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                         Luigi Auriemma

Application: Sauerbraten engine
http://sauerbraten.org
Versions: <= 2006_02_28 and current CVS
Platforms: Windows, *nix, *BSD and MacOS
Bugs: A] sgetstr() buffer-overflow
B] invalid memory access
C] clients crash through invalid map
D] crash through unconnected client
Exploitation: remote, versus both server and clients
Date: 06 Mar 2006
Author: Luigi Auriemma
e-mail: [email protected]
web: http://aluigi.altervista.org

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1) Introduction
2) Bugs
3) The Code
4) Fix

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1) Introduction

Sauerbraten is the evolution of the Cube engine
(http://www.cubeengine.com) developed by Wouter van Oortmerssen
(http://strlen.com), in fact can be defined also as "Next-Gen Cube" or
"Cube 2".
It supports both LAN and Internet multiplayer through its master
server.

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2) Bugs

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A] sgetstr() buffer-overflow

The game uses an unchecked function for reading the strings from the
incoming data.
The function is sgetstr() located in shared/cube.h:

#define sgetstr() { char *t = text; do { *t = getint(p); } while(*t+
+); }

The problem, which affects both server and clients, is that this code
copies the input data over the text buffer of size MAXTRANS (5000 bytes)
allowing possible malicious code execution.

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B] invalid memory access

sgetstr(), getint() and the instructions which call them don't check
the correct length of the input data.
In short is possible to force the server or the client to read over the
received data reaching unallocated zones of the memory and so crashing
immediately.

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C] clients crash through invalid map

In the Sauerbraten engine the players have the possibility to choose a
specific map on which playing, if there is only one player in the
server the map is changed immediately otherwise will be voted.
When a client tries to load an invalid map file it exits immediately
showing the "while reading map: header malformatted" error.
When the map is choosed all the clients add a .ogz extension to the
mapname received from the server and load the file.
The max size of the mapname is 260 bytes and the function which loads
the file uses a secure sprintf() which truncates the input mapname
(.ogz included) when the limit is reached.
Then the loading of the map is not sanitized versus possible directory
traversal exploitations so if an attacker (a player) specifies a
mapname of about 260 bytes he can force any client which will join the
server (due to the voting problem explained previously which limits the
exploitation of this bug) to load any file which is not a valid map and
so they will exit immediately.
As already said the exploitation happens with any new client which
joins the server since the new mapname will remain active in the server
for all the current match.

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D] crash through unconnected client

A partially connected client can easily crash the Sauerbraten server.
This bug is caused by the following instruction in engine/server.cpp:

int num = ((client *)event.peer->data)->num;

In short when the connection times out the server tries to show the
host of the disconnected client ignoring that it has never joined.
The effect is the reading of an unallocated zone of the memory.

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3) The Code

http://aluigi.altervista.org/poc/sauerburn.zip

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4) Fix

The developers will release a fix, only for the buffer-overflow bug,
soon.

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Luigi Auriemma
http://aluigi.altervista.org