CVE-2026-9812 Mattermost CVE debrief

Who should care

Mattermost Server administrators and users with run property-management access should assess exposure and prioritize patching to prevent potential disruption of Playbooks plugin functionality.

Why it matters

CVE-2026-9812 is a vulnerability in Mattermost Server that allows an authenticated user to crash the Playbooks plugin. Defenders should prioritize verifying and applying the vendor's patch to prevent potential disruption of Playbooks plugin functionality. The vulnerability requires verification of affected versions and patching priority.

• Potential disruption of Playbooks plugin functionality
• Need to verify and apply the vendor's patch
• Possible impact on business operations due to plugin disruption

Technical summary

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate that a property field belongs to the specified run before updating its value which allows an authenticated user with run property-management access to crash the Playbooks plugin via a REST request referencing a property field that belongs to a different run. The vulnerability requires verification of affected versions and patching priority to prevent potential disruption of Playbooks plugin functionality. Defenders should prioritize verifying and applying the vendor's patch.

Defensive priority

Defenders should prioritize verifying and applying the vendor's patch to prevent potential disruption of Playbooks plugin functionality.

Recommended defensive actions

• Verify and apply the vendor's patch for the vulnerability
• Restrict access to the Playbooks plugin to only necessary users
• Monitor Playbooks plugin functionality for potential disruptions
• 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 vulnerability is described in the Mattermost Advisory ID: MMSA-2026-00684 and CVE Program record. The NVD entry is currently Analyzed. Defenders should verify and apply the vendor's patch to prevent potential disruption of Playbooks plugin functionality. The vulnerability affects Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22.

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.