CVE-2026-44950

fs_read_glyphs() heap buffer overflow via cumulative glyph data overflow in libXfont2

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Published Sep 10, 2026 CVSS 4.0 · 9.5 Critical EPSS < 1% KEV · Not listed
Description
fs_read_glyphs() in the libXfont2 font-server client (src/fc/fserve.c) copies each glyph's bitmap into a single buffer. Existing checks validates only that the source slice (position, length) lies within the source bitmap buffer. It does not check whether the running destination cursor has exceeded the allocation.

A malicious font server can send overlapping source offsets -- for example 1000 glyphs each referencing {position:0, length:64} with nbytes=64. Each individual source range passes the existing validation, but the cumulative writes total 64000 bytes into a 64-byte destination buffer. This is a heap buffer overflow with attacker-controlled content.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

No advisories available
History
Type Values Removed Values Added
Weaknesses CWE-122
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}
Type Values Removed Values Added
Description A flaw was found in the libXfont2 font-server client. This heap buffer overflow vulnerability allows a malicious font server to send specially crafted glyph data. The fs_read_glyphs() function fails to properly validate the total size of the incoming data, leading to an overwrite of memory beyond the intended buffer. If the X server runs as a privileged user, this could result in privilege escalation, allowing an attacker to gain higher access. If the X server runs as an unprivileged user, it could lead to a denial of service, causing the system to crash. fs_read_glyphs() in the libXfont2 font-server client (src/fc/fserve.c) copies each glyph's bitmap into a single buffer. Existing checks validates only that the source slice (position, length) lies within the source bitmap buffer. It does not check whether the running destination cursor has exceeded the allocation. A malicious font server can send overlapping source offsets -- for example 1000 glyphs each referencing {position:0, length:64} with nbytes=64. Each individual source range passes the existing validation, but the cumulative writes total 64000 bytes into a 64-byte destination buffer. This is a heap buffer overflow with attacker-controlled content.
Title libxfonts2: libXfont2: Privilege Escalation via Heap Buffer Overflow in Font Server Client fs_read_glyphs() heap buffer overflow via cumulative glyph data overflow in libXfont2
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H'}

 cvssV4_0

{'score': 9.5, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H'}

cvssV3_1

{'score': 9, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H'}
Type Values Removed Values Added
First Time appeared Libxfonts2
Libxfonts2 libxfont2
Vendors & Products Libxfonts2
Libxfonts2 libxfont2
Type Values Removed Values Added
Description A flaw was found in the libXfont2 font-server client. This heap buffer overflow vulnerability allows a malicious font server to send specially crafted glyph data. The fs_read_glyphs() function fails to properly validate the total size of the incoming data, leading to an overwrite of memory beyond the intended buffer. If the X server runs as a privileged user, this could result in privilege escalation, allowing an attacker to gain higher access. If the X server runs as an unprivileged user, it could lead to a denial of service, causing the system to crash.
Title libxfonts2: libXfont2: Privilege Escalation via Heap Buffer Overflow in Font Server Client
Weaknesses CWE-120
References
Metrics threat_severity

None

 cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H'}

threat_severity

Important
Weaknesses

Sources

Status
PUBLISHED
Assigner
suse
Published
Updated
Sep 10, 2026, 6:26 PM
Reserved
May 8, 2026, 12:29 PM

View source JSON

Updated
Sep 10, 2026, 5:20 PM

JSON

Status
Awaiting Analysis
Published
Sep 10, 2026, 9:17 AM
Modified
Sep 10, 2026, 7:17 PM

View source JSON

Severity
Important
Public date
Aug 5, 2026, 10:00 AM

View source Bugzilla JSON

Updated
Sep 10, 2026, 11:30 PM

JSON