Product

Asterisk

Summary

SIP responses expose valid usernames

Nature of Advisory

Information leak

Susceptibility

Remote Unauthenticated Sessions

Severity

Minor

Exploits Known

No

Reported On

February 23, 2009

Reported By

Gentoo Linux Project: Kerin Millar ( kerframil on irc.freenode.net ) and Fergal Glynn < FGlynn AT veracode DOT com >

Posted On

April 2, 2009

Last Updated On

April 2, 2009

Advisory Contact

Tilghman Lesher < tlesher AT digium DOT com >

CVE Name

CVE-2008-3903

Description

In 2006, the Asterisk maintainers made it more difficult to scan for valid SIP usernames by implementing an option called "alwaysauthreject", which should return a 401 error on all replies which are generated for users which do not exist. While this was sufficient at the time, due to ever increasing compliance with RFC 3261, the SIP specification, that is no longer sufficient as a means towards preventing attackers from checking responses to verify whether a SIP account exists on a machine.

What we have done is to carefully emulate exactly the same responses throughout possible dialogs, which should prevent attackers from gleaning this information. All invalid users, if this option is turned on, will receive the same response throughout the dialog, as if a username was valid, but the password was incorrect.

It is important to note several things. First, this vulnerability is derived directly from the SIP specification, and it is a technical violation of RFC 3261 (and subsequent RFCs, as of this date), for us to return these responses. Second, this attack is made much more difficult if administrators avoided creating all-numeric usernames and especially all-numeric passwords. This combination is extremely vulnerable for servers connected to the public Internet, even with this patch in place. While it may make configuring SIP telephones easier in the short term, it has the potential to cause grief over the long term.

Resolution

Upgrade to one of the versions below, or apply one of the patches specified in the Patches section.

Affected Versions

Product

Release Series

Asterisk Open Source

1.2.x

All versions prior to 1.2.32

Asterisk Open Source

1.4.x

All versions prior to 1.4.24.1

Asterisk Open Source

1.6.0.x

All versions prior to 1.6.0.8

Asterisk Addons

1.2.x

Not affected

Asterisk Addons

1.4.x

Not affected

Asterisk Addons

1.6.x

Not affected

Asterisk Business Edition

A.x.x

All versions

Asterisk Business Edition

B.x.x

All versions prior to B.2.5.8

Asterisk Business Edition

C.1.x.x

All versions prior to C.1.10.5

Asterisk Business Edition

C.2.x.x

All versions prior to C.2.3.3

AsteriskNOW

1.5

Not affected

s800i (Asterisk Appliance)

1.3.x

All versions prior to 1.3.0.2

Corrected In

Product

Release

Asterisk Open Source

1.2.32

Asterisk Open Source

1.4.24.1

Asterisk Open Source

1.6.0.8

Asterisk Business Edition

B.2.5.8

Asterisk Business Edition

C.1.10.5

Asterisk Business Edition

C.2.3.3

s800i (Asterisk Appliance)

1.3.0.2

Patches

Patch URL

Version

http://downloads.digium.com/pub/asa/AST-2009-003-1.2.diff.txt

1.2

http://downloads.digium.com/pub/asa/AST-2009-003-1.4.diff.txt

1.4

http://downloads.digium.com/pub/asa/AST-2009-003-1.6.0.diff.txt

1.6.0

http://downloads.digium.com/pub/asa/AST-2009-003-1.6.1.diff.txt

1.6.1

Links

http://www.faqs.org/rfcs/rfc3261.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-2009-003.pdf and http://downloads.digium.com/pub/security/AST-2009-003.html

Revision History

Date

Editor

Revisions Made

2009-04-02

Tilghman Lesher

Initial release