Advisory: sudo 1.8 Format String Vulnerability

Phenoelit Advisory <wir-haben-auch-mal-was-gefunden #0815 ±-++>

[ Authors ]
joernchen <joernchen () phenoelit de>

    Phenoelit Group (http://www.phenoelit.de)

[ Affected Products ]
sudo 1.8.0 - 1.8.3p1 (http://sudo.ws)

[ Vendor communication ]
2012-01-24 Send vulnerability details to sudo maintainer
2012-01-24 Maintainer is embarrased
2012-01-27 Asking maintainer how the fixing goes
2012-01-27 Maintainer responds with a patch and a release date
of 2012-01-30 for the patched sudo and advisory
2012-01-30 Release of this advisory

[ Description ]

    Observe src/sudo.c:

void
sudo_debug(int level, const char *fmt, …)
{
va_list ap;
char *fmt2;

if (level > debug_level)
    return;

/* Backet fmt with program name and a newline to make it a single 
write */
easprintf(&fmt2, "%s: %s\n", getprogname(), fmt);
va_start(ap, fmt);
vfprintf(stderr, fmt2, ap);
va_end(ap);
efree(fmt2);

}

    Here getprogname() is argv[0] and by this user controlled. So 
    argv[0] goes to fmt2 which then gets vfprintf()ed to stderr. The
    result is a Format String vulnerability.   

[ Example ]
/tmp $ ln -s /usr/bin/sudo %n
/tmp $ ./%n -D9
*** %n in writable segment detected***
Aborted
/tmp $

   A note regarding exploitability: The above example shows the result
   of FORTIFY_SOURCE which makes explotitation painful but not 
   impossible (see [0]). Without FORTIFY_SOURCE the exploit is straight
   forward:
     1. Use formatstring to overwrite the setuid() call with setgid()
     2. Trigger with formatstring -D9 
     3. Make use of SUDO_ASKPASS and have shellcode in askpass script
     4. As askpass will be called after the formatstring has 
        overwritten setuid() the askepass script will run with uid 0
     5. Enjoy the rootshell

[ Solution ]
Update to version 1.8.3.p2

[ References ]
[0] http://www.phrack.org/issues.html?issue=67&amp;id=9

[ end of file ]