CVE-2026-55999 in xorg-server
Summary
by MITRE • 07/08/2026
Local attackers with a X connection able to provide PCX fonts to the X server xorg-server before 21.2.24 and xwayland before 24.1.13 could cause a heap buffer overflow via SetFont due to missing glyph boundary checks.
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Analysis
by VulDB Data Team • 07/08/2026
This vulnerability represents a critical heap buffer overflow condition affecting the xorg-server and xwayland components in X Window System implementations. The flaw occurs during font rendering operations when the server processes PCX format fonts through the SetFont function, creating an exploitable memory corruption scenario that can be triggered by local attackers with X connection privileges. The vulnerability stems from inadequate boundary validation within the glyph processing logic, specifically when handling font metrics and character data structures. This issue affects versions prior to xorg-server 21.2.24 and xwayland 24.1.13, indicating a widespread impact across multiple system configurations where X Window System services are deployed.
The technical implementation of this vulnerability involves the manipulation of font data structures during the SetFont operation, where the server fails to validate glyph boundaries before performing memory allocations. When processing PCX fonts, the system calculates required buffer sizes based on font metrics without proper input validation, leading to excessive memory allocation that exceeds allocated buffer limits. This creates a heap-based buffer overflow condition that can be exploited to overwrite adjacent memory regions, potentially enabling arbitrary code execution or denial of service scenarios. The vulnerability is classified as a heap overflow under CWE-122 and represents a classic improper input validation issue that violates secure coding practices for memory management.
Operational impact of this vulnerability extends beyond simple privilege escalation as local attackers with X connection access can leverage this flaw to compromise system integrity and availability. The attack vector requires only local access with existing X session privileges, making it particularly concerning for multi-user environments where users may have legitimate X connections but could exploit this weakness for malicious purposes. Successful exploitation could result in complete system compromise, allowing attackers to execute arbitrary code with the privileges of the X server process, potentially leading to privilege escalation to root level access depending on system configuration. The vulnerability also creates denial of service conditions that can crash the X server or xwayland processes, disrupting graphical user interfaces and affecting all running applications dependent on these services.
Mitigation strategies for this vulnerability require immediate patching of affected systems with updated versions of xorg-server and xwayland components, specifically upgrading to version 21.2.24 or later for xorg-server and 24.1.13 or later for xwayland. System administrators should also implement additional protective measures including restricting font loading capabilities for untrusted users, implementing proper input validation at the X server level, and monitoring for unusual font processing activities that might indicate exploitation attempts. Network segmentation and privilege separation can help reduce attack surface, while regular security audits should verify proper implementation of memory safety controls in graphical subsystems. Organizations should also consider implementing runtime protections such as address space layout randomization and stack canaries to make exploitation more difficult even if patches are not immediately available. The vulnerability aligns with ATT&CK technique T1068 which focuses on exploiting local privileges for system compromise, and represents a common vector for privilege escalation attacks in graphical environments where user session management is not properly secured against malicious font data processing.