# CVE-2026-59679: libXfont2 out-of-bounds heap access in fs_read_glyphs()

Source: https://junglewise.ai/threats/cve-2026-59679-libxfont2-out-of-bounds-heap-access-in-fs-read-glyphs  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-10. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-59679 · Severity: critical · CVSS 9 · Published 2026-09-10

libXfont2 is a font rendering library used by X11 display servers and desktop environments to load and render fonts from font servers. A malicious or compromised font server can exploit a validation flaw to trigger out-of-bounds memory reads and writes, potentially allowing remote code execution or denial of service on any system using the vulnerable library.

Technologies: [X.Org libXfont2](https://junglewise.ai/threats/technologies/libxfont2). Vendors: [X.Org](https://junglewise.ai/threats/vendors/x-org).

## Quick answers

**Is CVE-2026-59679 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59679 being exploited in the wild.

**How severe is CVE-2026-59679?** Critical severity, with a CVSS score of 9, published 10 September 2026.

**Which products and versions are affected?** X.Org libXfont2.

**Is there a fix for CVE-2026-59679?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-59679 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-59679?** Check whether you run X.Org libXfont2. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability exists in fs_read_glyphs() in src/fc/fserve.c, where the function indexes a per-character encoding[] array using num_chars from an FS_QueryXBitmaps16 reply. However, the array was allocated with a size derived from num_extents in a separate FS_QueryXExtents16 reply. These two CARD32 fields are never cross-checked, allowing a mismatch. An attacker controlling a font server can send a small num_extents (e.g., 1) in the extents reply followed by a large num_chars (e.g., 100,000) in the bitmaps reply, causing out-of-bounds heap read and write operations. No authentication is required, as the attack occurs at the font server protocol level. Exploitation leads to memory corruption and potential code execution.

## Affected products

- X.Org libXfont2

## Timeline

- 2026-09-10: disclosed

## References

- [https://bugzilla.suse.com/show_bug.cgi?id=CVE-2026-59679](https://bugzilla.suse.com/show_bug.cgi?id=CVE-2026-59679)

## Related threats

- [X.Org libXfont2 heap buffer overflow in fs_read_glyphs](https://junglewise.ai/threats/cve-2026-44950-x-org-libxfont2-heap-buffer-overflow-in-fs-read-glyphs)
- [X.Org libXfont2 heap buffer overflow in ComputeScaledProperties](https://junglewise.ai/threats/cve-2026-56003-x-org-libxfont2-heap-buffer-overflow-in-computescaledproperties)
- [X.Org libXfont2 heap buffer overflow in pcfReadFont](https://junglewise.ai/threats/cve-2026-56002-x-org-libxfont2-heap-buffer-overflow-in-pcfreadfont)
- [X.Org libXfont2 heap buffer overflow in BitmapScaleBitmaps](https://junglewise.ai/threats/cve-2026-56001-x-org-libxfont2-heap-buffer-overflow-in-bitmapscalebitmaps)