CVE-2026-5755 Mattermost CVE debrief

Who should care

Users of Mattermost Server versions 10.11.x through 11.6.x should apply patches to prevent denial of service attacks. System administrators and security teams responsible for Mattermost deployments should review and implement recommended mitigations.

Technical summary

The vulnerability occurs because Mattermost fails to validate the TIFF IFD offset in the image header before allocating memory. This allows authenticated users with file upload or posting permissions to cause a denial of service (server OOM) via uploading a crafted TIFF file or posting a URL that serves one. The issue is related to image processing and memory management in Mattermost Server. Affected product deployments exist in environments where Mattermost Server versions 10.11.x through 11.6.x are used. Users with file upload or posting permissions can exploit this vulnerability. Defenders should verify affected systems and apply patches or mitigations as recommended by the vendor.

Defensive priority

Medium

Recommended defensive actions

• Apply patches to Mattermost Server versions 10.11.x through 11.6.x
• Restrict file upload and posting permissions to trusted users
• Monitor server resources for signs of OOM
• 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 2026-05-22T11:16:23.287Z and last modified on 2026-07-23T16:10:00.137Z. The NVD entry is currently Analyzed. Evidence is limited to public sources and may not reflect the full scope or details of the vulnerability. Defenders should verify affected systems and apply patches or mitigations as recommended by the vendor.

Official resources