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.