SAML RelayState Integrity Issue in Mattermost by Mattermost
CVE-2026-16453

7.1HIGH

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
16 October 2026

What is CVE-2026-16453?

A vulnerability exists in Mattermost versions that fail to properly validate the integrity of the SAML RelayState POST parameter. This issue permits authenticated users to manipulate the team_id or invite_id fields, thereby gaining unauthorized access to any team, including those that are set as invite-only. This security flaw significantly undermines the intended access controls, posing a risk to team confidentiality and user security. Mattermost has issued an advisory to address this critical security lapse.

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:
7.1
Severity:
HIGH
Confidentiality:
High
Integrity:
Low
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Doyensec
.