CVE-2026-50258 in X11 Server
Summary
by MITRE • 06/05/2026
A stack-based buffer overflow flaw was found in the X.Org X server and Xwayland. The X server has multiple stack buffers sized XkbMaxShiftLevel * XkbNumKbdGroups but CheckKeyTypes() does not verify or clamp non-canonical key types to XkbMaxShiftLevel. A client can change key types to excessive shift levels and trigger stack overflows. This is caused by an incomplete fix of CVE-2025-26597. This may be used to crash the server, or for privilege escalation if the X server runs as root.
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Analysis
by VulDB Data Team • 06/06/2026
This vulnerability represents a critical stack-based buffer overflow in the X.Org X server and Xwayland implementations that stems from inadequate input validation within the keyboard handling subsystem. The flaw manifests when the CheckKeyTypes() function processes key type configurations without proper bounds checking against the XkbMaxShiftLevel constant, which defines the maximum allowable shift levels for keyboard layouts. The vulnerability specifically affects the X server's handling of keyboard groups and shift levels through the X Keyboard Extension protocol, where multiple stack buffers are allocated based on the formula XkbMaxShiftLevel * XkbNumKbdGroups. When a malicious client sends malformed keyboard type data with excessive shift level values, the system fails to clamp these values to the defined maximum, leading to buffer overflows in the stack-allocated memory regions.
The technical execution of this vulnerability leverages the incomplete remediation of a previous CVE-2025-26597, indicating that the initial fix was insufficient to address all attack vectors within the keyboard processing pipeline. The flaw operates through the X server's keyboard extension mechanism where client applications can submit keyboard configuration data that specifies key types with shift levels exceeding the legitimate maximum. This creates a scenario where the stack buffer allocation calculation becomes invalid, as the system allocates memory based on potentially excessive values while the validation logic fails to constrain these inputs properly. The vulnerability is classified under CWE-121 Stack-based Buffer Overflow, which is a well-documented weakness in software systems where data written to a buffer can overwrite adjacent stack memory locations.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to potentially enable privilege escalation when the X server operates with elevated privileges. When the X server runs as root or with administrative privileges, an attacker can exploit this buffer overflow to execute arbitrary code within the server's memory space, potentially gaining root access to the underlying system. The crash potential manifests through stack corruption that can cause the X server process to terminate unexpectedly, disrupting graphical sessions and potentially affecting system stability. This vulnerability is particularly concerning in multi-user environments where untrusted clients might have access to X server connections, as it provides a direct path for privilege escalation attacks that could compromise entire systems.
Mitigation strategies for this vulnerability should focus on implementing comprehensive input validation and bounds checking within the X server's keyboard processing code. System administrators should immediately apply patches from the X.Org project that properly clamp keyboard type values to the defined maximum limits, ensuring that CheckKeyTypes() function validates all incoming key type configurations against the XkbMaxShiftLevel constant. Additionally, organizations should consider implementing network-level restrictions to limit access to X server connections, particularly when running with elevated privileges. The vulnerability aligns with ATT&CK technique T1068, which covers 'Exploitation for Privilege Escalation', and T1499, which covers 'Endpoint Denial of Service', making it a significant concern for both operational security and attack surface reduction. Organizations should also implement monitoring for unusual keyboard configuration changes and consider running X servers in restricted environments or using alternative display servers with more robust input validation mechanisms.