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Mattermost ServerApplication · Mattermost

CVE-2026-3590

MEDIUM · 6.5 CVSS v3.1 Published 2026-04-15
Fix available
A fix is available. Upgrade to 10.11.13 / 11.3.3 or later.
See remediation →
74/100
Remediation priority · Elevated
Remotely reachable No privileges 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 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

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 atomically consume guest magic link tokens, allowing the same token to be used for multiple concurrent authentication requests. This race condition enables an attacker with a valid token to establish multiple independent authenticated sessions instead of being limited to single-use.

MitigationUpgrade to patched Mattermost versions (10.11.13+, 11.5.1+, 11.4.3+, 11.3.3+) or implement application-layer rate limiting on magic link token validation as a temporary workaround.

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:>= 10.11.0, < 10.11.13>= 11.3.0, < 11.3.3>= 11.4.0, < 11.4.3>= 11.5.0, < 11.5.1

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
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

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

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 'mmctl version' or check the System Console > About page for the server version number
    Affected if Version is 10.11.0-10.11.12, 11.3.0-11.3.2, 11.4.0-11.4.2, or 11.5.0 (any build before 10.11.13, 11.3.3, 11.4.3, or 11.5.1)
  2. Determine if guest accounts are enabled
    Navigate to System Console > User Management > Guests, or check config.json for 'GuestAccountsSettings.Enable' set to true
    Affected if Guest accounts feature is enabled and magic link invitations are in use
  3. Verify magic link token configuration
    Check System Console > Authentication > Email > Enable sign up with email, or review config.json for 'EmailSettings.EnableSignUpWithEmail' and guest invitation settings
    Affected if Email-based sign-up and guest invitations are configured
  4. Confirm guest invitation method
    Review invitation emails sent to guest users containing magic links, or check if 'EmailSettings.EnableSignInWithEmail' is true in configuration
    Affected if Guest users receive magic link tokens for authentication

You are affected if your Mattermost Server version falls within any of the vulnerable ranges AND guest magic link invitations are enabled in your environment.

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 10.11.13 / 11.3.3 / 11.4.3 or later
Fixed in 10.11.1311.3.311.4.3
Interim mitigation

Upgrade to patched Mattermost versions (10.11.13+, 11.5.1+, 11.4.3+, 11.3.3+) or implement application-layer rate limiting on magic link token validation as a temporary workaround.

Recommended fix High confidence

Upgrade to 10.11.13 (for 10.11.x branch), 11.3.3 (for 11.3.x branch), 11.4.3 (for 11.4.x branch), or 11.5.1 (for 11.5.x branch) - choose the version corresponding to your current branch

  1. 1. Identify the currently running Mattermost Server version using the system console or mmctl command
  2. 2. Based on your current version, determine the appropriate upgrade target: 10.11.x users upgrade to 10.11.13, 11.3.x users upgrade to 11.3.3, 11.4.x users upgrade to 11.4.3, or 11.5.x users upgrade to 11.5.1
  3. 3. Back up the Mattermost database and configuration files before upgrading
  4. 4. Download the appropriate Mattermost Server fixed release from the official Mattermost download page
  5. 5. Follow Mattermost upgrade documentation to install the new version - typically involves stopping the server, replacing binaries, and restarting
  6. 6. Verify the upgrade was successful by checking the server version in the system console
  7. 7. Test that guest magic link authentication works correctly and that tokens are consumed atomically (single-use)
Caveat Standard Mattermost minor version upgrades typically have minimal breaking changes, but review the release notes for your target version for any migration notes or deprecation warnings

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation3.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
17.0 hours of engineering $3,000
Get the upgrade done

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Scan for this in your stack

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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-2026-3590 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