CVE-2026-5308 Mattermost CVE debrief

Who should care

Users of Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 should apply the patches or mitigations provided by the vendor to prevent potential denial of service attacks. This includes operators, administrators, and security teams responsible for maintaining and securing Mattermost Server deployments.

Technical summary

The vulnerability is caused by the lack of request body size limits on plugin HTTP endpoints in the affected Mattermost Server versions. This allows an attacker to send oversized HTTP requests, potentially causing a denial of service. The CVSS score for this vulnerability is 4.9, indicating a medium severity. Affected versions include Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14.

Defensive priority

Medium priority should be given to patching or mitigating this vulnerability, as it could potentially lead to denial of service attacks.

Recommended defensive actions

• Apply patches or updates provided by the vendor to enforce request body size limits on plugin HTTP endpoints.
• Implement compensating controls, such as monitoring and rate limiting, to detect and prevent potential denial of service attacks.
• Review and update incident response plans to include procedures for handling potential denial of service attacks.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.

Evidence notes

The CVE record and NVD detail provide information on the vulnerability, including its description, CVSS score, and affected versions. The vendor advisory provides guidance on patching and mitigation. Evidence limits suggest that additional information may exist but is not currently publicly available. Defenders should verify the affected versions, review the vendor advisory, and assess their 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.