised.
In addition, since a vote was cast, the protective counter
has been incremented; however, the protective counter
is subject to software manipulation and could easily be
rolled back if the attacker desires. Traces of the phantom
vote might also remain in the machine or operator logs; if
so, a stealthy exploit would have to remove these traces.
The attacker now reinserts the results cartridge and a
cartridge of the appropriate type into the auxiliary port
and navigates the menus to trigger the vulnerability described
in Section 5. Using a three-stage exploit as described
in Section 5, he takes control of the AVC Advantage
and can execute arbitrary (return-oriented) code.
Note that hardware miniaturization since the design
of the AVC Advantage makes possible the creation of
cartridges much smaller than legitimate cartridges with
orders of magnitude more storage. (Different parts of
memory could be paged in using a "secret knock" protocol.)
A smaller cartridge may allow the attacker to bypass
tamper-evident loops placed on the auxiliary port
guide rails that would prevent the insertion of a legitimate
cartridge (although we are not aware of a jurisdiction
that attempts to limit access to the auxiliary port in
this way); it may also allow him to leave an auxiliary
cartridge in place during voting while avoiding detection,
which would be useful for exploit payloads larger
than can fit in main memory and unused portions of the
results cartridge. (As noted below, our exploit payload
easily fits in main memory.)
The exploit first restores those parts of the machine's
state necessary to allow the election to begin again.
It copies the results cartridge's post-LAT voting files
(which are in their empty state) over the results cartridge's
election files so that the single ballot that was
cast in order to close the polls is erased. It then copies
(most of) the contents of the results cartridge into the internal
memory. At this point, a message is displayed on
the operator LCD instructing the attacker to remove the
auxiliary cartridge and turn off the power.
In order to convincingly simulate power off, we need
the power switch to be in the off position. Luckily, the
AVC Advantage has a soft power switch, so turning the
power knob just sets a flag that can be polled by the processor
at interrupt time to detect power off. So long as the
exploit code disables interrupts (while petting the watchdog
timer to keep it from firing) it can keep the machine
running; it can also detect when, later, the power switch
is turned to the on position. (By contrast, were the machine
actually to power down, the stack would be reset on
a subsequent power up and the attacker would lose control.)
The AVC Advantage features a large 110V battery
designed for 16 hours of operation that we believe will
allow it to remain overnight in this state [23]. Of all the
steps in our exploit, this is the one that most intimately
relies on the details of the AVC Advantage's hardware
implementation. We emphasize that we have tested on
the actual machine that our exploit code is able to survive
a power-down/power-up cycle in this way on battery
power alone.
When the exploit code detects that the power switch
has been turned to the off position, it simulates power
down. It turns off the LEDs in the voter panel, clears the
LCD displays, and turns off any status LEDs. In testing
on an AVC Advantage, we have been able to disable (via
return-oriented code) all indicators of power except the
LCD backlight on the operator panel. This is the most
visible sign of our attack; we are currently studying how
the backlight might be disabled.
The attacker now closes and locks the operator and
voter panels, removes the auxiliary cartridge, and leaves.
The next morning, poll workers open the machine and
use the power switch to turn it on. The exploit code
detects the change and simulates the machine's powerup
behavior, followed by the official election mode messages.
The exploit must now simulate the machine's normal
behavior when poll workers open and close the polls and
when voters cast votes. While it would be possible to
reimplement this behavior entirely using return-oriented
code, the design of the AVC Advantage's voting program
makes it possible for us to reuse large portions of the legitimate
code, making the exploit smaller, simpler, and
more robust. This would be more difficult to do if the
exploit modified votes as they were cast, but we have
instead chosen to wait until polls are closed and only
then change the cast votes retroactively. The absence of
a paper audit trail means that the vote modification will
not be detected. Other possible designs for vote-stealing
software are described by Appel et al. [3, Section I.5-6].
The main voting function is structured as a series
of function calls that can be separated into three main
groups, each called a single time in order in the normal
case. The first group of functions waits for the "open/-
close polls" switch to be set to open and prints the zero
tape. The second group of functions handles all of the
voting, including waiting for the activate button to be
pressed and handling all voter input. Once the polls are
closed, the third group of functions handles printing the
final results tape and all post-election tasks.
Our demonstration exploit uses the high-level functions
in the AVC Advantage's legitimate voting program
to handle all voting until the polls are closed. Then the
exploit reads the vote totals, moves half of the votes for
the second candidate to the first candidate, and changes
the cast vote records (CVRs) to match the vote totals.
(Obviously, any fraction of the votes could be modified.
Furthermore, while our exploit processes the CVR log in
order, changing every CVR cast for the disfavored candidate
until the desired shift has been effected, more sophisticated
strategies are possible.) The exploit now relinquishes
control for good, handing control over to the
legitimate AVC Advantage program to handle all postelection
behavior. When the "Official Election Results
Report" is printed it will reflect the results as modified
by the exploit.
The AVC Advantage contains routines to check the
consistency of its internal data structures. When the data
is inconsistent, e.g., the vote totals do not match the CVR
totals, this is noted in the Results Report. The exploit ensures
that all data structures in memory and on the results
cartridge that are checked by these routines are consistent
whenever the routines are executed.
Even after it has relinquished control, our exploit remains
in main memory until the machine is shut down.
Forensic analysis of the contents of the AVC Advantage's
RAM would be a nontrivial task; nevertheless, a stealthier
exploit would wipe itself from memory before returning
control to the legitimate program. If any portion of the
exploit code is stored on a cartridge, this must be wiped
as well. Because suspicious poll workers might remove
the cartridge before it can be wiped, anything stored on a
cartridge should be kept encrypted, and the exploit code
should scrub the key from RAM if it detects that the cartridge
has been removed.
Our vote-stealing demonstration exploit is just over
3:2kB in size, including all of the code to copy the files
and the memory cart. It fits entirely in RAM, as would
even a substantially more sophisticated exploit: There is
roughly an additional 10kB of unused heap space that
could be used. In addition, any code that is executed
only while the attacker is present need not actually stay
in the heap once it is finished and could be replaced with
additional code for modifying the election outcome.
7 Conclusions
A secure voting machine design must withstand attacks
devised throughout the machine's service lifetime. Can
real designs, even ones with promising security features,
provide such long-term security? In this paper, we have
answered this question in the negative in the case of
the Sequoia AVC Advantage (version 5.00D). We have
demonstrated that an attacker can exploit vulnerabilities
in the AVC Advantage software to install vote-stealing
malware by using a maliciously-formatted memory cartridge,
without replacing the system ROMs. Starting with
no source code, schematics, or nonpublic documentation,
we reverse engineered the AVC Advantage and developed
a working vote-stealing attack with less than 16
man-months of labor. Our exploit relies in a fundamental
way on return-oriented programming, a technique introduced
some two decades after the AVC Advantage
was designed. In mounting the attack, we have extended
return-oriented programming to the Z80 processor.
