CVE-2026-3590 Race Condition in Guest Magic Link Authentication Allows Token Reuse

What is CVE-2026-3590?

CVE-2026-3590 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 15 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests.. Mattermost Advisory ID: MMSA-2026-00624

Weakness: CWE-367

Which versions are affected by CVE-2026-3590?

14 open source packages ship an affected version, including github.com/mattermost/mattermost-server, github.com/mattermost/mattermost-server/v5 and github.com/mattermost/mattermost-server/v6. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-3590?

Upgrade github.com/mattermost/mattermost-server to a fixed version for your release line: 10.11.13, 10.11.13+incompatible, 11.3.3+incompatible or 11.4.3+incompatible.

Is CVE-2026-3590 being exploited?

There is no known exploitation of CVE-2026-3590 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-mh4x-rmrx-3hp4, GO-2026-5507.