Security Bulletin: Vulnerabilities in xorg-x11-server library (CVE-2026-50256, CVE-2026-50257, CVE-2026-50258, CVE-2026-50264, CVE-2026-59083, CVE-2026-55999) affect Power HMC.

Vulnerability Details

CVEID:   CVE-2026-55999
DESCRIPTION:   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.
CWE:   CWE-122: Heap-based Buffer Overflow
CVSS Source:   NVD
CVSS Base score:   7.8
CVSS Vector:   (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

CVEID:   CVE-2026-50256
DESCRIPTION:   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.
CWE:   CWE-121: Stack-based Buffer Overflow
CVSS Source:   secalert@redhat.com
CVSS Base score:   7.8
CVSS Vector:   (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

CVEID:   CVE-2026-50257
DESCRIPTION:   A use-after-free flaw was found in the X.Org X server and Xwayland in miSyncDestroyFence(). A client that sets up multiple fence triggers can trigger a use-after-free function pointer call. An attacker would connect to the X server to set up a fence and await that fence, then a second X connection destroys the fence, causing the use-after-free. This may be used to crash the server, or for privilege escalation if the X server runs as root.
CWE:   CWE-416: Use After Free
CVSS Source:   secalert@redhat.com
CVSS Base score:   7.8
CVSS Vector:   (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

CVEID:   CVE-2026-50258
DESCRIPTION:   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.
CWE:   CWE-121: Stack-based Buffer Overflow
CVSS Source:   secalert@redhat.com
CVSS Base score:   7.8
CVSS Vector:   (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

CVEID:   CVE-2026-50264
DESCRIPTION:   An out-of-bounds write flaw was found in the X.Org X server and Xwayland in DRIGetBuffers/DRIGetBuffersWithFormat. A client that requests multiple DRI2BufferBackLeft attachments and one DRI2BufferFrontLeft can trigger an out-of-bounds heap write. This may be used to crash the server, or for privilege escalation if the X server runs as root.
CWE:   CWE-787: Out-of-bounds Write
CVSS Source:   secalert@redhat.com
CVSS Base score:   7.8
CVSS Vector:   (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)