[UNIX] Kerberos Faulty Length Checks in xdrmem_getbytes

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From: support@securiteam.com
To: list@securiteam.com
Date: 23 Mar 2003 18:43:53 +0200

The following security advisory is sent to the securiteam mailing list, and can be found at the SecuriTeam web site: http://www.securiteam.com
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  Kerberos Faulty Length Checks in xdrmem_getbytes
------------------------------------------------------------------------

SUMMARY

The MIT Kerberos 5 implementation includes an RPC library derived from
SUNRPC. We have been notified that the xdrmem_getbytes() function contains
faulty length checks. These length checks are vulnerable to an integer
overflow, which may be exploitable to create denials of service or to gain
unauthorized access to sensitive information.

An attacker who has successfully authenticated to the Kerberos
administration daemon (kadmind) may be able to crash kadmind or induce it
to leak sensitive information, such as secret keys. For the attack to
succeed, it is believed that the configuration of the kadmind installation
must allow it to successfully allocate more than INT_MAX bytes of memory.

DETAILS

Vulnerable systems:
 * All releases of MIT Kerberos 5, up to and including krb5-1.2.7.

The xdrmem_getbytes() function decrements the private signed integer
"xdrs->x_handy" by the supplied length "len", which is an unsigned int. It
then verifies that the resulting value of "xdrs->x_handy" is non-negative.
Using a carefully chosen value of "len" (so that it is greater than
INT_MAX), it is possible for this check to succeed even if the value of
"len" would cause the buffer to be overrun on read. This overrun may
result in a segmentation fault, or in the unauthorized copying of
sensitive information.

A mitigating factor is that most call chains that end up calling
xdrmem_getbytes() first call malloc() (via the mem_alloc() macro) to
allocate a buffer of the requested length. This allocation of more than
INT_MAX bytes will fail on most configurations due to internal limitations
of malloc() or due to system resource limits. On systems where allocation
of more than INT_MAX bytes can succeed (possibly including 64-bit
environments), the probability of successful exploit is higher.

In MIT krb5, the vulnerable invocations of xdrmem_getbytes() inside
kadmind only occur after the user has successfully authenticated.
Additionally, any unauthorized copies of sensitive data obtained by
exercising this vulnerability are extremely unlikely to be returned to the
remote client.

Impact:
 * An attacker capable of authenticating to kadmind may be able to crash
kadmind.

 * Under extremely unlikely circumstances, an attacker capable of
authenticating to kadmind may be able to induce it to return sensitive
information, such as secret keys.

Fix:
Apply the following patch to src/lib/rpc/xdr_mem.c and rebuild your tree.

Index: xdr_mem.c
 ===================================================================
RCS file: /cvs/krbdev/krb5/src/lib/rpc/xdr_mem.c,v
retrieving revision 1.8
diff -c -r1.8 xdr_mem.c
*** xdr_mem.c 1998/02/14 02:27:24 1.8
- --- xdr_mem.c 2003/02/04 22:57:24
***************
*** 47,52 ****
- --- 47,54 ----
  #include <gssrpc/xdr.h>
  #include <netinet/in.h>
  #include <stdio.h>
+ #include <string.h>
+ #include <limits.h>
  
  static bool_t xdrmem_getlong();
  static bool_t xdrmem_putlong();
***************
*** 83,89 ****
  xdrs->x_op = op;
  xdrs->x_ops = &xdrmem_ops;
  xdrs->x_private = xdrs->x_base = addr;
! xdrs->x_handy = size;
  }
  
  static void
- --- 85,91 ----
  xdrs->x_op = op;
  xdrs->x_ops = &xdrmem_ops;
  xdrs->x_private = xdrs->x_base = addr;
! xdrs->x_handy = (size > INT_MAX) ? INT_MAX : size; /* XXX */
  }
  
  static void
***************
*** 98,105 ****
  long *lp;
  {
  
! if ((xdrs->x_handy -= sizeof(rpc_int32)) < 0)
  return (FALSE);
  *lp = (long)ntohl(*((rpc_u_int32 *)(xdrs->x_private)));
  xdrs->x_private += sizeof(rpc_int32);
  return (TRUE);
- --- 100,109 ----
  long *lp;
  {
  
! if (xdrs->x_handy < sizeof(rpc_int32))
  return (FALSE);
+ else
+ xdrs->x_handy -= sizeof(rpc_int32);
  *lp = (long)ntohl(*((rpc_u_int32 *)(xdrs->x_private)));
  xdrs->x_private += sizeof(rpc_int32);
  return (TRUE);
***************
*** 111,118 ****
  long *lp;
  {
  
! if ((xdrs->x_handy -= sizeof(rpc_int32)) < 0)
  return (FALSE);
  *(rpc_int32 *)xdrs->x_private = (rpc_int32)htonl((rpc_u_int32)(*lp));
  xdrs->x_private += sizeof(rpc_int32);
  return (TRUE);
- --- 115,124 ----
  long *lp;
  {
  
! if (xdrs->x_handy < sizeof(rpc_int32))
  return (FALSE);
+ else
+ xdrs->x_handy -= sizeof(rpc_int32);
  *(rpc_int32 *)xdrs->x_private = (rpc_int32)htonl((rpc_u_int32)(*lp));
  xdrs->x_private += sizeof(rpc_int32);
  return (TRUE);
***************
*** 125,132 ****
  register unsigned int len;
  {
  
! if ((xdrs->x_handy -= len) < 0)
  return (FALSE);
  memmove(addr, xdrs->x_private, len);
  xdrs->x_private += len;
  return (TRUE);
- --- 131,140 ----
  register unsigned int len;
  {
  
! if (xdrs->x_handy < len)
  return (FALSE);
+ else
+ xdrs->x_handy -= len;
  memmove(addr, xdrs->x_private, len);
  xdrs->x_private += len;
  return (TRUE);
***************
*** 139,146 ****
  register unsigned int len;
  {
  
! if ((xdrs->x_handy -= len) < 0)
  return (FALSE);
  memmove(xdrs->x_private, addr, len);
  xdrs->x_private += len;
  return (TRUE);
- --- 147,156 ----
  register unsigned int len;
  {
  
! if (xdrs->x_handy < len)
  return (FALSE);
+ else
+ xdrs->x_handy -= len;
  memmove(xdrs->x_private, addr, len);
  xdrs->x_private += len;
  return (TRUE);
***************
*** 179,185 ****
  {
  rpc_int32 *buf = 0;
  
! if (xdrs->x_handy >= len) {
  xdrs->x_handy -= len;
  buf = (rpc_int32 *) xdrs->x_private;
  xdrs->x_private += len;
- --- 189,195 ----
  {
  rpc_int32 *buf = 0;
  
! if (len >= 0 && xdrs->x_handy >= len) {
  xdrs->x_handy -= len;
  buf = (rpc_int32 *) xdrs->x_private;
  xdrs->x_private += len;

The patch was generated against krb5-1.2.7; patches to other releases may
apply with some offset.

This patch may also be found at:
 <http://web.mit.edu/kerberos/www/advisories/2003-003-xdr_patch.txt>
http://web.mit.edu/kerberos/www/advisories/2003-003-xdr_patch.txt

ADDITIONAL INFORMATION

The information has been provided by <mailto:tlyu@mit.edu> Tom Yu.

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