CVE-2011-2666 in Asterisk
Summary
by MITRE
The default configuration of the SIP channel driver in Asterisk Open Source 1.4.x through 1.4.41.2 and 1.6.2.x through 1.6.2.18.2 does not enable the alwaysauthreject option, which allows remote attackers to enumerate account names by making a series of invalid SIP requests and observing the differences in the responses for different usernames, a different vulnerability than CVE-2011-2536.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 02/10/2019
The vulnerability described in CVE-2011-2666 represents a significant security flaw in the Asterisk open source telephony platform that affects versions 1.4.x through 1.4.41.2 and 1.6.2.x through 1.6.2.18.2. This issue stems from the default configuration of the SIP channel driver which fails to enable the alwaysauthreject option, creating an information disclosure vulnerability that enables remote attackers to perform account enumeration attacks. The flaw operates by exploiting the inconsistent response behavior of the SIP server when processing invalid authentication requests from different usernames, allowing attackers to distinguish between valid and invalid account names through careful analysis of server responses. This vulnerability specifically relates to the SIP protocol implementation within Asterisk's channel driver architecture and constitutes a form of authentication bypass through information leakage.
The technical mechanism behind this vulnerability involves the SIP authentication process where the server responds differently to authentication attempts with valid versus invalid usernames when the alwaysauthreject option remains disabled. In a properly configured system with alwaysauthreject enabled, the server would respond identically to authentication requests regardless of whether the username exists, thus preventing attackers from inferring account existence. However, when this option is disabled, the server provides distinguishable responses that reveal account information, with valid accounts typically receiving different error codes or response patterns compared to invalid accounts. This behavior creates a side-channel information leak that directly violates the principle of least privilege and authentication security. The vulnerability is categorized under CWE-200 Information Exposure and aligns with ATT&CK technique T1212 Exploitation for Credential Access, as it enables attackers to gather credentials through indirect means rather than direct exploitation.
The operational impact of CVE-2011-2666 is substantial for organizations relying on Asterisk for voice communications, as it provides attackers with a straightforward method for account enumeration that can be automated and scaled. This vulnerability enables reconnaissance attacks that can be combined with other techniques such as password spraying or brute force attacks to compromise the system. The information disclosure aspect means that attackers can systematically test usernames against the SIP server to identify valid accounts, which significantly reduces the complexity of subsequent attacks. Organizations may experience unauthorized access to their telephony systems, potential eavesdropping on communications, and increased risk of denial of service attacks against valid accounts. The vulnerability also impacts the overall security posture of the communication infrastructure by providing attackers with intelligence that can be used to target specific accounts or conduct more sophisticated attacks.
The recommended mitigations for CVE-2011-2666 involve enabling the alwaysauthreject option in the Asterisk configuration files, specifically within the sip.conf or pjsip.conf configuration sections depending on the Asterisk version being used. Administrators should also implement additional security measures such as rate limiting for SIP authentication requests, implementing firewall rules to restrict access to SIP ports, and deploying intrusion detection systems to monitor for suspicious authentication patterns. Regular security audits of Asterisk configurations should be conducted to ensure that security best practices are maintained, including keeping the system updated to patched versions where available. Organizations should also consider implementing additional authentication mechanisms such as IP-based restrictions, two-factor authentication, and monitoring for unusual authentication patterns that could indicate enumeration attacks. This vulnerability highlights the importance of proper default security configurations and demonstrates how seemingly minor configuration omissions can create significant security risks in telephony systems. The remediation process should include comprehensive testing to ensure that the configuration changes do not negatively impact legitimate system functionality while providing the necessary security protections against account enumeration attacks.