CVE-2026-59679: libxfont2 Out-of-bounds Write

Vulnerability summary

CVE-2026-59679 is a critical (CVSS 9.2) Out-of-bounds Write vulnerability in libxfont2. EPSS estimates a 0.26% chance of exploitation in the next 30 days, higher than 16.2% of scored vulnerabilities.

Timeline

1. 10-09-2026
   PublishedVulnerability first disclosed.
2. 10-09-2026
   Last modifiedVulnerability information updated.

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.

• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.