X.org X11 Server CheckKeyTypes stack-based overflow

Summaryinfo

A vulnerability was found in X.org X11 Server. It has been rated as critical. This affects the function CheckKeyTypes. This manipulation causes stack-based overflow. This vulnerability appears as CVE-2026-50258. The attack requires local access. There is no available exploit.

Detailsinfo

A vulnerability classified as critical was found in X.org X11 Server (version unknown). Affected by this vulnerability is the function CheckKeyTypes. The manipulation with an unknown input leads to a stack-based overflow vulnerability. The CWE definition for the vulnerability is CWE-121. A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function). As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at access.redhat.com. This vulnerability is known as CVE-2026-50258 since 06/04/2026. The exploitation appears to be easy. An attack has to be approached locally. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 06/06/2026).

The vulnerability scanner Nessus provides a plugin with the ID 318748 (Linux Distros Unpatched Vulnerability : CVE-2026-50258), which helps to determine the existence of the flaw in a target environment.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the databases at Tenable (318748) and CERT Bund (WID-SEC-2026-1774). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Affected

• Open Source X.Org X11
• Open Source Xwayland

Productinfo

Type

• Windowing System Software

Vendor

• X.org

Name

• X11 Server

License

• open-source

Website

• Vendor: https://www.x.org/

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CPE 2.2info

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Entryinfo

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

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