Asterisk Project Security Advisory – AST-2017-005

Product Asterisk
Summary Media takeover in RTP stack
Nature of Advisory Unauthorized data disclosure
Susceptibility Remote Unauthenticated Sessions
Severity Critical
Exploits Known No
Reported On May 17, 2017
Reported By Klaus-Peter Junghanns
Posted On
Last Updated On August 30, 2017
Advisory Contact Joshua Colp
CVE Name

Description The “strictrtp” option in rtp.conf enables a feature of the
RTP stack that learns the source address of media for a
session and drops any packets that do not originate from
the expected address. This option is enabled by default in
Asterisk 11 and above.

The “nat” and “rtp_symmetric” options for chan_sip and
chan_pjsip respectively enable symmetric RTP support in the
RTP stack. This uses the source address of incoming media
as the target address of any sent media. This option is not
enabled by default but is commonly enabled to handle
devices behind NAT.

A change was made to the strict RTP support in the RTP
stack to better tolerate late media when a reinvite occurs.
When combined with the symmetric RTP support this
introduced an avenue where media could be hijacked. Instead
of only learning a new address when expected the new code
allowed a new source address to be learned at all times.

If a flood of RTP traffic was received the strict RTP
support would allow the new address to provide media and
with symmetric RTP enabled outgoing traffic would be sent
to this new address, allowing the media to be hijacked.
Provided the attacker continued to send traffic they would
continue to receive traffic as well.

Resolution The RTP stack will now only learn a new source address if it
has been told to expect the address to change. The RTCP
support has now also been updated to drop RTCP reports that
are not regarding the RTP session currently in progress. The
strict RTP learning progress has also been improved to guard
against a flood of RTP packets attempting to take over the
media stream.

Affected Versions
Product Release
Series
Asterisk Open Source 11.x 11.4.0
Asterisk Open Source 13.x All Releases
Asterisk Open Source 14.x All Releases
Certified Asterisk 11.6 All Releases
Certified Asterisk 13.13 All Releases

Corrected In
Product Release
Asterisk Open Source 11.25.2, 13.17.1, 14.6.1
Certified Asterisk 11.6-cert17, 13.13-cert5

Patches
SVN URL Revision
http://downloads.asterisk.org/pub/security/AST-2017-005-11.diff Asterisk
11
http://downloads.asterisk.org/pub/security/AST-2017-005-13.diff Asterisk
13
http://downloads.asterisk.org/pub/security/AST-2017-005-14.diff Asterisk
14
http://downloads.asterisk.org/pub/security/AST-2017-005-11.6.diff Certified
Asterisk
11.6
http://downloads.asterisk.org/pub/security/AST-2017-005-13.13.diff Certified
Asterisk
13.13

Links https://issues.asterisk.org/jira/browse/ASTERISK-27013

Asterisk Project Security Advisories are posted at
http://www.asterisk.org/security

This document may be superseded by later versions; if so, the latest
version will be posted at
http://downloads.digium.com/pub/security/AST-2017-005.pdf and
http://downloads.digium.com/pub/security/AST-2017-005.html

Revision History
Date Editor Revisions Made
May 30, 2017 Joshua Colp Initial Revision

Asterisk Project Security Advisory – AST-2017-005
Copyright (c) 2017 Digium, Inc. All Rights Reserved.
Permission is hereby granted to distribute and publish this advisory in its
original, unaltered form.