# CVE-2026-55999: xorg-server / xwayland glamor font atlas Heap Buffer Overflow

> Local attackers can trigger a heap buffer overflow in the X server via malicious PCX fonts, potentially leading to code execution or privilege escalation.

- **Severity:** High 8.5 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Jul 2026
- **Updated:** 9 Jul 2026
- **Also known as:** ALSA-2026:38486, ALSA-2026:38487, ALSA-2026:38488, AZL-92196, BELL-CVE-2026-55999, CLSA-2026-1784295645, DEBIAN-CVE-2026-55999, ECHO-d009-b514-1211, GHSA-h655-847g-q4f7, OESA-2026-3026, RHSA-2026:38486, RHSA-2026:38487, RHSA-2026:38488, RHSA-2026:49515, RHSA-2026:49516, RHSA-2026:49605, RHSA-2026:49606, RHSA-2026:49608, RHSA-2026:50116, RHSA-2026:50117, RHSA-2026:52392, RHSA-2026:52397, RHSA-2026:53450, RLSA-2026:38486, RLSA-2026:38487, RLSA-2026:38488, ROOT-OS-DEBIAN-11-CVE-2026-55999, ROOT-OS-DEBIAN-12-CVE-2026-55999, SUSE-SU-2026:2792-1, UBUNTU-CVE-2026-55999

## What is CVE-2026-55999?

CVE-2026-55999 is a high-severity memory corruption vulnerability in X.org X Server, published on 8 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Local attackers with a X connection able to provide PCX fonts to the X 
server xorg-server before 21.2.24 and xwayland before 24.1.13 could 
cause a heap buffer overflow via SetFont due to missing glyph boundary checks.

Weakness: CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-55999?

It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `xorg-x11-server-Xdmx` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xephyr` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xnest` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xorg` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xvfb` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xwayland` | AlmaLinux:8 | All versions | 21.1.3-20.el8_10.3 |
| `xorg-x11-server-common` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-devel` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-source` | AlmaLinux:8 | All versions | 1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xdmx` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-Xephyr` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-Xnest` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-Xorg` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-Xvfb` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-common` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-devel` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server-source` | AlmaLinux:9 | All versions | 1.20.11-34.el9_8.3 |
| `xorg-x11-server` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xdmx` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xdmx-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xephyr` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xephyr-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xnest` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xnest-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xorg` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xorg-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xvfb` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xvfb-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xwayland` | Red Hat:enterprise_linux:8::appstream | All versions | 0:21.1.3-20.el8_10.3 |
| `xorg-x11-server-Xwayland-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:21.1.3-20.el8_10.3 |
| `xorg-x11-server-Xwayland-debugsource` | Red Hat:enterprise_linux:8::appstream | All versions | 0:21.1.3-20.el8_10.3 |
| `xorg-x11-server-common` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-debugsource` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xdmx-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xephyr-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xnest-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xorg-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-Xvfb-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:1.20.11-28.el8_10.3 |
| `xorg-x11-server-debuginfo` | Red Hat:enterprise_linux:8::crb | All versions | 0:1.20.11-28.el8_10.3 |

Products: X.org X Server, X.org Xwayland.

## How do I fix CVE-2026-55999?

Apply the vendor's patch.

## Is CVE-2026-55999 being exploited?

There is no known exploitation of CVE-2026-55999 in the wild and no public exploit code.

## Other vulnerabilities in X.org X Server

- [CVE-2026-56000](https://insights.sbomify.com/CVE-2026-56000/): xorg-x11-server / xwayland GLX contextTags Use-After-Free in CommonMakeCurrent() (critical 9.0)
- [CVE-2017-10971](https://insights.sbomify.com/CVE-2017-10971/): Memory corruption in n/a (high 8.8)
- [CVE-2022-2319](https://insights.sbomify.com/CVE-2022-2319/): Memory corruption in xorg-x11-server (high 7.8)
- [CVE-2022-2320](https://insights.sbomify.com/CVE-2022-2320/): Memory corruption in xorg-x11-server (high 7.8)
- [CVE-2020-14360](https://insights.sbomify.com/CVE-2020-14360/): Memory corruption in xorg-x11-server (high 7.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55999 and fix it if it is affected.

What it is: CVE-2026-55999 is a high-severity memory corruption vulnerability in X.org X Server, published on 8 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-55999 in the wild and no public exploit code.

Affected product: X.org X Server. 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-2026-55999/ (Markdown: https://insights.sbomify.com/CVE-2026-55999/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55999
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55999
- OSV: https://osv.dev/vulnerability/CVE-2026-55999

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55999/
Summaries are generated and may contain mistakes; the official description above is the record.