Advisory: sudo 1.8 Format String Vulnerability



Hi,

FYI, see attached.

cheers,

joernchen
--
joernchen ~ Phenoelit
<joernchen@xxxxxxxxxxxx> ~ C776 3F67 7B95 03BF 5344
http://www.phenoelit.de ~ A46A 7199 8B7B 756A F5AC
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&id=9

[ end of file ]