CVE-2026-56002 in libXfont2
Summary
by MITRE • 07/08/2026
A heap bufferflow in pcfReadFont() due to missing glyph bounds checking in libXfont2 before 2.0.8 allows attackers authenticated as X client to execute code within the X server.
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability described represents a critical heap buffer overflow condition within the libXfont2 library affecting versions prior to 20.8. This flaw exists in the pcfReadFont() function where insufficient validation of glyph bounds occurs during font processing operations. The vulnerability specifically impacts X Window System implementations where client applications can interact with the X server through authenticated connections.
The technical exploitation of this vulnerability stems from the absence of proper input validation when reading PCF (Portable Compiled Format) font files. When an authenticated X client submits malformed font data to the X server, the pcfReadFont() function fails to properly bounds-check glyph dimensions and memory allocations. This omission creates a scenario where attacker-controlled data can overwrite adjacent heap memory regions, potentially leading to arbitrary code execution within the context of the X server process. The vulnerability is particularly dangerous because it requires only authenticated access from an X client, which is commonly granted in typical desktop environments.
The operational impact of this vulnerability extends beyond simple privilege escalation as it provides attackers with direct control over the X server's execution environment. This presents significant risks for graphical user interfaces and remote desktop implementations that rely on the X Window System. Attackers can leverage this flaw to execute malicious code with the privileges of the X server process, potentially compromising entire graphical sessions or enabling further attacks against underlying systems. The vulnerability affects all versions of libXfont2 prior to 2.0.8, making it a widespread concern for Linux distributions and Unix-like systems that utilize X11.
This vulnerability maps directly to CWE-121 Heap-based Buffer Overflow and CWE-787 Out-of-bounds Write within the Common Weakness Enumeration framework. From an adversarial perspective, this flaw aligns with ATT&CK technique T1059.007 Command and Scripting Interpreter: Unix Shell and T1068 Exploitation for Privilege Escalation as it enables code execution in privileged contexts. The attack surface is particularly concerning given that X server processes often run with elevated privileges and may have access to sensitive system resources.
Mitigation strategies should include immediate patching of libXfont2 to version 2.0.8 or later, which implements proper bounds checking for glyph data processing. System administrators should also consider implementing additional security controls such as X server access controls, network segmentation, and monitoring for anomalous font loading activities. The patch addresses the root cause by introducing comprehensive validation of glyph boundaries before memory allocation occurs, preventing the overflow condition that previously allowed attacker-controlled data to overwrite heap memory regions. Additional defensive measures include restricting X client privileges where possible and implementing proper input sanitization at the application level when processing font files.