Product

Asterisk

Summary

Unathenticated calls in chan_sip

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

December 8, 2016

Last Updated On

December 13, 2016

Advisory Contact

Mmichelson AT digium DOT com

CVE Name

CVE-2016-9938

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This document may be superseded by later versions; if so, the latest version will be posted at http://downloads.digium.com/pub/security/ASTERISK-2016-009.pdf and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

December 13, 2016

Kevin Harwell

Added description and CVE