CVE-2026-94286 in libXtst
Summary
by MITRE • 09/28/2026
An out-of-bounds read in libXtst's RECORD reply parser in libXtst before 1.2.6 could be used by malicious X servers to crash attached X clients.
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Analysis
by VulDB Data Team • 09/28/2026
The vulnerability identified as an out-of-bounds read within the libXtst library, specifically affecting versions prior to 1.2.6, represents a critical security flaw in how client applications process extended input events from the X Window System server. The RECORD extension is designed to allow clients to record and replay user input events, facilitating accessibility features and automated testing tools. However, the parser responsible for handling replies from this extension failed to adequately validate the length of incoming data structures before accessing them. This lack of boundary checking allows a malicious or compromised X server to craft specially crafted reply packets that contain malformed lengths or offsets pointing beyond the allocated memory buffer associated with the event record structure.
From a technical perspective, this flaw is classified under CWE-125, which denotes an out-of-bounds read vulnerability. When the parser encounters these maliciously constructed replies, it attempts to access memory locations outside the bounds of the intended data array. In many execution environments, particularly those utilizing standard C libraries without advanced memory protection mechanisms enabled by default, this illegal memory access triggers a segmentation fault or similar hardware exception. Consequently, the affected X client process terminates abruptly, resulting in a denial of service condition for the end user. While modern operating systems with Address Space Layout Randomization and stack canaries may mitigate some aspects of exploitation, the immediate operational impact remains the disruption of application availability.
The attack vector requires an active connection to a potentially untrusted or compromised X server, which aligns with ATT&CK technique T1498, Network Denial of Service. This scenario is particularly relevant in multi-user environments such as Linux desktop sessions where users may connect to shared displays or virtual machines managed by less trusted entities. An attacker controlling the X server can exploit this weakness to crash specific client applications that utilize libXtst for input recording, thereby degrading system stability and potentially causing data loss if unsaved work is present in those clients. The vulnerability highlights the risks associated with trusting network-facing components of graphical subsystems when they interact with external or virtualized display servers.
Mitigation strategies primarily involve upgrading to version 1.2.6 or later of libXtst, where the developers have implemented proper bounds checking within the RECORD reply parser. System administrators should ensure that all client applications linked against this library are updated through their respective package managers. Additionally, organizations relying on critical desktop infrastructure should monitor for vendor advisories and apply security patches promptly to prevent exploitation in production environments. Implementing strict network segmentation between untrusted X servers and sensitive client workstations can also reduce the attack surface by limiting exposure to potential malicious actors capable of injecting malformed packets into the display protocol stream.