CVE-2011-2536 in Asterisk
Summary
by MITRE
chan_sip.c in the SIP channel driver in Asterisk Open Source 1.4.x before 1.4.41.2, 1.6.2.x before 1.6.2.18.2, and 1.8.x before 1.8.4.4, and Asterisk Business Edition C.3.x before C.3.7.3, disregards the alwaysauthreject option and generates different responses for invalid SIP requests depending on whether the user account exists, which allows remote attackers to enumerate account names via a series of requests.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 11/14/2021
The vulnerability described in CVE-2011-2536 represents a critical account enumeration flaw within the SIP channel driver of Asterisk Open Source and Business Edition systems. This vulnerability affects multiple versions of the Asterisk telephony platform, specifically targeting the chan_sip.c module that handles Session Initiation Protocol communications. The flaw manifests when the system fails to properly respect the alwaysauthreject configuration option, creating inconsistent response behaviors that inadvertently reveal account existence information to remote attackers. The issue stems from the SIP channel driver's inconsistent handling of invalid authentication requests, where the system provides different response patterns depending on whether the requested user account exists within the system's configuration.
The technical implementation of this vulnerability exploits the fundamental security principle of information disclosure through response timing and content variations. When remote attackers send malformed or unauthorized SIP requests to the affected Asterisk systems, the server's response behavior reveals whether a specific username exists in the system's user database. This occurs because legitimate accounts receive different authentication responses compared to non-existent accounts, creating a side-channel attack vector that can be systematically exploited through repeated request patterns. The vulnerability specifically impacts the authentication flow within the SIP protocol implementation, where the system's failure to consistently apply the alwaysauthreject setting creates predictable response patterns that attackers can analyze and exploit. This type of flaw aligns with CWE-200, which categorizes information exposure vulnerabilities, and represents a classic example of how improper error handling can create security weaknesses.
The operational impact of this vulnerability extends far beyond simple account enumeration, as it provides attackers with a systematic method to discover valid user accounts within the telephony infrastructure. Once attackers have compiled a list of valid usernames, they can proceed with more sophisticated attacks including password brute force attempts, credential stuffing attacks, or social engineering operations targeting specific users. The vulnerability affects organizations using Asterisk systems for VoIP communications, potentially compromising not just the telephony infrastructure but also the broader network security posture. Attackers can leverage this information to target specific users for phishing campaigns or to focus their efforts on accounts that are more likely to have valid credentials. The exposure of account information through this vulnerability can lead to unauthorized access to voice communications, potential eavesdropping on sensitive conversations, and broader compromise of the organization's communication infrastructure.
Mitigation strategies for CVE-2011-2536 require immediate implementation of version upgrades to patched Asterisk releases, specifically targeting versions 1.4.41.2, 1.6.2.18.2, 1.8.4.4, and C.3.7.3 respectively. Organizations should also ensure that the alwaysauthreject configuration option is properly enabled and enforced within their SIP channel driver settings, which will standardize authentication responses regardless of account existence. Network-level mitigations including firewall rules to restrict SIP traffic to trusted sources, rate limiting to prevent automated enumeration attempts, and intrusion detection systems configured to monitor for suspicious SIP request patterns can provide additional protection layers. Security monitoring should include logging and alerting for repeated authentication failures or unusual SIP request patterns that may indicate enumeration attempts. The vulnerability demonstrates the importance of consistent security configuration management and proper error handling in telephony systems, aligning with ATT&CK techniques that involve credential access and reconnaissance activities. Organizations should conduct thorough security assessments of their telephony infrastructure to identify any other potential information disclosure vulnerabilities that could be exploited in similar fashion.