WebSocket Event Handling Flaw in Mattermost by Mattermost
CVE-2026-19621

6.5MEDIUM

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
16 October 2026

What is CVE-2026-19621?

Certain versions of Mattermost exhibit a vulnerability where action integration secrets are not adequately stripped from post payloads before broadcasting post-deletion events via web sockets or returning responses from the thread API. This oversight allows authenticated channel members to potentially access sensitive server-side webhook URLs and related authentication contexts from interactive-message posts. This mismanagement poses a significant risk to the integrity and confidentiality of communications within the Mattermost platform, calling for immediate attention and remediation.

Affected Version(s)

Mattermost 11.10.0

Mattermost 11.7.0 <= 11.7.9

Mattermost 11.9.0 <= 11.9.1

References

https://mattermost.com/security-updates
vendor-advisory

CVSS V3.1

Score:
6.5
Severity:
MEDIUM
Confidentiality:
High
Integrity:
None
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Edgar Bellot MicĂł
.