STEAM BROWSER PROTOCOL INSECURITY
(WHEN LOCAL BUGS GO REMOTE)
Luigi Auriemma1and Donato Ferrante2
ReVuln
http://revuln.com
info@revuln.com
http://twitter.com/revuln
15 October 2012
Abstract In this paper we will uncover and demonstrate a novel and
interesting way to convert local bugs and features in remotely exploitable
security vulnerabilities by using the well known Steam3platform as attack
vector against remote systems.
1 STEAM
From Wikipedia: "Steam is a digital distribution, digital rights management, multi-
player and communications platform developed by Valve Corporation".
With Steam users can buy games, download demos and free-to-play games, find
multiplayer matches, communicate with other users, share stats and so on. Steam
is the digital delivery platform having the biggest user base (approximately 50
millions users) and supporting several platforms: Windows, MacOS, PS3, mobile
devices and Linux.
2 STEAM BROWSER PROTOCOL
Steam, like other software, uses its own URL handler to enhance experience by
integrating web-based functionality directly in its own platform.
Steam uses the steam:// URL protocol in order to:
•Install and uninstall games
•Backup, validate and defrag game files
•Connect to game servers
•Run games
•Reach various pages and sections where it’s possible to buy or activate games,
download tools, read news, check user profiles and so on
The Steam Browser Protocol has several commands, most of them are listed on
Valve4website along with a non-updated list5of games using Steam as platform.
The list of commands is not a complete reference of all the commands available
1http://twitter.com/luigi_auriemma
2http://twitter.com/dntbug
3http://steampowered.com
4https://developer.valvesoftware.com/wiki/Steam_browser_protocol
5https://developer.valvesoftware.com/wiki/Steam_Application_ID
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with the Steam Browser Protocol, as several commands are partially documented or
not documented at all on Valve website.
In the next sections we are going to cover how Steam URLs are handled by web
browsers and other software, in order to get a good understanding of the possible
ways in which it’s possible to trigger remote attacks via such URLs. Figure 1 gives
an overview of one of the possible attack scenario.
Figure 1: Remote Steam Protocol Commands exploitation: overview
2.1 STRATEGY 1: WEB BROWSERS
First we checked all of the most known web browsers in order to verify how they
react while handling external (not handled by the browser itself) URL protocols
(i.e. rtsp://,mms://,steam:// and so on).
According to the results reported in Table 1 all the browsers that execute exter-
nal URL handlers directly without warnings and those based on the Mozilla engine
(like Firefox and SeaMonkey) are a perfect vector to perform silent Steam Browser
Protocol calls. Additionally for browsers like Internet Explorer and Opera it’s still
possible to hide the dodgy part of the URL from being showed in the warning mes-
sage by adding several spaces into the steam:// URL itself.
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Internet Explorer Warning including the URL, and in case of IE9 a possible
additional warning ("protected mode") without any detail
Firefox No URL visualized, only request for confirmation (no
warnings)
Chrome Warning with a detailed description or the URL and the
program to call
Opera Warning with only 40 chars of the URL visualized
Safari Direct execution without warnings
Webkit Direct execution without warnings
MaxThon Direct execution without warnings
Avant Direct execution without warnings
Lunascape Direct execution without warnings
SeaMonkey See Firefox
PaleMoon See Firefox
SRWare Iron See Chrome
Table 1: Web browser and Steam protocol survey
2.2 STRATEGY 2: ALTERNATIVES
Apart from web browsers, there is additional software that may be used to perform
calls to external protocol handlers. Most of them rely on the default browser but
some of them don’t.
The following are some of the software (tested during our research) that doesn’t
show any warnings while performing external URL protocol calls:
•Steam browser (Steam’s custom web browser)
•RealPlayer embedded browser
•Other software able to process html pages
In our opinion the Steam browser is a very interesting alternative to the com-
mon browsers, except for the following limitations:
•The websites one can visit from within the Steam browser are generally lim-
ited to locations owned by Valve like steampowered.com and steamcommu-
nity.com domains
•Valve prevents steam:// protocol injections by performing several checks on
users provided links
•References to external websites get redirected via steam://openurl/website
calls, which rely on the default browser instead of the Steam one
For the sake of completeness, it’s worth mentioning that the web browser used
in the in-game Overlay Interface of Steam acts differently and it allows all the web-
sites, except steam:// links get ignored completely, so this browser flavor can’t be
used as vector. As you may have argued, we want to be able to open links that we
are not supposed to open by using the Steam browser.
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If you are familiar with Steam, you know that every user gets a personal profile
page and on this page it’s possible to include information like pictures and videos.
While pictures provided by the users get uploaded on Steam, videos are just links
to YouTube videos. If a user tries to view a video attached to a profile, the user will
get a page in which there is only the video, so no comments or description coming
from YouTube. But if the user clicks on the title of the video (i.e. to leave comments
on the YouTube video) then a new window is opened with all the details about the
video including comments and description. So a malicious user can include links
to external hosts, which can remotely invoke Steam commands by using the usual
steam:// URLs. With this strategy the Steam browser will execute the protocol
handler calls without any warnings. Please see Figure 2 for a graphical recap of
this approach.
Figure 2: Remote exploitation via Steam browser and YouTube bouncing
3 INSECURITY TIME
At this point we know the pros and cons of various ways to launch steam:// URLs,
so we can start exploring some security vulnerabilities and what we can achieve
with them. In the following sections we will report some new vulnerabilties we
found during this research, please note that all of them are exploitable remotely by
simply using the Steam Browser Protocol as trigger.
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3.1 STEAM BROWSER PROTOCOL COMMANDS
The retailinstall command is an undocumented feature (not a bug) of the Steam
Browser Protocol that allows installing and restoring backups from a local directory.
One of its parameter is path that is used to specify this local directory but obvi-
ously this directory can be a Windows network folder available on a remote host.
When Steam executes the retailinstall command, Steam checks and loads two files:
splash.tga (an image) and sku.sis (an install file). The splash image gets displayed
immediately (Figure 3 ) to the user as soon as the command gets executed.
Figure 3: Steam loading splash.tga while executing the retailinstall command
The Steam function in charge of processing the splash images is vulnerable to
an integer overflow vulnerability while processing malformed TGA files. The prob-
lem is located in LoadTGA function of vgui2_s.dll that loads TGA files in memory
(Figure 4). The result is a heap-based buffer-overflow that may allow executing
malicious code on the Steam process.
Figure 4: ASM code related to the integer overflow condition in LoadTGA
3.2 STEAM BROWSER PROTOCOL GAME COMMAND-LINE PARAMETERS
In Steam it’s possible to launch installed games by using one of the following
steam:// commands:
•run
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•rungameid
•runsafe
•rungame
As with most of the software, the games available on Steam accept command
line arguments. Steam allows you to pass such arguments to games but there is
no official documentation about any strategy to do that, except for the -applaunch
command that can’t be used in a universal and silent way, because of different URL
encoding strategies used by web browsers.
Most of the four commands that can be used to run games via Steam URLs are
undocumented, anyway the following are their formats:
•steam://run/id/language/url_encoded_parameters
•steam://rungameid/id/language_bug/url_encoded_parameters
•steam://runsafe/id
•steam://rungame/id/lobby_id/parameters
The only commands suitable for remote environments are run and rungameid
where url_encoded_parameters is an URL encoded string passed to the Q_URLDecode
function that stores the decoded result in a buffer of 128 bytes. The Q_URLDecode
function allows you to use any character and also demonstrates that there are some
commands designed to be used remotely via browser. The limitation of 128 chars
for the parameters doesn’t affect exploitation of any of the following bugs, because
if we need more room we can just use some JavaScript to join chunks of commands.
3.2.1 GAME EXPLOITATION 1: SOURCE ENGINE
As first example of game exploitation via Steam we have chosen the game engine
with the biggest user base: Source6.
The following are the most known games based on such engine: Half-Life 2,
Counter-Strike: Source, Half-Life: Source, Day of Defeat: Source, Team Fortress 2,
Portal 2, Left 4 Dead 2, Dota 2, Alien Swarm, SiN Episodes, Dark Messiah of Might
and Magic, The Ship, Zombie Panic! Source, Age of Chivalry, Synergy, D.I.P.R.I.P.,
Eternal Silence, Pirates Vikings & Knights II, Dystopia, Insurgency, Nuclear Dawn
and Smashball.
Most of them include the basis commands7available in the Source engine,
which we are going to use for writing files with custom content in arbitrary lo-
cations. For exploiting this engine we have opted for the following command-line
options:
•+con_logfile, allows you to specify a file that will receive the content of the
console (it can’t be a Windows remote share)
6http://source.valvesoftware.com
7https://developer.valvesoftware.com/wiki/Command_Line_Options
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•+echo, used to put custom data in the log file
•+quit, (optional) closes the game
•-hijack, (optional) useful in case the user already has an instance of the game
running and we want to send additional commands that are limited by the
Q_URLDecode 128 chars
Our choice for exploiting this bug is to create a .bat file in the Startup folder of
the user account which will execute our commands injected through +echo at the
next login of the user on the system. There is also an interesting scenario against
dedicated servers by specifying the motd.txt of the game as logfile and launching
the cvarlist command that will dump all the game variables in such file that is
visible to any player who joins the server. Team Fortress 28is one of the most
played games based on this engine and it’s free-to-play.
3.2.2 GAME EXPLOITATION 2: UNREAL ENGINE
Figure 5: Remote exploitation via Steam of Unreal Engine
For games based on the Unreal Engine9we opted for exploiting a real security
vulnerability that occurs while loading content that resides on remote computers
(Windows remote WebDAV or SMB share) which we can load via command-line
parameters:
steam://run/ID/server \\HOST\evil.upk -silent
Indeed this engine is affected by many integer overflow vulnerabilities (maybe
we will document them one of these days) that allow execution of malicious code.
A full list of command-line parameters available for the Unreal Engine 3 is avail-
able online10.
3.2.3 GAME EXPLOITATION 3: APB RELOADED
All Points Bulletin11 is a well known Massive Multiplayer Online (MMO) game that
includes a customizable auto-update feature. In this case we decide an arbitrary
8http://www.teamfortress.com
9http://www.unrealengine.com
10 http://udn.epicgames.com/Three/CommandLineArguments.html
11 http://www.gamersfirst.com/apb/
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update server via command-line and exploit a directory traversal for overwriting or
creating any file we desire with our custom content.
On Steam there are tons of MMO games free-to-play like APB so the user base
is very big and most of them can be exploited with such techniques. Additionally
most of these games use anti-cheating solutions and require to be launched with
Administrator permissions (we are in the gaming world where people don’t have
security knowledge, having such privileges is quite common) so the whole system
can be compromised.
3.2.4 GAME EXPLOITATION 4: MICROVOLTS
MicroVolts12 is another example of known MMO game exploitable via auto-update,
just another directory traversal vulnerability.
3.3 PROOF-OF-CONCEPT
Please refer to vimeo.com/revuln13 channel, for a proof-of concept video14, illus-
trating all the issues reported in this paper.
4 POSSIBLE FIX AND WORKAROUND
In this section we propose some solutions to avoid or reduce the impact of the the
issues we found during our research.
4.1 USER-SIDE
The issue can be limited by disabling the steam:// URL handler or using a browser
that doesn’t allow the direct execution of the Steam Browser Protocol.
4.2 STEAM-SIDE
A solution would be avoiding to pass command-line arguments to third party soft-
ware and undocumented commands accessible from external and untrusted sources
like the Internet.
4.3 GAME-SIDE
The main problem of the Steam Browser Protocol is that it allows abusing local
features of games (like using log files) so the developers can’t do much in this
situation, except trying to reduce possible related issues, by:
•Adopting secure programming techniques also in non-network related code
•Using certificates validation while performing auto-patching
5 CONCLUSION
In this paper we proved that the current implementation of the Steam Browser Pro-
tocol handling mechanism is an excellent attack vector, which enables attackers to
exploit local issues in a remote fashion. We also detailed as proof of the effective-
ness of this new attack vector, five new remotely exploitable issues, including one
12 http://www.microvolts.com
13 http://vimeo.com/revuln
14 http://vimeo.com/51438866
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in Steam, and two in widely used game engines (Source and Unreal). Because of
the big audience (more than 50 million people), the support for several different
platforms including Windows, MacOs and Linux and the amount of effort required
to exploit bugs via Steam Browser Protocol commands, Steam can be considered a
high-impact attack vector.
6 FAQ
•Is this a Windows-only issue?
–No. If you can install and run Steam on your OS then you are
vulnerable.
•Is this a browser-only issue?
–No. Anything that is able to process common URL links can be used as
a trigger.
•Is this a Safari-only issue?
–No. Safari is just one of the possible triggers.
•Will browsers always show a warning/popup to the user?
–No. Users can suppress the warning for steam:// links by using the
browser settings. This is quite common for gamers that use the Steam
protocol to join online game servers.
•Are the issues related only to the four games you listed?
–No. Games usually share the same engine (i.e. Source Engine, Unreal
Engine, and so on), so an engine related bug affects several games.
•Did you test all the games available on Steam?
–No. Our only purpose was to detail the Steam Browser Protocol issues.
Moreover with our examples we covered two different engines (Source
and Unreal) and two well known MMOs.
•Is the retailinstall issue remotely exploitable?
–Yes.
•Does the victim user have to click on a malicious steam:// link?
–No. In fact all the links used in our PoC video point to normal HTML
pages.
•Does the victim user always see the real link shown in the browser status
bar?
–No.
•Are users using only the Steam browser safe?
–No. As demonstrated in the YouTube bouncing scenario.
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•I can’t replicate some of the bugs shown in the video. Why?
–Because after our public paper, Valve has just limited the con_logfile
command15 and APB has just removed a legacy command16.
7 REVISION HISTORY
•18 October 2012: Version 1.1 released: FAQ section added.
•15 October 2012: Version 1.0 released.
15http://store.steampowered.com/news/9120/
16http://www.reddit.com/r/Games/comments/11kib1/steam_security_issue_
crosspost_rgaming/c6nrr0i
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