Asterisk Project Security Advisory – ASTERISK-2016-009

Product Asterisk
Summary
Nature of Advisory Authentication Bypass
Susceptibility Remote unauthenticated sessions
Severity Minor
Exploits Known No
Reported On October 3, 2016
Reported By Walter Doekes
Posted On
Last Updated On December 8, 2016
Advisory Contact Mmichelson AT digium DOT com
CVE Name

Description The chan_sip channel driver has a liberal definition for
whitespace when attempting to strip the content between a
SIP header name and a colon character. Rather than
following RFC 3261 and stripping only spaces and horizontal
tabs, Asterisk treats any non-printable ASCII character as
if it were whitespace. This means that headers such as

Contact\x01:

will be seen as a valid Contact header.

This mostly does not pose a problem until Asterisk is
placed in tandem with an authenticating SIP proxy. In such
a case, a crafty combination of valid and invalid To
headers can cause a proxy to allow an INVITE request into
Asterisk without authentication since it believes the
request is an in-dialog request. However, because of the
bug described above, the request will look like an
out-of-dialog request to Asterisk. Asterisk will then
process the request as a new call. The result is that
Asterisk can process calls from unvetted sources without
any authentication.

If you do not use a proxy for authentication, then this
issue does not affect you.

If your proxy is dialog-aware (meaning that the proxy keeps
track of what dialogs are currently valid), then this issue
does not affect you.

If you use chan_pjsip instead of chan_sip, then this issue
does not affect you.

Resolution chan_sip has been patched to only treat spaces and
horizontal tabs as whitespace following a header name. This
allows for Asterisk and authenticating proxies to view
requests the same way

Affected Versions
Product Release
Series
Asterisk Open Source 11.x All Releases
Asterisk Open Source 13.x All Releases
Asterisk Open Source 14.x All Releases
Certified Asterisk 13.8 All Releases

Corrected In
Product Release
Asterisk Open Source 11.25.1, 13.13.1, 14.2.1
Certified Asterisk 11.6-cert16, 13.8-cert4

Patches
SVN URL Revision

Links

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This document may be superseded by later versions; if so, the latest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/ASTERISK-2016-009.pdf and
http://downloads.digium.com/pub/security/ASTERISK-2016-009.html

Revision History
Date Editor Revisions Made
November 28, 2016 Mark Michelson Initial writeup

Asterisk Project Security Advisory – ASTERISK-2016-009
Copyright (c) 2016 Digium, Inc. All Rights Reserved.
Permission is hereby granted to distribute and publish this advisory in its
original, unaltered form.