Is CVE-2026-16453 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-16453 being exploited in the wild.

How severe is CVE-2026-16453?

High severity, with a CVSS score of 7.1, published 16 October 2026.

Which products and versions are affected?

Mattermost Server 11.7.x through 11.7.9, 11.8.x through 11.8.5, 11.9.x through 11.9.1, 11.10.x through 11.10.0.

Is there a fix for CVE-2026-16453?

Yes: fixed in . Until then: Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-16453 attacked?

It is attacked over the network, in a web application.

Am I affected by CVE-2026-16453?

Check whether you run Mattermost Server at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-16453: Mattermost SAML RelayState integrity verification bypass

CVE-2026-16453 · Severity: high · CVSS 7.1 · Published 2026-10-16

Mattermost, a team collaboration and messaging platform, fails to properly validate SAML authentication tokens in certain versions. An authenticated user with valid SAML credentials can bypass team access restrictions and join invite-only teams by forging authentication parameters, potentially gaining unauthorized access to sensitive team information and communications.

Technologies: Mattermost Server. Vendors: Mattermost.

Quick answers

Is CVE-2026-16453 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-16453 being exploited in the wild.
How severe is CVE-2026-16453?: High severity, with a CVSS score of 7.1, published 16 October 2026.
Which products and versions are affected?: Mattermost Server 11.7.x through 11.7.9, 11.8.x through 11.8.5, 11.9.x through 11.9.1, 11.10.x through 11.10.0.
Is there a fix for CVE-2026-16453?: Yes: fixed in <UNKNOWN>. Until then: <UNKNOWN> Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-16453 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-16453?: Check whether you run Mattermost Server at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

Fixed in: <UNKNOWN>

Technical details

The vulnerability exists in SAML RelayState POST parameter handling, where Mattermost fails to verify the integrity of the RelayState value. An authenticated SAML user can forge the team_id or invite_id fields within the RelayState to circumvent team membership restrictions, including joining invite-only teams. This requires an active SAML authentication session but allows escalation of team access without additional credentials.

Affected products

Mattermost Mattermost Server 11.7.x through 11.7.9, 11.8.x through 11.8.5, 11.9.x through 11.9.1, 11.10.x through 11.10.0

Timeline

2026-10-16: disclosed

References

https://mattermost.com/security-updates

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