Webhook Notification Vulnerability in Mattermost Product Suite
CVE-2026-10556

5.3MEDIUM

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
14 September 2026

What is CVE-2026-10556?

Mattermost versions 10.11.x up to 10.11.22 and 11.7.x through 11.9.0 have a vulnerability that fails to adequately validate null entries in Microsoft Graph webhook notifications. This flaw allows unauthenticated attackers to send a malicious {{POST}} request to the public webhook endpoint, potentially leading to a crash of the Microsoft Calendar plugin process. Consequently, this can disrupt the calendar integration functionality for all users on the affected Mattermost instance, resulting in a denial of service.

Affected Version(s)

Mattermost 11.9.0

Mattermost 11.8.0 <= 11.8.4

Mattermost 11.7.0 <= 11.7.7

References

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

CVSS V3.1

Score:
5.3
Severity:
MEDIUM
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

kamil-sawicki
.