CVE-2026-59679

fs_read_glyphs() heap OOB read/write via encoding array index mismatch in libXfont2

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Published Sep 10, 2026 CVSS 4.0 · 9.2 Critical EPSS < 1% KEV · Not listed
Description
fs_read_glyphs() in the libXfont2 font-server client (src/fc/fserve.c) indexes the per-character encoding[] array using num_chars from the FS_QueryXBitmaps16 reply, but that array was allocated with a size derived from num_extents in the separate FS_QueryXExtents16 reply. The two CARD32 fields are never cross-checked.
A malicious or compromised font server can send a small num_extents (e.g. 1) in the extents reply, then a large num_chars (e.g. 100000) in the bitmaps reply. This causes attacker-controlled out-of-bounds heap read and writes.

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
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. A remote attacker, by operating a malicious font server, could exploit an out-of-bounds read/write vulnerability. This occurs because the client incorrectly handles font data, leading to an out-of-bounds memory access. This can lead to privilege escalation if the X server runs with root privileges, or a denial of service (crash) if it runs as an unprivileged user. fs_read_glyphs() in the libXfont2 font-server client (src/fc/fserve.c) indexes the per-character encoding[] array using num_chars from the FS_QueryXBitmaps16 reply, but that array was allocated with a size derived from num_extents in the separate FS_QueryXExtents16 reply. The two CARD32 fields are never cross-checked. A malicious or compromised font server can send a small num_extents (e.g. 1) in the extents reply, then a large num_chars (e.g. 100000) in the bitmaps reply. This causes attacker-controlled out-of-bounds heap read and writes.
Title libxfont2: Font Server Client encoding[] Out-Of-Bounds Read/Write fs_read_glyphs() heap OOB read/write via encoding array index mismatch 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': 9.2, 'vector': 'CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N'}

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 Libxfont2
Libxfont2 libxfont2
Vendors & Products Libxfont2
Libxfont2 libxfont2
Type Values Removed Values Added
Weaknesses CWE-119
CWE-787
Type Values Removed Values Added
Description A flaw was found in the libXfont2 font-server client. A remote attacker, by operating a malicious font server, could exploit an out-of-bounds read/write vulnerability. This occurs because the client incorrectly handles font data, leading to an out-of-bounds memory access. This can lead to privilege escalation if the X server runs with root privileges, or a denial of service (crash) if it runs as an unprivileged user.
Title libxfont2: Font Server Client encoding[] Out-Of-Bounds Read/Write
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, 12:45 PM
Reserved
Jul 6, 2026, 11:59 AM

View source JSON

Updated
Sep 10, 2026, 12:45 PM

JSON

Status
Awaiting Analysis
Published
Sep 10, 2026, 9:17 AM
Modified
Sep 10, 2026, 3:43 PM

View source JSON

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

View source Bugzilla JSON

Updated
Aug 7, 2026, 9:58 AM

JSON