Overview and details about the security issues found in the Steam voice
ReVuln Ltd.
Revision 3
Steam and Steamworks __________________________________________________________________________________ 1
The voice chat ____________________________________________________________________________________________ 3
Technical overview of the voice chat ___________________________________________________________________ 5
Security issues ___________________________________________________________________________________________ 9
Security Impact _________________________________________________________________________________________ 12
FAQ _______________________________________________________________________________________________________ 13
Changelog _______________________________________________________________________________________________ 15
Company Information __________________________________________________________________________________ 16
Steam VOIP security
Steam and Steamworks
is the gaming platform developed by Valve Software and used by millions of players around the
world to buy and play games, for multiplayer matchmaking and for its gaming-related social network.
The most interesting feature of Steam is its framework called Steamworks
. It contains the APIs used by
most of the games available on this platform for integrating the following main features
Stats & Achievements
User Authentication & Ownership
Multiplayer Matchmaking
DLC and Content
Peer-to-peer networking
Voice chat
The main aspect of the framework, from a security perspective, is it’s role in increasing the attack surface of
the games that use its API and making them remotely exploitable through its security vulnerabilities.
Additionally the framework and its operations are completely transparent to the final user (the player)
who doesn’t know what level of interaction is allowed from the other players or what are the external
inputs that can be used to access and communicate with the framework.
The target of this security auditing has been just the Voice chat feature that allows the players to
communicate via the voip system integrated in Steam and used on many games.
This research has been commissioned just by the developers of one of the games that use this Steamworks
feature, Epic Games
for the Unreal Engine 4
Steam VOIP security
This security auditing has been performed black-box (no Steam source code available) against the current
versions of Steam and Steamworks available at the moment of the work:
Steam package versions: 1401381906
Steam package versions: 1404163764
Fixes available in Steam from 03 Jul 2014.
Quick considerations about the vulnerable Steamclient.dll:
It supports DEP
It doesn’t support ASLR, which instead is supported by steamclient64.dll but is not used by Steam
It’s digitally signed
The functions affected by stack-related vulnerabilities do not use stack cookies
Steam VOIP security
The voice chat
The VOIP framework available in Steamworks can be used in two ways:
As a voice chat with the Steam friends through the Steam chat interface
In-game with the games that support it
Using the voice chat in Steam requires that the other endpoint is one of our friends and he accepts the voice
This sort of “restriction” doesn’t involve the games where, instead, any player can speak to the other people
inside the same lobby or server, often the members of the same team for those games in which there are
two or more teams.
Additionally there is no need to configure settings, when a player joins a lobby both his voice and the one of
the other players are already active.
Steam VOIP security
The following is an example of users speaking while playing the game Grid 2
The own voice can be captured via push-to-talk or automatically while talking, it depends by the default
settings of the game and in both the cases it’s necessary to have a certain level of input volume to activate
the capturing, just like a squelch
Receiving voice data is enabled by default on all the games and must be manually disabled or limited by the
user if allowed by the game, for example in Grid 2 the game gives automatically “voice to all” everytime we
join and change a lobby.
That means any user playing on the same server, lobby or team of the attacker will be targeted by any
malicious voip data broadcasted by the attacker through the server or peer-to-peer.
Steam VOIP security
Technical overview of the voice chat
Technically the in-game voice chat works in the following way:
All the code necessary to handle the data is located in steamclient.dll and steamclient64.dll
This DLL is located in the Steam folder and a copy, got from the Steamworks framework used by
the game, is located in the game folder
The game loads the DLL of the Steam folder
In this case the DLL works as a wrapper interfaced via IPC (events, shared memory and named
pipes) to the running Steam.exe process
So when the game calls an API, all the operation is executed inside the Steam.exe process through
the steamclient.dll loaded in it (both Steam and the game use this DLL)
A security vulnerability in the API compromises Steam and causes the freezing of the game which
is waiting a reply from the IPC interface
Steam offers some APIs to use its network code but it’s up to the game to use it or their own
protocol to transmit and receive the audio data
The IsteamUser APIs that handle the voice data are very easy to use:
GetVoice for capturing and compressing the voice data from the microphone
DecompressVoice for decoding the received audio data in 16-bit, signed integer PCM format
What a game does is using its own protocol for sending and receiving the audio data without touching or
modifying the content of the data generated by GetVoice or received from the network. The role of a server
is just broadcasting the received data “as is” to the other clients.
The clients call DecompressVoice on the received data to decode it.
There are no limitations about the size of the voip data, so it’s possible to use DecompressVoice with
chunks of any size and it’s all up to the game. For example Unreal Engine 4 uses a buffer of 8192 bytes
while Portal 2 and Counter Strike Global Offensive use 18432, Team Fortress 2 and Half-Life 2 use 2048 and
all the others like SteamworksExample project and Grid 2 use 1024.
So, just to recap, if an attacker sends malformed voip data to the chat of a game, it’s Steam that will be
Please note that the Steam chat doesn’t use GetVoice API to capture the audio, but it uses DecompressVoice
for decoding the incoming data and so it’s vulnerable to the vulnerabilities described in this paper.
Steam VOIP security
The DecompressVoice API is our target because the data sent by the other players is received and passed to
this function “as is”.
The following are the prototype and the comments from Steamworks:
// Decompresses a chunk of compressed data produced by GetVoice().
// nBytesWritten is set to the number of bytes written to pDestBuffer unless the return value is
// In that case, nBytesWritten is set to the size of the buffer required to decompress thegiven
// data. The suggested buffer size for the destination buffer is 22 kilobytes.
// The output format of the data is 16-bit signed at the requested samples per second.
// If youre upgrading from an older Steamworks API, youll want to pass in 11025 to nDesiredSampleRate
virtual EvoiceResult DecompressVoice(
const void *pCompressed, < INPUT DATA
uint32 cbCompressed, < SIZE OF INPUT DATA
void *pDestBuffer, > OUTPUT BUFFER
uint32 cbDestBufferSize, > MAXIMUM SIZE OF OUTPUT BUFFER
uint32 *nBytesWritten, > DECODED BYTES WRITTEN
uint32 nDesiredSampleRate | STEAMWORKS LIMITS IT TO MAX 49000
) = 0;
Our input data is not just raw compressed audio data, it contains a header and opcodes with various fields.
The main header is composed by a 32bit field followed by three flags packed as bitfields:
typedef struct {
uint32 id;
uint32 flag1:20;
uint32 flag2:4;
uint32 flag3:8;
Steam VOIP security
This header is followed by a sequence of opcodes called nPayloadType:
At the end of the data is located the 32bit CRC, a classical checksum calculated on the whole data before it.
Arguments: uint16 samples
Used as a way to create silence by transmitting only the 16bit number containing the desired amount of
samples to fill.
It uses the same function of nPayloadType 1, 3 and 4 by passing it an allocated buffer filled with zeroes
used as input data and choosing the codec 3 (raw PCM).
The temporary buffer used by this function to contain the decompressed chunk is located on the stack and
has a size of 16428 bytes.
Arguments: uint16 samples followed by data
Type 1 is a codec no longer available, probably Miles
Type 3 is the uncompressed 16bit PCM data, copied “as is” to the destination buffer.
Steam VOIP security
Type 4 is the Silk
codec, the function performs its initialization using the default samplerate (11025) or
the one specified by nPayloadType 11.
The Silk codec has been introduced in Steam since 2011
replacing Miles. Steam uses the SILK SDK
provided by Skype.
These opcodes check the number of samples specified in the packet to avoid that what is specified is more
than the data available in the chunk.
Arguments: none
End of voip data.
Arguments: uint8 bytes[2]
Arguments: uint16 samplerate
Used for specifying the sample rate (the frequency in hertz) of the input data.
Steam VOIP security
Security issues
By using nPayloadType 0 we can decide the 16bit size of an input buffer containing zeroes that will be
copied directly in a stack buffer of 16428 bytes:
The problem is caused by the function that handles the data of the codec because it doesn’t check if the
input data is bigger than the available stack buffer, resulting in a stack-based buffer-overflow.
The possibility of specifying the exact number of zeroes to write on the stack and the lack of stack cookies,
allow an attacker to modify the lower part of the saved addresses, for example by using “(16428 – 0x4) / 2”
as number of samples.
Anyway code execution doesn’t seem possible on the Windows version we tested.
There is also a way to exploit the previous vulnerability using controlled content.
The only check performed by nPayloadType 1, 3 and 4 is related to the amount of samples and the size of
the input data, but there are no checks performed by the function seen before.
.text:00102291 push 0
.text:00102293 push esi
.text:00102294 push 0
.text:00102296 lea ecx, [ebp+var_14]
.text:00102299 call calloc_like
.text:0010229E lea eax, [esi+esi]
.text:001022A1 push eax ; size_t
.text:001022A2 push 0 ; int
.text:001022A4 push [ebp+var_10] ; void *
.text:001022A7 call _memset
; int __stdcall sub_101F30(void *, size_t, int, int)
.text:00101F30 push ebp
.text:00101F31 mov ebp, esp
.text:00101F33 mov eax, 402Ch ; 16428
.text:00101F38 call __alloca_probe
.text:00101F8E mov eax, [ebp+arg_8]
.text:00101F91 push edi
.text:00101F92 mov edi, ebx
.text:00101F94 shr edi, 1
.text:00101F96 cmp eax, 3
.text:00101F99 jnz short loc_101FEA
.text:00101F9B push ebx ; size_t
.text:00101F9C push [ebp+arg_0] ; void *
.text:00101F9F lea eax, [ebp+var_402C]
.text:00101FA5 push eax ; void *
.text:00101FA6 call _memmove_0
Steam VOIP security
So if a game can receive an audio chunk of more than 16428 bytes, it’s possible to exploit the relative stack-
based buffer-overflow using the provided data. The Steamworks documentation recommends to use a
buffer of 8 kilobytes or larger for the compressed audio collected with GetVoice.
Portal 2 and Counter Strike Global Offensive are some of the games tested by us that support packets bigger
than that stack buffer size, exactly 18432 bytes. Other games may be vulnerable too.
With nPayloadType 11 we can set the desired sample rate of our audio data and it can be any number
between 0 and 65535. If we set the sample rate to zero we are able to cause an endless loop in the
following cycle:
The Steam process remains freezed with the assigned core of the CPU at 100% and must be killed from the
Task Manager:
.text:001024F0 ; int __stdcall sub_1024F0(int, int, double)
.text:00102523 loc_102523: ; CODE XREF: sub_1024F0+A8
.text:00102523 fld st
.text:00102525 call __ftol2_sse
.text:0010252A mov esi, eax
.text:0010252C sub esp, 8
.text:0010252F movsx ecx, word ptr [ebx+esi*2]
.text:00102533 mov [ebp+arg_4], ecx
.text:00102536 fild [ebp+arg_4]
.text:00102539 fstp [ebp+var_18]
.text:0010253C fstp [esp+34h+var_34] ; double
.text:0010253F call _floor
.text:00102544 fsubr [ebp+var_8]
.text:00102547 movsx eax, word ptr [ebx+esi*2+2]
.text:0010254C add esp, 8
.text:0010254F mov [ebp+arg_4], eax
.text:00102552 fild [ebp+arg_4]
.text:00102555 fstp [ebp+var_10]
.text:00102558 fld [ebp+var_10]
.text:0010255B fld [ebp+var_18]
.text:0010255E fsub st(1), st
.text:00102560 fxch st(2)
.text:00102562 fmulp st(1), st
.text:00102564 faddp st(1), st
.text:00102566 call __ftol2_sse
.text:0010256B movzx eax, ax
.text:0010256E lea ecx, [edi+30h]
.text:00102571 mov [ebp+arg_0], eax
.text:00102574 lea eax, [ebp+arg_0]
.text:00102577 push 2 ; int
.text:00102579 push eax ; void *
.text:0010257A call sub_399FD0
.text:0010257F fld [ebp+var_8]
.text:00102582 fadd [ebp+arg_8]
.text:00102585 add dword ptr [edi+7D5Ch], 2
.text:0010258C fst [ebp+var_8]
.text:0010258F fld [ebp+var_20]
.text:00102592 fxch st(1)
.text:00102594 fcomi st, st(1)
.text:00102596 fstp st(1)
.text:00102598 jb short loc_102523
Steam VOIP security
The first 32bit field of the voip header is used as an ID, and the Steam process allocates new resources
everytime a new ID is parsed.
These resources remain allocated for all the time Steam is running and an attacker can saturate all the
memory of the Steam process that is limited to less than 2 Gigabytes since it’s a 32bit program.
When there is no longer memory available for the process, Steam terminates with the following error
Steam VOIP security
Security Impact
Considering the types of security issues found during this auditing, we think that no previous security
assessment has been performed on such code.
The minimum risk derived from these issues is a Denial of Service affecting not only the Steam process but
also the target game and any other game using the Steamworks API because all the Steamworks operation
are handled by the Steam process.
Particularly interesting is the endless loop.
Code execution may be possible.
The most critical part of these issues is that it’s not needed to develop a proof-of-concept or an exploit
specific for the target game.
In fact we created a very simple proof-of-concept consisting of a DLL that is injected in the running
Steam.exe process and replaces the original GetVoice function (the one called via IPC) with ours that fills
the buffer with the desired malformed data.
The result is that any game supporting the Steam voip can automatically exploit any remote player
reachable by the malformed voip data.
The proof-of-concept for steamclient.dll is available as source code and pre-compiled dll:
Read the header of steamute.c for information and details on how to use it.
Steam VOIP security
How much critical are these issues?
Denial of Service and possible code execution from remote without user interaction.
The issues affect the Steam process and, as side effect, cause a Denial of Service in the game.
A vulnerability exploitable inside the Steam process is much worst than one affecting the game due to the
single target (Steam.exe) for tuning the own code and the amount of personal information and interaction
possible through this platform in case of possible code execution.
Do I need to authorize a player to be vulnerable?
No, all the games automatically allow other players to send voice data, in some games the attacker must be
in the same team of the victim.
Blacklisting or whitelisting a player is optional and may not be implemented in some games.
Do I need to have Steam running to be vulnerable?
Steam is launched automatically by the supported games.
Is it easy for an attacker to exploit these issue?
Our proof-of-concept consists of some lines of code injected in the Steam process and being able to test
automatically any game that uses DecompressVoice.
So, yes, it’s very easy.
Additionally doesn’t matter if the game is compiled as 64bit because Steam is 32bit and so uses only
steamclient.dll, not steamclient64.dll.
Why Steam crashes when I exploit these vulnerabilities?
That’s caused by how Steam and Steamworks operate: the APIs of Steamworks communicate with the
Steam process via IPC so if you exploit these vulnerabilities when you are playing a game like Grid 2 the
effect will be the crash of Steam.exe and the game no longer responding.
Steam VOIP security
Is the game *** vulnerable?
All the games that rely on Steamworks for handling the voice data are vulnerable, and obviously also Steam
itself through its integrated voice chat.
Currently we don’t have an exaustive list of games using such feature but some of the most played games
we tested that use the DecompressVoice API are the following:
Half-Life 2 (basically any Valve game, with the only exception of DOTA 2)
Portal 2
Counter Strike: Global Offensive / Counter Strike / Counter Strike: Source / Garry’s Mod
Left for Dead 2
Team Fortress 2
Borderlands 2
Grid 2
Dirt 3 Showdown
the recent games of the Worms series
Unreal Engine 4
Is DOTA 2 vulnerable?
No, DOTA 2 doesn’t use the DecompressVoice API of Steamworks.
I’m a game developer who uses Steamworks and my software uses many secure memory protections.
The role of the game is just passing the malformed audio data received from the network to the Steam.exe
process via IPC so any memory protection inside the game is completely useless.
My game uses version *** of Steamworks, am I vulnerable?
The version of Steamworks used by the game doesn’t matter, the vulnerability is exploited in the
steamclient.dll library loaded by the Steam.exe process.
Please check the “Vulnerable versions” section of this paper to know what’s the latest known version of
steamclient.dll that is affected by these issues. The current version of Steam is fixed.
Steam VOIP security
20 Jun 2014 vulnerabilities reported to Epic Games and then Valve
25 Jun 2014 vulnerabilities fully fixed in Steam beta client
03 Jul 2014 fixes implemented in the stable Steam client
04 Jul 2014 public release of this document
Steam VOIP security
Company Information
ReVuln Ltd.
Level 3, Theuma House, 302, St.Paul Street,
Valletta VLT1213