CVE-2026-50259 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. _XkbSetMapChecks() declares a fixed-size stack buffer mapWidths[256] indexed by key type index. The helper function CheckKeyTypes() writes to this buffer at a client-controlled offset, allowing a stack buffer overflow. 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 components that stems from improper bounds checking in the keyboard mapping functionality. The flaw exists within the _XkbSetMapChecks() function where a fixed-size stack buffer named mapWidths[256] is declared to store keyboard mapping data. This buffer is indexed using a key type index that can be controlled by client applications, creating a scenario where malicious input can cause writes beyond the allocated buffer boundaries. The vulnerability is particularly dangerous because it occurs in the X server's core keyboard handling code, which is responsible for managing keyboard input across the graphical interface. When CheckKeyTypes() function writes data to this buffer at an offset controlled by client input, it can overwrite adjacent stack memory, potentially corrupting program execution flow and leading to arbitrary code execution.
The operational impact of this vulnerability extends beyond simple crash conditions to include potential privilege escalation scenarios. Since the X server typically runs with elevated privileges, particularly when operating as root, successful exploitation could allow unprivileged users to gain root access to the system. This makes the vulnerability particularly attractive to attackers seeking to establish persistent access or escalate their privileges within the target environment. The vulnerability affects systems running X.Org X server versions that include the problematic code path, making it relevant to a wide range of Linux distributions and desktop environments that rely on X11 for graphical display management. The stack overflow occurs during keyboard mapping operations, which are fundamental to user interaction with graphical interfaces, making exploitation potentially straightforward for attackers who can establish a connection to the X server.
From a cybersecurity perspective, this vulnerability maps directly to CWE-121 Stack-based Buffer Overflow, which is classified as a fundamental memory safety issue in software development practices. The ATT&CK framework categorizes this as a privilege escalation technique through exploitation of software vulnerabilities, specifically targeting the execution of malicious code with elevated privileges. The vulnerability also aligns with ATT&CK technique T1068, which covers the use of privilege escalation methods through local exploitation of system vulnerabilities. Mitigation strategies should include immediate patching of the X.Org X server components to address the buffer overflow condition, implementing proper bounds checking in the keyboard mapping functions, and considering privilege separation techniques to reduce the impact if exploitation occurs. System administrators should also consider restricting X server access through network firewalls or using alternative display protocols that don't expose the same attack surface. Additionally, monitoring for unusual keyboard mapping requests and implementing application-level sandboxing for X server components can help detect and prevent exploitation attempts. The vulnerability underscores the importance of rigorous input validation and memory safety practices in system-level software components that handle untrusted input from network clients.