CVE-2011-4597 in Asterisk
Summary
by MITRE
The SIP over UDP implementation in Asterisk Open Source 1.4.x before 1.4.43, 1.6.x before 1.6.2.21, and 1.8.x before 1.8.7.2 uses different port numbers for responses to invalid requests depending on whether a SIP username exists, which allows remote attackers to enumerate usernames via a series of requests.
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Analysis
by VulDB Data Team • 11/28/2021
The vulnerability described in CVE-2011-4597 represents a significant information disclosure weakness in the Asterisk open source telephony system's Session Initiation Protocol implementation over User Datagram Protocol. This flaw affects multiple versions of the Asterisk PBX software, specifically targeting the 1.4.x series before 1.4.43, 1.6.x series before 1.6.2.21, and 1.8.x series before 1.8.7.2. The issue stems from inconsistent port number handling in SIP response mechanisms when processing invalid requests, creating a side-channel information leakage vector that can be exploited by remote attackers.
The technical mechanism behind this vulnerability involves the SIP over UDP implementation's differential behavior in response handling based on the presence or absence of valid SIP usernames. When Asterisk receives invalid SIP requests, it responds with different UDP port numbers depending on whether the requested username exists in its system. This behavioral inconsistency creates a predictable pattern that attackers can exploit through systematic request manipulation to determine which usernames are valid within the target system. The vulnerability operates at the application layer of the OSI model, specifically affecting the signaling protocol implementation rather than underlying network protocols.
From an operational perspective, this vulnerability enables remote attackers to perform user enumeration attacks against SIP-based telephony systems, potentially compromising the security of voice communication networks. The impact extends beyond simple information disclosure as it provides attackers with a foundation for more sophisticated attacks including credential brute-forcing, social engineering, and targeted exploitation of valid user accounts. The vulnerability is particularly concerning in enterprise environments where Asterisk serves as a core communication infrastructure component, as successful exploitation can lead to unauthorized access to voice services and potential compromise of the entire telephony infrastructure.
The vulnerability aligns with CWE-200, which describes "Information Exposure," and can be mapped to ATT&CK technique T1087.001 for account discovery through enumeration attacks. Organizations using affected Asterisk versions should implement immediate mitigations including upgrading to patched versions, implementing network-level access controls, and deploying intrusion detection systems to monitor for suspicious SIP traffic patterns. The fix typically involves standardizing the response port handling regardless of username validity, ensuring consistent behavior that does not leak information about system users through network response characteristics. Security teams should also conduct comprehensive audits of their telephony infrastructure to identify any other potential information disclosure vulnerabilities in SIP implementations and related VoIP systems.