CVE-2026-0999 Authentication bypass via userID login when email and username login are disabled
Authenticated users can bypass SSO-only login restrictions by using userID-based authentication when email and username logins are disabled.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-0999?
CVE-2026-0999 is a medium-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 16 Feb 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to properly validate login method restrictions which allows an authenticated user to bypass SSO-only login requirements via userID-based authentication. Mattermost Advisory ID: MMSA-2025-00548
Weakness: CWE-303
Which versions are affected by CVE-2026-0999?
17 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.
| Package | Affected | Fixed in |
|---|---|---|
github.com/mattermost/mattermost-server Go | All versions | None |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20251212052346-61651b0df7ea |
github.com/mattermost/mattermost-server Go | All versions | 5.3.2-0.20251212052346-61651b0df7ea |
github.com/mattermost/mattermost-server Go | From 10.11.0 | None |
github.com/mattermost/mattermost-server Go | From 10.11.0 | None |
github.com/mattermost/mattermost-server Go | From 10.11.0+incompatible | None |
11 more packages
github.com/mattermost/mattermost-server Go | From 11.1.0 | None |
github.com/mattermost/mattermost-server Go | From 11.1.0 | None |
github.com/mattermost/mattermost-server Go | From 11.1.0+incompatible | None |
github.com/mattermost/mattermost-server Go | From 11.2.0 | None |
github.com/mattermost/mattermost-server Go | From 11.2.0 | None |
github.com/mattermost/mattermost-server Go | From 11.2.0+incompatible | None |
github.com/mattermost/mattermost-server/v5 Go | All versions | None |
github.com/mattermost/mattermost-server/v6 Go | All versions | None |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20251212052346-61651b0df7ea |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20251212052346-61651b0df7ea |
github.com/mattermost/mattermost/server/v8 Go | All versions | 8.0.0-20251212052346-61651b0df7ea |
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-0999?
Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20251212052346-61651b0df7ea or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-0999 being exploited?
There is no known exploitation of CVE-2026-0999 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-3c9r-7f29-qp32, GO-2026-4520.