Product

Asterisk

Summary

IAX2 Call Number Resource Exhaustion

Nature of Advisory

Denial of Service

Susceptibility

Remote unauthenticated sessions

Severity

Major

Exploits Known

Yes - Published by Blake Cornell < blake AT remoteorigin DOT com > on voip0day.com

Reported On

June 22, 2008

Reported By

Noam Rathaus < noamr AT beyondsecurity DOT com >, with his SSD program, also by Blake Cornell

Posted On

September 3, 2009

Last Updated On

September 3, 2009

Advisory Contact

Russell Bryant < russell AT digium DOT com >

CVE Name

CVE-2009-2346

Description

The IAX2 protocol uses a call number to associate messages with the call that they belong to. However, the protocol defines the call number field in messages as a fixed size 15 bit field. So, if all call numbers are in use, no additional sessions can be handled.

A call number gets created at the start of an IAX2 message exchange. So, an attacker can send a large number of messages and consume the call number space. The attack is also possible using spoofed source IP addresses as no handshake is required before a call number is assigned.

Resolution

Upgrade to a version of Asterisk listed in this document as containing the IAX2 protocol security enhancements. In addition to upgrading, administrators should consult the users guide section of the IAX2 Security document (IAX2-security.pdf), as well as the sample configuration file for chan_iax2 that have been distributed with those releases for assistance with new options that have been provided.

Discussion

A lot of time was spent trying to come up with a way to resolve this issue in a way that was completely backwards compatible. However, the final resolution ended up requiring a modification to the IAX2 protocol. This modification is referred to as call token validation. Call token validation is used as a handshake before call numbers are assigned to IAX2 connections.

Call token validation by itself does not resolve the issue. However, it does allow an IAX2 server to validate that the source of the messages has not been spoofed. In addition to call token validation, Asterisk now also has the ability to limit the amount of call numbers assigned to a given remote IP address.

The combination of call token validation and call number allocation limits is used to mitigate this denial of service issue.

An alternative approach to securing IAX2 would be to use a security layer on top of IAX2, such as DTLS [RFC4347] or IPsec [RFC4301].

Affected Versions

Product

Release Series

Asterisk Open Source

1.2.x

All versions

Asterisk Open Source

1.4.x

All versions

Asterisk Open Source

1.6.x

All versions

Asterisk Business Edition

B.x.x

All versions

Asterisk Business Edition

C.x.x

All versions

s800i (Asterisk Appliance)

1.3.x

All versions

Corrected In

Product

Release

Asterisk Open Source

1.2.35

Asterisk Open Source

1.4.26.2

Asterisk Open Source

1.6.0.15

Asterisk Open Source

1.6.1.6

Asterisk Business Edition

B.2.5.10

Asterisk Business Edition

C.2.4.3

Asterisk Business Edition

C.3.1.1

S800i (Asterisk Appliance)

1.3.0.3

Patches

Link

Branch

http://downloads.asterisk.org/pub/security/AST-2009-006-1.2.diff.txt

1.2

http://downloads.asterisk.org/pub/security/AST-2009-006-1.4.diff.txt

1.4

http://downloads.asterisk.org/pub/security/AST-2009-006-1.6.0.diff.txt

1.6.0

http://downloads.asterisk.org/pub/security/AST-2009-006-1.6.1.diff.txt

1.6.1

Links

http://www.rfc-editor.org/authors/rfc5456.txt

https://issues.asterisk.org/view.php?id=12912

http://www.beyondsecurity.com/ssd.html

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-2009-006.pdf and http://downloads.digium.com/pub/security/AST-2009-006.html

Revision History

Date

Editor

Revisions Made

2009-09-03

Russell Bryant

Initial release