CVE-2026-50256: Xorg-x11-server: xorg-x11-server-xwayland: xorg-x11-server: stack buffer overflow in font alias resolution due to libxfont2 name length mismatch
A mismatch between the X server and the libXfont2 library's maximum font name length can cause a stack buffer overflow during font alias resolution. The server allocates a 256 byte stack buffer but libXfont2's alias target name length is 1024 bytes. A font alias name between 257 and 1023 bytes causes the X server to copy that name into the undersized stack buffer without further checks.
Any X client that can connect to the server can trigger this issue. This may be used to crash the server, or for privilege escalation if the X server runs as root.
Components affected: xorg-x11-server, xorg-x11-server-Xwayland Versions affected: xorg-x11-server <= 21.1.22, xorg-x11-server-Xwayland <= 24.1.9
Fixed upstream in xorg-server-21.1.23 and xwayland-24.1.12. Fix: https://gitlab.freedesktop.org/xorg/xserver/-/commit/bb5158f962dc935e58ef8b4b5fcb31be201a6e07
Reported via ZDI-CAN-30136 (Trend Micro Zero Day Initiative). Tracking: PSIRTSUPT-16950.
Other sources
A stack-based buffer overflow flaw was found in the X.Org X server and Xwayland. A mismatch between the X server and the libXfont2 library's maximum font name length can cause a stack buffer overflow during font alias resolution. The server allocates a 256 byte stack buffer but libXfont2's alias target name length is 1024 bytes. A font alias name between 257 and 1023 bytes causes the X server to copy that name into the undersized stack buffer without further checks. This may be used to crash the server, or for privilege escalation if the X server runs as root.
— NVD
Xorg-x11-server: xorg-x11-server-xwayland: xorg-x11-server: stack buffer overflow in font alias resolution due to libxfont2 name length mismatch
— Microsoft
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 24.1.12-1 - Upgrade
Upgrade
xorg-x11-serverto a version that resolves this vulnerability.Fixed in 21.1.23 - Upgrade
Upgrade
xorg-x11-server-Xwaylandto a version that resolves this vulnerability.Fixed in 24.1.12 - Compensating control
Restrict network access to the X server so that only trusted X clients can connect, since any X client that can connect to the server can trigger the issue.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-50256?
The severity of CVE-2026-50256 is categorized as high with a score of 7.8.
How do I fix CVE-2026-50256?
To fix CVE-2026-50256, upgrade to the latest version of the X.Org X server and Xwayland that has addressed the buffer overflow issue.
What type of vulnerability is CVE-2026-50256?
CVE-2026-50256 is a stack-based buffer overflow vulnerability.
Which software is affected by CVE-2026-50256?
CVE-2026-50256 affects the X.Org X server and Xwayland.
What causes the buffer overflow in CVE-2026-50256?
The buffer overflow in CVE-2026-50256 is caused by a mismatch in the maximum font name length between the X server and the libXfont2 library.