CVE-2016-9937 in Asterisk
Summary
by MITRE
An issue was discovered in Asterisk Open Source 13.12.x and 13.13.x before 13.13.1 and 14.x before 14.2.1. If an SDP offer or answer is received with the Opus codec and with the format parameters separated using a space the code responsible for parsing will recursively call itself until it crashes. This occurs as the code does not properly handle spaces separating the parameters. This does NOT require the endpoint to have Opus configured in Asterisk. This also does not require the endpoint to be authenticated. If guest is enabled for chan_sip or anonymous in chan_pjsip an SDP offer or answer is still processed and the crash occurs.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability described in CVE-2016-9937 represents a critical buffer overflow condition affecting Asterisk Open Source versions 13.12.x through 13.13.0 and 14.x through 14.2.0. This issue manifests specifically when processing Session Description Protocol (SDP) offers or answers containing the Opus audio codec with format parameters separated by spaces rather than the standard semicolon delimiter. The flaw resides in the SDP parsing logic that fails to properly validate parameter separation mechanisms, creating a recursive function call pattern that eventually exhausts system resources and causes application termination. The vulnerability operates at the protocol level within the media session handling components of Asterisk's SIP channel drivers, particularly affecting chan_sip and chan_pjsip modules that process incoming SDP negotiations.
The technical execution of this vulnerability leverages a classic stack overflow pattern through recursive function calls, where the parsing routine encounters malformed SDP parameter formatting and enters an infinite recursion loop. This recursive descent continues until the system stack becomes exhausted, resulting in an application crash that terminates the Asterisk service. The flaw is particularly dangerous because it operates at the network protocol parsing layer and does not require any authentication credentials or specific endpoint configuration to exploit. Attackers can trigger this condition simply by sending a malformed SDP message containing Opus codec parameters with space separators, making it highly accessible to remote unauthenticated attackers.
The operational impact of CVE-2016-9937 extends beyond simple service disruption to encompass potential denial of service across entire telephony infrastructure deployments. Organizations relying on Asterisk for voice communication services face significant risk of operational outages when this vulnerability is exploited, particularly in environments where guest access or anonymous SIP registration is enabled. The vulnerability affects both chan_sip and chan_pjsip channel drivers, meaning that deployments using either SIP implementation are equally at risk. The crash occurs during normal SDP processing, making it difficult to distinguish between legitimate network traffic and malicious exploitation attempts, which complicates intrusion detection and response efforts.
Mitigation strategies for this vulnerability require immediate patch application to upgrade Asterisk installations to versions 13.13.1 or 14.2.1 where the SDP parsing logic has been corrected to properly handle space-separated parameters. Organizations should also implement network-level filtering to detect and block malformed SDP content, particularly when Opus codec parameters are present in SDP offers or answers. Additionally, security teams should review and disable guest access or anonymous registration in SIP channel configurations where possible, reducing the attack surface for unauthenticated exploitation attempts. The vulnerability aligns with CWE-674, which describes the issue as an uncontrolled recursion condition, and maps to ATT&CK technique T1499.004 for network denial of service attacks, emphasizing the need for both preventive and detective security controls to address this critical weakness in telephony infrastructure.