Friday, February 19, 2021

Asterisk Project Security Advisory - AST-2021-005

               Asterisk Project Security Advisory - AST-2021-005

Product Asterisk
Summary Remote Crash Vulnerability in PJSIP channel driver
Nature of Advisory Denial of Service
Susceptibility Remote Unauthenticated Sessions
Severity Moderate
Exploits Known No
Reported On December 4, 2020
Reported By Mauri de Souza Meneguzzo (3CPlus)
Posted On February 8, 2021
Last Updated On February 8, 2021
Advisory Contact Jcolp AT sangoma DOT com
CVE Name CVE-2021-26906

Description Given a scenario where an outgoing call is placed from
Asterisk to a remote SIP server it is possible for a
crash to occur.

The code responsible for negotiating SDP in SIP
responses incorrectly assumes that SDP negotiation
will always be successful. If a SIP response
containing an SDP that can not be negotiated is
received a subsequent SDP negotiation on the same call
can cause a crash.

If the “accept_multiple_sdp_answers” option in the
“system” section of pjsip.conf is set to “yes” then
any subsequent non-forked SIP response with SDP can
trigger this crash.

If the “follow_early_media_fork” option in the
“system” section of pjsip.conf is set to “yes” (the
default) then any subsequent SIP responses with SDP
from a forked destination can trigger this crash.

If a 200 OK with SDP is received from a forked
destination it can also trigger this crash, even if
the “follow_early_media_fork” option is not set to

In all cases this relies on a race condition with
tight timing where the second SDP negotiation occurs
before termination of the call due to the initial SDP
negotiation failure.
Modules Affected res_pjsip_session.c, PJSIP

Resolution The issue has been fixed in PJSIP by changing the behavior
of the pjmedia_sdp_neg_modify_local_offer2 function. If SDP
was previously negotiated the code no longer assumes that it
was successful and instead checks that SDP was negotiated.

This issue can only be resolved by upgrading to a fixed
version or applying the provided patch.

Affected Versions
Product Release Series
Asterisk Open Source 13.x All versions
Asterisk Open Source 16.x All versions
Asterisk Open Source 17.x All versions
Asterisk Open Source 18.x All versions
Certified Asterisk 16.x All versions

Corrected In
Product Release
Asterisk Open Source 13.38.2, 16.16.1, 17.9.2, 18.2.1
Certified Asterisk 16.8-cert6

Patch URL Revision Asterisk
13 Asterisk
16 Asterisk
17 Asterisk
18 Certified


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
Date Editor Revisions Made
February 8, 2021 Joshua Colp Initial revision

Asterisk Project Security Advisory - AST-2021-005
Copyright © 2021 Digium, Inc. All Rights Reserved.
Permission is hereby granted to distribute and publish this advisory in its
original, unaltered form.

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Thanks for everything Templateism - You should have written the code a little more complicated