# CVE-2026-18551: Mattermost privilege escalation in guest and admin role validation

Source: https://junglewise.ai/threats/cve-2026-18551-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-2026-18551 · Severity: medium · CVSS 6.5 · Published 2026-10-16

Mattermost Server, a team collaboration platform, fails to properly validate user roles when modifying member permissions. An authenticated attacker can exploit this flaw to escalate a guest account to team or channel administrator status through a crafted API request, gaining unauthorized access to administrative functions and sensitive data.

Technologies: [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-18551 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-18551 being exploited in the wild.

**How severe is CVE-2026-18551?** Medium severity, with a CVSS score of 6.5, published 16 October 2026.

**Which products and versions are affected?** Mattermost Server 11.7.x <= 11.7.9, 11.8.x <= 11.8.5, 11.9.x <= 11.9.1, 11.10.x <= 11.10.0.

**Is there a fix for CVE-2026-18551?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-18551 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-18551?** 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.

## Technical details

The vulnerability exists in the member roles API endpoint validation logic, which does not properly validate the combination of guest and admin scheme roles. An authenticated attacker can send a crafted PUT request to escalate a guest account to team or channel admin status, bypassing intended role restrictions. A fix is available in updated versions across all affected release branches.

## Affected products

- Mattermost Mattermost Server 11.7.x <= 11.7.9, 11.8.x <= 11.8.5, 11.9.x <= 11.9.1, 11.10.x <= 11.10.0

## Timeline

- 2026-10-16: disclosed: Security advisory published with CVE-2026-18551

## References

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

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