CVE-2026-55999 X.Org CVE debrief

Who should care

Users of xorg-server before version 21.2.24 and xwayland before version 24.1.13 may be vulnerable to a local attack. Operators of systems using these versions should review their deployments and assess potential exposure. Security teams should prioritize patching or mitigating this vulnerability due to its high severity and potential for local exploitation.

Technical summary

The vulnerability exists in the X server, specifically in the SetFont function, where missing glyph boundary checks can lead to a heap buffer overflow when PCX fonts are provided by a local attacker with a X connection. This vulnerability affects xorg-server before version 21.2.24 and xwayland before version 24.1.13. The high CVSS score of 8.5 indicates a high severity vulnerability that requires immediate attention.

Defensive priority

High priority due to the high CVSS score of 8.5 and the potential for local attackers to exploit this vulnerability.

Recommended defensive actions

• Update xorg-server to version 21.2.24 or later
• Update xwayland to version 24.1.13 or later
• Restrict access to the X server to trusted users only
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-07-08T09:16:29.800Z and has not been modified since then. The NVD entry is currently being reviewed. Evidence is limited to public sources and may not reflect the full scope of affected products or potential impacts. Defenders should verify the official CVE record and NVD entry for the latest information. Additional verification is recommended to confirm affected deployments and assess potential exposure.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.