CVE-2026-19621 Mattermost CVE debrief

Who should care

Defenders responsible for Mattermost instances, security teams, and administrators should assess exposure and prioritize remediation based on the sensitivity of the exposed information. This includes reviewing webhook URLs and authentication context, monitoring post-deletion web socket events and thread API responses, and verifying the integrity of their Mattermost instances.

Why it matters

CVE-2026-19621 allows authenticated channel members to obtain sensitive information due to Mattermost's failure to strip action integration secrets.

• Authenticated channel members may obtain server-side webhook URLs and authentication context
• Post-deletion web socket events and thread API responses may expose sensitive information

Technical summary

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to strip action integration secrets from post payloads before broadcasting post-deletion web socket events and returning thread API responses. This vulnerability allows authenticated channel members to obtain server-side webhook URLs and authentication context, potentially exposing sensitive information. Defenders should prioritize verifying and remediating affected Mattermost instances to prevent unauthorized access to sensitive information.

Defensive priority

Defenders should prioritize verifying and remediating affected Mattermost instances.

Recommended defensive actions

• Verify Mattermost instance versions and apply patches
• Review and update webhook URLs and authentication context
• Monitor post-deletion web socket events and thread API responses
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• 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

Evidence notes

Evidence from Mattermost Advisory ID: MMSA-2026-00766 and CVE Program record. The advisory provides details on the affected versions and the nature of the vulnerability, while the CVE Program record offers additional context on the vulnerability's severity and potential impact. Defenders should verify the affected versions and assess the potential exposure of their Mattermost instances.

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.