CVE-2026-27769 Connected Workspaces: Malicious remote server can manipulate arbitrary user's status

What is CVE-2026-27769?

CVE-2026-27769 is a low-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 15 Apr 2026. An attacker needs administrator access to exploit it, with no action from the victim.

Official description

Mattermost versions 10.11.x <= 10.11.12 fail to validate whether users were correctly owned by the correct Connected Workspace which allows a malicious remote server connected using the Conntexted Workspaces feature to change the displayed status of local users via the Connected Workspaces API.. Mattermost Advisory ID: MMSA-2026-00603

Weakness: CWE-862

Which versions are affected by CVE-2026-27769?

3 open source packages ship an affected version, including github.com/mattermost/mattermost-server. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-27769?

Upgrade github.com/mattermost/mattermost-server to 8.0.0-20260316060126-bc1a2b34b1f9 or later.

Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.

Is CVE-2026-27769 being exploited?

There is no known exploitation of CVE-2026-27769 in the wild and no public exploit code.

Other vulnerabilities in Mattermost Mattermost Server

• CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers Critical 9.9
• CVE-2025-12421 Account Takeover via Code Exchange Endpoint Critical 9.9
• CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User Critical 9.9
• CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards Critical 9.9
• CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive Critical 9.9

All Mattermost Mattermost Server vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-mxxh-fmjq-j6x4, GO-2026-5522.