Asterisk Project Security Advisory - AST-2007-014




Stack buffer overflow in IAX2 channel driver

Nature of Advisory

Exploitable Stack Buffer Overflow


Remote Unauthenticated Sessions



Exploits Known


Reported On

July 12, 2007

Reported By

Russell Bryant, Digium, Inc.

Posted On

July 17, 2007

Last Updated On

August 21, 2007

Advisory Contact

Russell Bryant <>

CVE Name



The Asterisk IAX2 channel driver, chan_iax2, has a remotely exploitable stack buffer overflow vulnerability. It occurs when chan_iax2 is passed a voice or video frame with a data payload larger than 4 kB. This is exploitable by sending a very large RTP frame to an active RTP port number used by Asterisk when the other end of the call is an IAX2 channel. Exploiting this issue can cause a crash or allow arbitrary code execution on a remote machine.

The specific conditions that trigger the vulnerability are the following:

  • iax2_write() is called with a frame with the following properties

    • a voice or video frame

    • Its 4-byte timestamp has the same high 2 bytes as the previous frame that was sent

    • Its format is the one currently expected

    • Its data payload is larger than 4 kB

iax2_write() calls iax2_send() to send the frame. Inside of iax2_send(), there is a conditional check to determine whether the frame should be sent immediately (the now variable) or queued for transmission later.

If the frame is going to be transmitted later, an iax_frame struct is dynamically allocated with a data buffer that has the exact buffer size needed to accommodate for the provided ast_frame data. However, if the frame is being sent immediately, it uses a stack allocated iax_frame, with a data buffer size of 4096 bytes.

Later, the iax_frame_wrap() function is used to copy the data from the ast_frame struct into the iax_frame struct. This function assumes the iax_frame data buffer has enough space for all of the data in the ast_frame.


This issue is only exploitable when the system is configured in such a way that calls between channels that use RTP and IAX2 channels are possible. Also, some additional protection against arbitrary code execution is provided if the call involves transcoding between audio formats as this will change the contents of the frame payload.

All users that have systems that connect calls between channels that use RTP and IAX2 channels should immediately update to versions listed in the corrected in section of this advisory.

Affected Versions


Release Series

Asterisk Open Source


All versions

Asterisk Open Source


All versions prior to 1.2.22

Asterisk Open Source


All versions prior to 1.4.8

Asterisk Business Edition


All versions

Asterisk Business Edition


All versions prior to B.2.2.1



All versions prior to beta7

Asterisk Appliance Developer Kit


All versions prior to 0.5.0

s800i (Asterisk Appliance)


All versions prior to 1.0.2

Corrected In



Asterisk Open Source

1.2.22 and 1.4.8, available from

Asterisk Business Edition

B.2.2.1, available from the Asterisk Business Edition user portal on or

via Digium Technical Support


Beta7, available from Beta5 and Beta6 users can update using the system update feature in the appliance control panel.

Asterisk Appliance Developer Kit

0.5.0, available from

s800i (Asterisk Appliance)



Asterisk Project Security Advisories are posted at

This document may be superseded by later versions; if so, the latest version will be posted at and

Revision History



Revisions Made

July 17, 2007

Initial Release

July 17, 2007

Minor Spelling Fix

August 21, 2007

Changed name prefix from ASA to AST, changed to

Asterisk Project Security Advisory - AST-2007-014
Copyright © 2007 Digium, Inc. All Rights Reserved.
Permission is hereby granted to distribute and publish this advisory in its original, unaltered form.