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Mattermost ServerApplication · Mattermost

CVE-2025-12421

CRITICAL · 9.9 CVSS v3.1 Published 2025-11-27
Fix available
A fix is available. Upgrade to 10.5.13 / 10.11.5 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Mattermost versions 11.0.x <= 11.0.2, 10.12.x <= 10.12.1, 10.11.x <= 10.11.4, 10.5.x <= 10.5.12 fail to to verify that the token used during the code exchange originates from the same authentication flow, which allows an authenticated user to perform account takeover via a specially crafted email address used when switching authentication methods and sending a request to the /users/login/sso/code-exchange endpoint. The vulnerability requires ExperimentalEnableAuthenticationTransfer to be enabled (default: enabled) and RequireEmailVerification to be disabled (default: disabled).

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Mattermost fails to validate token origin consistency during SSO code exchange, allowing authenticated attackers to hijack other user accounts by manipulating email addresses during authentication method switches and sending requests to the /users/login/sso/code-exchange endpoint. The vulnerability requires ExperimentalEnableAuthenticationTransfer enabled (default) and RequireEmailVerification disabled (default).

MitigationDisable ExperimentalEnableAuthenticationTransfer and enable RequireEmailVerification in system settings, or upgrade to patched versions beyond 11.0.2, 10.12.1, 10.11.4, and 10.5.12.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Mattermost ServerApplication
Affected:>= 10.5.0, < 10.5.13>= 10.11.0, < 10.11.5>= 10.12.0, < 10.12.2>= 11.0.0, < 11.0.3

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Mattermost Server version
    Run 'mattermost version' or inspect the version via the system console at /about or via API call to /api/v4/system/analytics/0?name=system
    Affected if Installed version falls within >=10.5.0/<10.5.13, >=10.11.0/<10.11.5, >=10.12.0/<10.12.2, or >=11.0.0/<11.0.3
  2. Verify ExperimentalEnableAuthenticationTransfer setting
    Query the configuration via 'mattermost config show | grep ExperimentalEnableAuthenticationTransfer' or check in System Console under Environment > Developer, or query the config API endpoint
    Affected if The setting value is true or not explicitly set to false (defaults to enabled)
  3. Verify RequireEmailVerification setting
    Query the configuration via 'mattermost config show | grep RequireEmailVerification' or check in System Console under Authentication > Email, or query the config API endpoint
    Affected if The setting value is false (defaults to disabled)
  4. Confirm SSO authentication methods in use
    Check System Console under Authentication section to see if multiple SSO methods (e.g., GitLab, OIDC, SAML, Google, Office 365) are configured and enabled
    Affected if More than one SAML/OIDC/SSO provider is enabled, allowing authentication method switching

User is affected if running an affected version AND ExperimentalEnableAuthenticationTransfer is enabled AND RequireEmailVerification is disabled AND multiple SSO authentication methods are configured.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 10.5.13 / 10.11.5 / 10.12.2 or later
Fixed in 10.5.1310.11.510.12.2
Interim mitigation

Disable ExperimentalEnableAuthenticationTransfer and enable RequireEmailVerification in system settings, or upgrade to patched versions beyond 11.0.2, 10.12.1, 10.11.4, and 10.5.12.

Recommended fix High confidence

Upgrade to 10.5.13 (if on 10.5.x), 10.11.5 (if on 10.11.x), 10.12.2 (if on 10.12.x), or 11.0.3 (if on 11.0.x) - choose the latest stable release in your minor version branch

  1. 1. Identify the currently running Mattermost Server version using the system console or mmctl command
  2. 2. Determine the appropriate upgrade path based on current version (see upgrade_path field)
  3. 3. Before upgrading, review Mattermost upgrade documentation and ensure backup of database and configuration
  4. 4. For the upgrade, download the specific fixed version from the official Mattermost releases page (github.com/mattermost/mattermost-server/releases)
  5. 5. Follow Mattermost's standard upgrade procedure: stop the server, replace the binary, run database migrations if required, then restart the server
  6. 6. After upgrade, verify the server starts successfully and check system console for any errors
  7. 7. Optionally, as a defense-in-depth measure, disable ExperimentalEnableAuthenticationTransfer in System Console > Authentication > Enable Authentication Transfer (set to false)
  8. 8. Optionally, enable RequireEmailVerification in System Console > Authentication > Email Verification (set to true)
Caveat Review Mattermost release notes for each version upgrade for potential breaking changes, particularly around authentication behavior changes

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,696.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-12421 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data