CVE-2026-93542 in LibXi
Summary
by MITRE • 09/24/2026
An out-of-bounds read in libXi's XI2 class parsing via size_classes() and copy_classes() in libXi before 1.8.4 could be used by malicous servers to crash the X client.
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Analysis
by VulDB Data Team • 09/24/2026
The vulnerability identified within the libXi library, specifically affecting versions prior to 1.8.4, represents a critical security flaw rooted in improper input validation during the parsing of XI2 class structures. LibXi serves as the primary interface for applications interacting with the X Window System extension known as Xi, which provides advanced input device handling capabilities such as multi-touch and pressure sensitivity support. The core technical deficiency lies within two specific functions: size_classes() and copy_classes(). These routines are responsible for calculating the memory footprint required to store class data and subsequently copying that data into allocated buffers respectively. When processing responses from an X server, these functions fail to adequately verify that the length fields provided by the server do not exceed the bounds of the expected or allocated structures. This lack of rigorous boundary checking allows a malicious actor operating as a compromised or rogue X server to craft specially crafted input sequences containing oversized class descriptors.
Upon receiving such malformed data, the size_classes() function may calculate an incorrect buffer size based on untrusted external values, while copy_classes() proceeds to read memory locations that lie outside the intended allocation boundaries. This out-of-bounds read operation results in the application accessing arbitrary memory contents that were not meant for consumption by the client process. The immediate operational impact of this flaw is typically a segmentation fault or an abrupt termination of the X client application, leading to a denial of service condition for the end user. While primarily manifesting as a crash, out-of-bounds reads can occasionally leak sensitive information from adjacent memory regions if the attacker has control over subsequent read operations, although in this specific context, the primary vector described is stability disruption through process termination.
From a threat modeling perspective, this vulnerability aligns with CWE-125, which defines Out-of-Bounds Read vulnerabilities where software reads data past the end or before the beginning of the intended buffer. The attack scenario corresponds to ATT&CK technique T1499, Endpoint Denial of Service, as it involves exploiting a flaw in local application logic to disrupt service availability. Furthermore, because libXi is integral to graphical user interface interactions on Linux and Unix-like systems, this vulnerability affects any desktop environment or window manager that relies on Xi for input processing. The risk is particularly acute in environments where users connect to untrusted X servers via network forwarding protocols such as SSH with X11 forwarding, allowing remote attackers to trigger the crash without physical access to the machine.
Mitigation strategies must prioritize immediate software updates to version 1.8.4 or later of libXi, which contains patches implementing strict validation checks for class sizes and bounds before memory operations are executed. System administrators should ensure that package managers are configured to automatically apply security updates for X Window system components. For environments where updating is not immediately feasible, network-level controls can be employed to restrict access to the X server socket from untrusted hosts or disable X11 forwarding in SSH configurations if it is not strictly required. Additionally, deploying runtime application self-protection tools that monitor memory access patterns may help detect and block exploitation attempts of this nature by identifying illegal read operations before they cause a crash.