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Mattermost ServerApplication · Mattermost

CVE-2025-21088

MEDIUM · 6.5 CVSS v3.1 Published 2025-01-15
Fix available
A fix is available. Upgrade to 9.11.6 / 10.0.4 or later.
See remediation →
71/100
Remediation priority · Elevated
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 10.2.x <= 10.2.0, 9.11.x <= 9.11.5, 10.0.x <= 10.0.3, 10.1.x <= 10.1.3 fail to properly validate the style of proto supplied to an action's style in post.props.attachments, which allows an attacker to crash the frontend via crafted malicious input.

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 · moderate confidence

Mattermost fails to validate the style parameter supplied to action styles within post.props.attachments, allowing an attacker to inject crafted malicious proto input that causes a denial of service by crashing the frontend.

MitigationUpgrade Mattermost to versions 10.2.1+, 9.11.6+, 10.0.4+, and 10.1.4+ or later to receive the security patch; as a temporary workaround, implement server-side input validation for attachment style parameters before rendering.

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:>= 9.11.0, < 9.11.6>= 10.0.0, < 10.0.4>= 10.1.0, < 10.1.4= 10.2.0

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
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/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 'mmctl version' or check the System Console under About > Mattermost, or inspect the /opt/mattermost/version file
    Affected if The installed version is >= 9.11.0 and < 9.11.6, OR >= 10.0.0 and < 10.0.4, OR >= 10.1.0 and < 10.1.4, OR exactly 10.2.0
  2. Confirm interactive messages are in use
    Check if the server has any plugins or integrations (such as webhooks, apps, or custom plugins) that can create posts with attachments containing the style parameter
    Affected if The style parameter vulnerability is only exploitable when posts with attachments are created, typically through webhooks, apps, or custom integrations
  3. Review post attachment processing
    Inspect server logs for any crash reports or frontend errors related to attachment rendering in the webapp logs
    Affected if Any frontend crashes or errors mentioning 'attachments' or 'style' parameter processing are observed

The environment is affected if Mattermost Server version falls within any of the vulnerable ranges (9.11.0-9.11.5, 10.0.0-10.0.3, 10.1.0-10.1.3, or 10.2.0) and the server processes interactive messages or webhook payloads with attachments.

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 9.11.6 / 10.0.4 / 10.1.4 or later
Fixed in 9.11.610.0.410.1.4
Interim mitigation

Upgrade Mattermost to versions 10.2.1+, 9.11.6+, 10.0.4+, and 10.1.4+ or later to receive the security patch; as a temporary workaround, implement server-side input validation for attachment style parameters before rendering.

Recommended fix High confidence

9.11.6 (for 9.11.x), 10.0.4 (for 10.0.x), 10.1.4 (for 10.1.x), latest 10.2.x patch (for 10.2.0)

  1. Identify the currently running Mattermost Server version using the system console or mmctl command
  2. Take a complete backup of the Mattermost database and configuration files
  3. For Mattermost 9.11.x: Upgrade to version 9.11.6 or later
  4. For Mattermost 10.0.x: Upgrade to version 10.0.4 or later
  5. For Mattermost 10.1.x: Upgrade to version 10.1.4 or later
  6. For Mattermost 10.2.0: Upgrade to the latest 10.2.x patch that addresses this vulnerability
  7. After upgrade, verify the server is running and test that post attachments with actions render correctly
  8. Review Mattermost release notes for any additional security fixes included in the upgrade
Caveat Standard Mattermost upgrade considerations apply - review migration guides for major version jumps; ensure plugin compatibility before upgrading

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
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
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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-2025-21088 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