# CVE-2026-59679: fs_read_glyphs() heap OOB read/write via encoding array index mismatch in libXfont2

> A malicious font server can trigger heap out-of-bounds read and write in libXfont2, potentially leading to code execution or denial of service.

- **Severity:** Critical 9.2 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Sep 2026
- **Updated:** 10 Sep 2026
- **Also known as:** AZL-101730, DEBIAN-CVE-2026-59679, GHSA-g44m-wpf7-gr7x, UBUNTU-CVE-2026-59679

## What is CVE-2026-59679?

CVE-2026-59679 is a critical memory corruption vulnerability in Container suse/kiosk/tigervnc-x11vnc:1.14-63.8, published on 10 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-59679?


## How do I fix CVE-2026-59679?

Apply the vendor's patch.

## Is CVE-2026-59679 being exploited?

There is no known exploitation of CVE-2026-59679 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59679 and fix it if it is affected.

What it is: CVE-2026-59679 is a critical memory corruption vulnerability in Container suse/kiosk/tigervnc-x11vnc:1.14-63.8, published on 10 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-59679 in the wild and no public exploit code.

Affected product: Container suse/kiosk/tigervnc-x11vnc:1.14-63.8. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-59679/ (Markdown: https://insights.sbomify.com/CVE-2026-59679/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59679
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59679
- OSV: https://osv.dev/vulnerability/CVE-2026-59679

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59679/
Summaries are generated and may contain mistakes; the official description above is the record.