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

Source: https://junglewise.ai/threats/cve-2026-16453-mattermost-versions-11-10-x-11-10-0-11-7-x-11-7-9-11-9-x-11-9-1  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-16. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/server-public). Vendors: [Mattermost](https://junglewise.ai/threats/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>

<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](https://mattermost.com/security-updates)

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