AST-2011-013: Possible remote enumeration of SIP endpoints with differing NAT settings Dec 08 2011 10:47PM
Asterisk Security Team (security asterisk org)
Asterisk Project Security Advisory - AST-2011-013

Product Asterisk
Summary Possible remote enumeration of SIP endpoints with
differing NAT settings
Nature of Advisory Unauthorized data disclosure
Susceptibility Remote unauthenticated sessions
Severity Minor
Exploits Known Yes
Reported On 2011-07-18
Reported By Ben Williams
Posted On
Last Updated On December 7, 2011
Advisory Contact Terry Wilson <twilson (at) digium (dot) com [email concealed]>
CVE Name

Description It is possible to enumerate SIP usernames when the general
and user/peer NAT settings differ in whether to respond to
the port a request is sent from or the port listed for
responses in the Via header. In 1.4 and 1.6.2, this would
mean if one setting was nat=yes or nat=route and the other
was either nat=no or nat=never. In 1.8 and 10, this would
mean when one was nat=force_rport or nat=yes and the other
was nat=no or nat=comedia.

Resolution Handling NAT for SIP over UDP requires the differing
behavior introduced by these options.

To lessen the frequency of unintended username disclosure,
the default NAT setting was changed to always respond to the
port from which we received the request-the most commonly
used option.

Warnings were added on startup to inform administrators of
the risks of having a SIP peer configured with a different
setting than that of the general setting. The documentation
now strongly suggests that peers are no longer configured
for NAT individually, but through the global setting in the
"general" context.

Affected Versions
Product Release Series
Asterisk Open Source All All versions

Corrected In
As this is more of an issue with SIP over UDP in general, there is no
fix supplied other than documentation on how to avoid the problem. The
default NAT setting has been changed to what we believe the most
commonly used setting for the respective version in Asterisk 1.4.43,, and


Asterisk Project Security Advisories are posted at

This document may be superseded by later versions; if so, the latest
version will be posted at
http://downloads.digium.com/pub/security/AST-2011-013.pdf and

Revision History
Date Editor Revisions Made

Asterisk Project Security Advisory - AST-2011-013
Copyright (c) 2011 Digium, Inc. All Rights Reserved.
Permission is hereby granted to distribute and publish this advisory in its
original, unaltered form.

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