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Vulnerability of Asterisk: user detection via IAX2

Synthesis of the vulnerability 

An attacker can use information returned during the IAX2 authentication in order to detect if a user name is valid.
Vulnerable products: Asterisk Open Source, Debian, Fedora.
Severity of this weakness: 2/4.
Creation date: 09/01/2009.
Références of this bulletin: AST-2009-001, BID-33174, CERTA-2009-AVI-010, CVE-2009-0041, DSA-1952-1, FEDORA-2009-0973, FEDORA-2009-0984, VIGILANCE-VUL-8376.

Description of the vulnerability 

The Asterisk product implements the IAX2 protocol (Inter-Asterisk Exchange version 2) to transmit streaming over IP.

The IAX2 authentication uses several methods:
 - IAX_AUTH_PLAINTEXT : text
 - IAX_AUTH_MD5 : md5 hash
 - IAX_AUTH_RSA : RSA encryption

When the user name is invalid, the error message uses a text authentication. When the username is valid and the password is invalid, the error message uses one of user's authentication methods.

An attacker can therefore use this difference in order to detect if the username is valid. The attacker can thus construct a list of valid users.
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This weakness impacts software or systems such as Asterisk Open Source, Debian, Fedora.

Our Vigil@nce team determined that the severity of this vulnerability announce is medium.

The trust level is of type confirmed by the editor, with an origin of intranet client.

An attacker with a expert ability can exploit this threat bulletin.

Solutions for this threat 

Asterisk: version 1.2.31.
Version 1.2.31 is corrected:
  http://www.asterisk.org/

Debian: new asterisk packages.
New packages are available:
  http://security.debian.org/pool/updates/main/a/asterisk/asterisk*_1.4.21.2~dfsg-3+lenny1_*.deb

Fedora 9: new dahdi-tools packages.
New packages are available:
  dahdi-tools-2.0.0-1.fc9

Fedora 9: new libresample packages.
New packages are available:
  libresample-0.1.3-9.fc9

Fedora: new asterisk packages.
New packages are available:
  asterisk-1.6.0.5-2.fc9
  asterisk-1.6.0.5-2.fc10
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