CVE-2016-7952 in XRecord
Summary
by MITRE
X.org libXtst before 1.2.3 allows remote X servers to cause a denial of service (infinite loop) via a reply in the (1) XRecordStartOfData, (2) XRecordEndOfData, or (3) XRecordClientDied category without a client sequence and with attached data.
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Analysis
by VulDB Data Team • 09/22/2022
The vulnerability identified as CVE-2016-7952 affects X.org libXtst version 1.2.2 and earlier, representing a significant denial of service weakness in the X Window System's X Test extension. This flaw exists within the X11 protocol implementation that handles recording and replaying of X protocol requests, specifically impacting how the system processes certain reply messages in the XRecordStartOfData, XRecordEndOfData, and XRecordClientDied categories. The vulnerability stems from insufficient validation of incoming X protocol messages, particularly those that lack proper client sequence numbers while containing attached data payloads.
The technical flaw manifests when remote X servers send specially crafted replies that conform to the XRecord protocol specifications but omit essential client sequence identifiers. These malformed messages trigger an infinite loop condition within the libXtst library's message processing routines, causing the affected X server to consume excessive CPU resources and become unresponsive to legitimate client requests. The vulnerability is particularly dangerous because it operates at the protocol level, allowing attackers to exploit it without requiring authentication or elevated privileges, making it accessible to anyone who can establish a connection to the vulnerable X server.
The operational impact of this vulnerability extends beyond simple service disruption, as it can effectively render X server instances unusable for legitimate users and applications. Systems running vulnerable versions of libXtst become vulnerable to remote exploitation, potentially affecting desktop environments, remote desktop services, and any applications that rely on X11 forwarding or recording capabilities. The infinite loop condition can persist until the system is manually restarted or the affected process is terminated, creating extended downtime that can disrupt critical business operations, especially in environments where X11 services are extensively used for graphical applications or remote access.
This vulnerability maps to CWE-835, which specifically addresses the issue of infinite loops in software implementations, and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks. The flaw demonstrates how protocol-level vulnerabilities in widely-used system libraries can create cascading effects across entire desktop environments and network infrastructures. Organizations should prioritize patching their X.org libXtst installations to version 1.2.3 or later, which includes proper validation of sequence numbers in XRecord replies. Additional mitigations include implementing network segmentation to restrict access to X servers, disabling unnecessary X11 forwarding capabilities, and monitoring for unusual CPU usage patterns that might indicate exploitation attempts. The vulnerability serves as a reminder of the critical importance of proper input validation in protocol implementations and the potential for seemingly minor flaws to create significant operational disruptions in graphical computing environments.