08 December 2018

Exploiting Steam Lobbies and Matchmaking [18 Sep 2014]




Description of the security vulnerabilities that affected the Steam lobbies and all the games using the Steam Matchmaking functionalities.

Revision 1 



"Steam is an internet-based digital distribution, digital rights management, multiplayer, and communications platform developed by Valve Corporation. It is used to distribute games and related media from small, independent developers and larger software houses online." (link)

It's not easy to define Steam because it's not just a platform for buying games but also a social network, a market for game items, a framework for integrating various functionalities in games, an anti-cheat, a cloud and more.

But the most important and attractive feature, from a security point of view, is its incredible diffusion (link)(link).


Steam offers a simple and efficient way to allow games to provide online multiplayer functionalities to their users by using Steam Matchmaking.

Steam Matchmaking can be compared to a chat server where any user can create his own room (the “lobby”) that will appear in a public online list and other players can join it. 

It’s possible to configure the lobby in various ways, for example adding custom parameters like name and game data, maximum number of joinable players, making it non-joinable or private or for friends only, sending chat messages, running a game server and more. 

The interaction with this matchmaking system is granted by the set of Steamworks APIs contained in the IsteamMatchmaking class, so any game can use this feature. 

Many games use the Steam lobbies for online gaming: Counter Strike Global Offensive, Left for Dead 1 and 2, Borderlands 2, Payday 2, the Codemasters games (Dirt, Grid and F1 series) and any multiplayer game sold on Steam that is not based on the Source engine or proprietary solutions. 

Steam Matchmaking gained some notoriety in the last years due to the “migration” performed by the developers/publishers of many games from a master server architecture, private or hosted by Gamespy, to the Steam one. This solution granted some of them to survive from the Gamespy shutdown of May 2014.

Steam lobbies and security risks


In technical terms the concept of the Steam lobbies is quite simple:
  • An user starts a lobby (CreateLobby)
  • He sets some lobby parameters (SetLobbyData)
  • The other users can view the new lobby when they query the list of public lobbies (GetLobbyByIndex)
  • The users join the lobby (JoinLobby)
  • At this point joining the game server (which is separate from the Steam lobby) is game dependent, some games use SetLobbyGameServer, others get the lobby owner’s SteamID (GetLobbyOwner), others put that ID in a lobby parameter, others specify the IP and port of the server instead of the SteamID and so on
  • When the clients have the owner’s ID, they can join his game server using the Steam Networking API (SendP2Ppacket)
What is visualized in-game to the players is not different than any other “master server“-based game, additionally Steam automatically sorts the lobbies based on the geographic distance between the lobby’s owner and the user who requested the list to allow the quick-matchmaking feature (auto-joining servers with best ping and maybe with players of same nationality). 


The vulnerabilities in the Steam Matchmaking have been found during a research commissioned by Epic Games regarding the third-party libraries and services used in their Unreal Engine 4

The tests have been started the 25th July 2014. 

Some issues were discovered with the following security effects:
  • Takeover of the lobby owned by other users
  • Forcing all the players in a lobby to leave it and joining an inexistent game server
  • Setting custom parameters of any lobby
  • Making any lobby not publicly visible
  • Performing these operations without even joining the lobby

The main effect of these vulnerabilities, affecting the Steam back-end network, is that an attacker can deny the online gaming of several known and played multiplayer games with a simple and silent attack performed in a couple of seconds.


The Steam back-end network that handles the lobbies was vulnerable till the 17th September 2014.


Currently all the reported issues have been fixed.
An undefined number of old games has been left vulnerable (“whitelisted”) due to how they implement the Steam Matchmaking, probably because their P2P oriented gaming requires that any user can act as co-owner of the lobby. For these games may be released game-related patches in future if necessary.
No further details are available.

Description of the issues


Joining a game server, after having joined a lobby, is a game-dependent operation. 

Steamworks in its SpaceWar example game, used to show to the game developers how to implement the Steamworks API, suggests to use the SetLobbyGameServer API and automatically joining the server upon the execution of a specific callback.

When that API is executed Steam sends an event to all the users in the target lobby that will execute the LobbyGameCreated_t callback and adds the following lobby parameters:
  • __gameserverIP – IP address of the game server or 0
  • __gameserverPort – port of the game server or 0
  • __gameserverSteamID – SteamID of the user running the server or 0

Valve suggests that the default behavior is leaving the lobby and connecting to the game server:
// Purpose: A game created a game for all the members of the lobby to join,
//          as triggered by a SetLobbyGameServer()
//          it's up to the individual clients to take action on this; the usual
//          game behavior is to leave the lobby and connect to the specified game server

struct LobbyGameCreated_t
    enum { k_iCallback = k_iSteamMatchmakingCallbacks + 9 };

    uint64 m_ulSteamIDLobby;         // the lobby we were in
    uint64 m_ulSteamIDGameServer;    // the new game server that has been created or found for
                                     // the lobby members
    uint32 m_unIP;                   // IP & Port of the game server (if any)
    uint16 m_usPort;

That’s the default behavior that happens with SpaceWar, AlienSwarm, Borderlands 2 and some other games. 

That API can be called not only by the owner of the lobby but also by any other user that joins that lobby, this is the reason why this feature can be abused to force the other players to leave the lobby trying to join an arbitrary IP or SteamID. 

Performing this operation against all the available lobbies of an affected game, will result in the absence of online lobbies and in clients that try to connect to inexistent servers. In some games like Alien Swarm there are no visible effects for the owner of the lobby and other players, they will silently leave the lobby (that will be automatically deleted when left by the owner) but nothing is shown to the players.


Steamworks provides various ways to the users for controlling and customizing their lobbies:

- SetLobbyData and DeleteLobbyData
Adds, modifies and deletes the lobby parameters, for example “name”

- SetLobbyMemberLimit
Limits the amount of users who can join the lobby

- SetLobbyType
Allows to set the lobby as:
  • Private: invisible to the public list and to the friends
  • FriendsOnly: invisible to the public list, but visible to the friends
  • Public: default
  • Invisible: allows an user to join two lobbies 

Allows to make the lobby non-joinable

- SetLobbyGameServer
The API seen before 

The following are some real examples of Steam lobbies taken from Borderlands 2, F1 2013, XCom-Enemy-Unknown and Payday 2, they are useful to understand better what are the lobby parameters:
lobby 109775241376664452 - 459508612 393216 8 1
  BuildUniqueString: BORDERLANDS2-1.8.3W
  CurrMission: 7
  CurrPlotMission: 7
  DlcFlag: 1
  DlcMapContentId: 0
  DlcMapPackageId: 0
  gameMode: 0
  HostExpLevel: 31
  IsPublic: 1
  OwningPlayerName: TRUCKERBOX
  PlayThrough: 1
  __gameserverIP: 0
  __gameserverPort: 0
  __gameserverSteamID: 765611981062*****

lobby 109775241376111944 - 458956104 393216 8 1
  268435458: 65365
  536870936: 0
  SteamLobbyGameMode: 0
  SteamLobbyGameType: 0
  SteamLobbyHostId: 765611980157*****
  SteamLobbyHostName: ICEMAN
  SteamLobbyOpenSlots: 15
  SteamLobbyVisibility: 0

lobby 109775241373641018 - 456485178 393216 8 1
  268435468: 0
  268435469: 0
  268435470: 90
  268435471: 10000
  268435472: 0
  268435474: 0
  268435488: 1724
  268435489: 28398179
  32779: 0
  553648128: 9212610293214#24968160127#
  bIsDedicated: False
  BotPlayerCount: 0
  bUsesStats: True
  GameSettings: XComOnlineGameSettingsDeathmatchRanked
  GameTags: XComMPLobbyGame
  MapName: XComShell
  MaxPlayerCount: 1
  NumOpenPrivateConnections: 0
  NumOpenPublicConnections: 1
  NumPrivateConnections: 0
  NumPublicConnections: 2
  OwningPlayerId: 765611980191*****
  OwningPlayerName: Kharon
  PasswordProtected: 0
  ServerName: Kharon
  SteamEngineVersion: 8916
  __gameserverIP: 0
  __gameserverPort: 0
  __gameserverSteamID: 900914655904*****

lobby 109775241376713535 - 459557695 393216 8 1
  difficulty: 5
  drop_in: 1
  job_class_max: 80
  job_class_min: 80
  job_id: 28
  kicking_allowed: 1
  level: 57
  lobby_type: public
  min_level: 0
  num_players: 1
  owner_id: 765611980432*****
  owner_name: rendoman
  payday2_v1.12.4: true
  permission: 1
  state: 3
  __gameserverIP: 0
  __gameserverPort: 0
  __gameserverSteamID: 765611980432*****

These APIs can be called by any user, not only the lobby owner and, moreover, they can use used even from outside the lobby. That means an attacker is able to silently delete any online lobby without even joining them and resulting in a multiplayer game without online matches to join.

The proof-of-concept

A proof-of-concept is available as reference for the issues:
Please note that the issues have been fixed and that proof-of-concept no longer works, except for the whitelisted games.


What was the impact of these issues?
A single attacker, without particular network or bandwidth requirements, was able to make many multiplayer games unplayable online with zero lobbies/matches to which connecting. The attack was silent and performed in some seconds without even joining the target lobbies.

Were these issues critical?
Yes, without Steam lobbies it’s not possible to play online with many multiplayer games sold on Steam.

Are these issues fixed now?
Yes, all the issues have been definitely fixed the 17th September 2014.
Some old games have been left whitelisted by Valve due to backward compatibility (basically their multiplayer has been designed to work in that way) and so they may be still vulnerable “by design”.

Was the attack performed against the users’ computers?
No, the Steam lobbies are handled by the Steam back-end network.

Does the attacker need to own the target games to attack them?
It depends by the game, retrieving the list of online lobbies is an operation usually available to who owns the game but some games can be queried even from accounts that don’t own them.

Was/is the game X vulnerable?
There is a short list of some tested games in the Introduction section.
If you want to know if a game uses the Steam lobbies you can use some tools, while if you are interested to test it you can use the proof-of-concept provided in the previous section of this paper.


  • 25 Jul 2014 Security issues initially found
  • 04 Aug 2014 Vulnerabilities reported to Valve after more tests on various games
  • 12 Aug 2014 The APIs can be no longer called from outside the lobbies without joining
  • 23 Aug 2014 Some mitigations implemented by Valve, still possible to make lobbies private
  • 17 Sep 2014 After many e-mails all the remaining issues have been fixed, only the owners of the lobbies can perform operations on them

Steam Service Security [10 Jul 2014]




How a malware or an exploit can use the Steam local service to escalate its
ReVuln Ltd.

Revision 1



"Steam is an internet-based digital distribution, digital rights management, multiplayer, and communications platform developed by Valve Corporation. It is used to distribute games and related media from small, independent developers and larger software houses online." (link)

It's not easy to define Steam because it's not just a platform for buying games but also a social network, a market for game items, a framework for integrating various functionalities in games, an anti-cheat, a cloud and more.

But the most important and attractive feature, from a security point of view, is its incredible diffusion (link)(link).


In 2007 Valve introduced a new local Windows service in Steam for handling the tasks that require Administrator privileges and maintaining the main client process Steam.exe under the normal limited privileges of the current user.

This is a common practice adopted by many software developers moreover after Microsoft introduced the UAC technology from Windows Vista. In fact the secondary job of such service is avoiding to annoy the user with continuous Windows popups requiring the confirmation for using higher privileges.

The service is used also for monitoring the processes of the running games and it's part of Valve anti-cheat (VAC).

In Steam the local service that performs these operations is called "Steam Client Service", a Manual service with SYSTEM privileges. The service can be started by any user but it will terminate immediately if some requirements are not met.

The service is automatically started by Steam when launched and it remains active till Steam is working.

The "Steam Client Service" is a required component.


What's interesting about this service is that it can be abused by malicious programs (malware) for performing various tasks with high privileges and that's quite important considering that Steam is one of the most diffused software available.


Steam package versions: 1404163764


As a personal project, this document has been released publicly without contacting Valve.

How Steamservice works

The service uses an IPC interface for communicating with the Steam process, the access to the interface is performed using events and shared memory. Named pipes were used in past versions.

Exist many ways to perform IPC and the following are the current steps for starting to communicate with the SteamService:
  • create a Global\Valve_SteamIPC_Class event
  • create a Steam3Master_SharedMemFile mapped file
  • create a Steam3Master_SharedMemLock event
  • launch the service, any user without privileges can do it
  • open the Global\SteamClientService_SharedMemLock event
  • open the Global\SteamClientService_SharedMemFile mapped file
  • take the handles from the structure located on the mapped file
Note that such steps are necessary only if we use a stand-alone tool to access SteamService, so if Steam is already running we can inject our code in its process or we can just kill it and use the IPC or replace some Steam libraries and so on.

The service will verify that our process has the steam.exe name and that its own steamservice.dll shared library (C:\Program Files (x86)\Steam\bin\steamservice.dll) is correctly signed by Valve.

If steamservice.dll doesn't have a signature or it's signed with a different certificate, the service will terminate immediately.

No signature verification – DLL hijacking


The verification of the signature of steamservice.dll is performed for security reasons because the folder where is located the service executable cannot be modified by the user but the Steam folder used by the dll is fully writable and cannot be trusted.

But this check is completely useless because steamservice.dll depends by other libraries located in the Steam folder that are not verified at all and can be replaced by a malware to execute its code inside the service with SYSTEM privileges.

The following are the libraries that can be used by the malware:
  • crashhandler.dll
  • dbghelp.dll
  • tier0_s.dll
  • vstdlib_s.dll
  • dnsapi.dll (Windows dll searched in the local folder)
  • version.dll (Windows dll searched in the local folder)
  • winmm.dll (Windows dll searched in the local folder)
DLL hijacking is an issue which is quite common and it’s perfect to escalate privileges to SYSTEM, at this point a malware requires nothing else for its job.


Testing this behaviour is very simple:
  • terminate Steam
  • create your own dll
  • copy your dll in the Steam folder with the name winmm.dll
  • start the service: sc start "Steam Client Service"
Example of custom winmm.dll executing cmd.exe as SYSTEM:

Abusing the service for privileged tasks


Even if it’s easy to execute the own code as SYSTEM with the previous design issue, this paper would like to focus on how it's possible to abuse a legitimate service without using security vulnerabilities and design issues. So let's take a look at what is possible to do with its features in a scenario in which DLL hijacking is not possible or should be avoided.


The IPC protocol is composed by some commands, the following are the functions of the main one:
  • IClientInstallUtils::SetUniverse
  • IClientInstallUtils::AddShortcut
  • IClientInstallUtils::RemoveShortcut
  • IClientInstallUtils::RemoveFromGameExplorer
  • IClientInstallUtils::AddRichSavedGames
  • IClientInstallUtils::RemoveRichSavedGames
  • IClientInstallUtils::AddToMediaCenter
  • IClientInstallUtils::RemoveFromMediaCenter
  • IClientInstallUtils::AddUninstallEntry
  • IClientInstallUtils::RemoveUninstallEntry
  • IClientInstallUtils::AddToFirewall
  • IClientInstallUtils::RemoveFromFirewall
  • IClientInstallUtils::RegisterSteamProtocolHandler
  • IClientInstallUtils::FixupSteamClientShortcuts
  • IClientInstallUtils::RunInstallScript
  • IClientInstallUtils::AddInstallScriptToWhiteList
  • IClientInstallUtils::GetInstallScriptExitCode
  • IClientModuleManager::LoadModule
  • IClientModuleManager::UnloadModule
  • IClientModuleManager::CallFunctionAsync
  • IClientModuleManager::CallFunction
  • IClientModuleManager::PollResponseAsync
  • IClientProcessMonitor::RegisterProcess
  • IClientProcessMonitor::UnregisterProcess
  • IClientProcessMonitor::TerminateProcess
  • IRegistryInterface::BGetValueUint
  • IRegistryInterface::BSetValueBin
  • IRegistryInterface::BDeleteValue
  • IRegistryInterface::BDeleteKey
  • IRegistryInterface::BKeyExists
  • IRegistryInterface::BSetValueStr
  • IRegistryInterface::BSetValueUint
  • IRegistryInterface::BGetSubKeys
  • IRegistryInterface::BGetValues
  • IRegistryInterface::BEnumerateKey
  • IRegistryInterface::BGetValueStr
  • IRegistryInterface::BGetValueBin
  • IRegistryInterface::BenumerateValue



It's used for placing a “link” file anywhere we desire, it’s commonly used by games for placing their links on the Desktop of all the users.
If there is an Administrator account in the system, it's possible to create a link to our malware in the Startup folder to automatically execute it with such privileges:
  • before Vista - c:\Documents and Settings\Administrator\Start Menu\Programs\Startup\evil.lnk
  • Vista/Win7/8 - c:\Users\Administrator\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\evil.lnk
When we call the command RemoveShortcut the extension of the link file must be ".lnk" but with AddShortcut we have no restrictions and so we can also use it to overwrite any file on the system deleting the original content. 

Function used for adding a firewall rule to allow a specific program to receive incoming TCP connections and UDP packets, a solution for giving network access to a malware.

It terminates any desired process by specifying its PID.

It can be used for terminating the privileged processes of  various defensive solutions, for example before downloading the malicious code that may be identified by them or to avoid logging and so on.

If we kill the lsass.exe process we will force the system to reboot, useful if we make certain changes to the registry and the filesystem. 

Functions that write data in the registry but are limited by a set of whitelisted registry locations, so the service can write only under the registry keys listed in the file registrykeys.vdf.

This file is located in the Steam folder and so it's writable but it contains a digital signature "kvsignatures" at its end, this signature is verified by the service.

Even if there is such limitation it's still possible to have room for executing the own code with higher privileges when it's performed the manual uninstalling of software and in some cases during their updating.

One of the whitelisted keys is “HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall" so we are able to set WindowsInstaller to zero and the UninstallString value with the path of our malware, and we can perform this operation for all the available registry entries with the effect of executing the malware automatically when one of them is uninstalled. 

This command is used together with AddInstallScriptToWhiteList for automatically executing executables decided by the installscript.vdf  script files  of the games, for example to install the necessary  pre-requisites like DirectX, VC runtime and so on (the interesting aspect is that these IPC commands are used by Steam only when it runs in “Big Picture” mode, instead normally the Steam application launches the steamservice.exe executable (so doesn’t interact with the service) using the /installscript argument and the user must confirm the privileged operation). 

AddInstallScriptToWhiteList adds the executables listed in the script, usually called installscript.vdf, into the whitelist and RunInstallScript executes them only if the same locations are used also in the runasadmin.vdf script.

The installation scripts have a digital signature and currently doesn’t seem possible to execute scripts that don’t have this field, but an attacker can use an already signed legit script to execute his own code located in any folder he desires.
In fact if we specify an unexistent game ID, Steam will consider the current folder as “installation folder” of the game allowing us to launch batch and executables located in any local and remote webdav/shared folder we desire.

The executables will have the same privileges of the Steam Service, SYSTEM.

Possible steps for an attacker

For the design issue related to the unverified libraries in the Steam folder it's enough to put our malware in the Steam folder with a specific filename and we will have SYSTEM privileges immediately after we start the "Steam Client Service". 

If this issue will be fixed or limited in future and we must rely on the features of the service, we may think to the following steps (remember that they are performed ever with SYSTEM privileges):
  • execute programs
  • use TerminateProcess to kill some antivirus processes, please note that this is not enough for defensive solutions that work via drivers 
  • download the core of the malware
  • use AddToFirewall to add a firewall rule for the malware
  • check if there is an Administrator account and use AddShortcut to put a link to the malware in its StartUp folder to automatically execute it when he will login
  • use the BSetValue* functions to perform the Uninstall trick and being able to execute the malware when a software will be uninstalled by the user
Other features of the service may be abused, those are just the main ones.

The proof-of-concept

A proof-of-concept tool has been created to test the IPC interface, the following are the links for the source code and the binary:
It’s a simple command-line tool called Steam.exe (necessary) that allows to call the service functions listed above and passing them any argument you desire, for example:
  • Steam.exe AddToFirewall "c:\windows\notepad.exe" "malicious_firewall_rule"
  • Steam.exe TerminateProcess 12345678
The tool works also with a text file containing the list of commands to execute, in the provided package there is an example called example_commands.ini which contains some example commands to test but please don’t use it if you have no idea of what it does, example:
  • Steam.exe example_commands.ini
There is also an archive called example_execution.zip which contains all the necessary files to execute a custom executable with SYSTEM privileges using the RunInstallScript function. You need to create a temporary partition or ramdisk on Z:\ for the quick test or you have to edit both the text files example_commands.ini and RunAsAdmin.vdf replacing z:\ with the full path you would like to use. 

Then launch “Steam.exe example_commands.ini” and you should see calc.exe spawning through a batch script, so you can easily edit the “Microsoft .NET Framework 4.0.cmd” script to execute what you desire. 

Please note that the IPC mechanism used in the tool is not complete and so the Steam Client Service process will remained freezed and you MUST kill it manually. This tool must be considered only a proof-of-concept.


Is this issue a security vulnerability?
There is a design issue consisting in the missing verification of the signatures of the shared libraries loaded by the Steam Service that allows dll hijacking.
Additionally the service can be abused to perform various operations and launching executables as SYSTEM without using security vulnerabilities.

Can the service be accessed from remote?

What operating systems are affected?
The issue is not dependent by the system, anyway both Windows 7 and Windows 8 have been tested.
On the operating system before Vista, like Windows XP, the Steam Client Service was not strictly necessary but Steam needs to have Administrator privileges in that case.

What's the main scenario for this issue?
It’s a typical post-exploitation scenario, for example a malware/exploit running with the privileges of the current limited user can abuse the Steam Service for gaining SYSTEM privileges or performing some operations with such high privileges with the target of hiding itself and remaining persistent in the system.

What can be done by a malware after it gets SYSTEM privileges?
It's possible to have full access to the Windows registry and the disk allowing the malware to hide itself and disabling any security software just like the Administrator account.

Is this issue critical?
No, but the huge diffusion of Steam and the possibility of becoming SYSTEM without security vulnerabilities or complex exploits and executing certain operations through a trusted service is quite interesting for a malware.

Do I need to have Steam or the Steam Client Service running on my computer for being vulnerable?
No, it's enough that Steam has been installed, doesn't matter when and if it's used or not.

What about SteamMachine and the Linux/MacOSX/PlayStation3 versions of Steam?
Sorry, they have not been tested.

How can Steam rebuild the registrykeys.vdf file?
When Steam is launched it automatically verifies all the local files and restores them by taking the original copies from the lzma compressed ZIP files located in the Package folder, that's why registrykeys.vdf is restored (and not rebuilt) by Steam everytime it gets modified.
Steam doesn't contain any private key or certificate.

Why SteamService.exe is located in both Steam\bin and Common Files\Steam?
The service is located in “C:\Program Files (x86)\Common Files\Steam” which is not writable by the normal limited user and it’s the exact copy of the one located in the bin folder of Steam. When the service runs it checks if there is a different steamservice.exe file in the bin folder (which is writable by the user), then checks the digital signature of that file and then executes it with the /Update argument. The update is executed with the same privileges of the service (SYSTEM), that’s why the new executable will be copied in the “Common Files\Steam” folder without user interaction, first as SteamServiceTmp.exe and then overwriting the original SteamService.exe.
That’s how the update process of the service works.

Have you reported these issues to Valve?


  • Mar 2013 initial research on this topic
  • 01 Jun 2014 returned on the research to confirm the issues
  • 10 Jul 2014 public release

Steam Voip Security [04 Jul 2014]




Overview and details about the security issues found in the Steam voice framework.

ReVuln Ltd.

Revision 3

Steam and Steamworks


Steam 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
  • Community
  • DLC and Content
  • Peer-to-peer networking
  • Cloud
  • Anti-Cheat
  • Voice chat
  • DRM

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


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

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 chat.

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.

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.

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 exploited.

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.


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 k_EvoiceResultBufferTooSmall.
// 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 you’re upgrading from an older Steamworks API, you’ll 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;

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.  
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.

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:

.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

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.
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:
.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

The Steam process remains freezed with the assigned core of the CPU at 100% and must be killed from the Task Manager:


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 message:

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.


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.

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.


  • 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