Asterisk SIP Implementation Issue

["@stake Advisories" <advisories () atstake com>]

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                             @stake Inc.
                          www.atstake.com

                         Security Advisory

Advisory Name: Asterisk SIP Implementation Issue
Release Date: 09/04/2003
Application: Asterisk
   Platform: Linux (x86)
   Severity: An attacker is able to obtain remote access
             to the host in question prior to authentication
    Authors: Ollie Whitehouse [ollie () atstake com]
             Graham Murphy [gmurphy () atstake com]
             Stephen Kapp    [skapp () atstake com]
Vendor Status: Informed / CVS Updated 15th of August 2003
CVE Candidate: CAN-2003-???? (Pending)
  Reference: www.atstake.com/research/advisories/2003/a090403-1.txt


Overview:

Asterisk (http://www.asterisk.org/) is a complete PBX (Private
Branch eXchange) in software. It runs on Linux and provides all of the
features you would expect from a PBX and more. Asterisk does voice over IP
in three protocols (SIP, IAX (v1 and v2)) and H323, and can interoperate
with almost all standards-based telephony equipment using relatively
inexpensive hardware.

The Session Initiation Protocol (SIP) is an application-layer control
(signaling) protocol for creating, modifying and terminating sessions
with one or more participants. These sessions include Internet
multimedia conferences, Internet telephone calls, multimedia
distribution and instant messaging. The SIP protocol is described in
RFC3261 (with extensions contained in RFC3265).

While conducting a source code review of the SIP protocol implementation
within Asterisk, @stake found a vulnerability that could allow an attacker
to obtain remote and unauthenticated access to the host in question.

This is a good example of a vulnerability that would be difficult to
identify in the process of automated fault injection (fuzzing).


Details:

@stake discovered that if a specially crafted SIP request of a
specific size was sent (body length of 1024 bytes) with a particular
request
type (MESSAGE and INFO) that the following function could be exploited:

-------[chan_sip.c fragment start]------
static int get_msg_text(char *buf, int len, struct sip_request *req)
{
 int x;
 strcpy(buf, "");
 for (x=0;x<req->lines;x++) {
  strncat(buf, req->line[x], len - strlen(buf) - 5);
  strcat(buf, "\n");
 }
 return 0;
}
-------[chan_sip.c fragment end]------

Therefore, when a specially crafted request with a body size of 1024 bytes
is received, the end of the internal buffer used will be over-written.
This happens because a negative number is passed to strncat() instead
of the following (example):

0x080483b9 <go+93>:     push   $0xa        - Copy 10 bytes
0x080483bb <go+95>:     pushl  0x8(%ebp)

The number is decremented past 0x0 and wraps to 0xFFFFFFFF to become
(example):

0x080483e5 <go+137>:    push   $0xfffffffc    - Copy a lot more than 10
0x080483e7 <go+139>:    pushl  0x8(%ebp)

So this causes the strncat() to copy this many bytes, however due to
a null being located in our page of memory, we don't get a page fault.
Instead it simply overwrites the save return address. Thus this becomes
an easily exploitable buffer overflow.

Although SIP supports authentication, both the MESSAGE and INFO messages
will be processed without any authentication. This allows any user who can
send SIP messages to Asterisk to take advantage of the vulnerability.

By exploiting this vulnerability, @stake managed to obtain access to the
remote host in question. The access level attained by exploiting this
vulnerability is that of the user that started the Asterisk services,
which
in the default installation is as the root user.
 

Recommendation:

@stake notified the author of this particular code on the
15th of August. The author developed and deployed a patch silently
to the CVS on the 15th of August.

@stake would recommend that if you have not deployed a CVS version
since the 15th of August 2003 to immediately do so.


Common Vulnerabilities and Exposures (CVE) Information:

The Common Vulnerabilities and Exposures (CVE) project has assigned
the following names to these issues.  These are candidates for
inclusion in the CVE list (http://cve.mitre.org), which standardizes
names for security problems.

CAN-2003-??? Asterisk SIP implementation issue


@stake Vulnerability Reporting Policy:
http://www.atstake.com/research/policy/

@stake Advisory Archive:
http://www.atstake.com/research/advisories/

PGP Key:
http://www.atstake.com/research/pgp_key.asc

@stake is currently seeking application security experts to fill
several consulting positions.  Applicants should have strong
application development skills and be able to perform application
security design reviews, code reviews, and application penetration
testing.  Please send resumes to jobs () atstake com.

Copyright 2003 @stake, Inc. All rights reserved.

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