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Mattermost ServerApplication · Mattermost

CVE-2026-4915

MEDIUM · 6.5 CVSS v3.1 Published 2026-05-25
Fix available
A fix is available. Upgrade to 10.11.15 / 11.4.5 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 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to filter nil elements from outgoing webhook attachment payloads before processing, which allows an authenticated user to cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.. Mattermost Advisory ID: MMSA-2026-00641

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 filter nil/null elements from outgoing webhook attachment payloads before processing. An authenticated user can send a crafted webhook callback response containing a null attachment entry, which causes the server process to terminate, resulting in denial of service.

MitigationUpgrade to Mattermost versions 11.6.1, 11.5.4, 11.4.5, and 10.11.15 or later which include proper nil element filtering in webhook attachment processing.

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, < 10.11.15>= 11.4.0, < 11.4.5>= 11.5.0, < 11.5.4>= 11.6.0, < 11.6.1

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
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. Determine installed Mattermost Server version
    Run `mattermost version` or `mmctl version` from the server command line, or check the About page in the System Console under the mattermost logo
    Affected if The version falls within any of these ranges: 10.11.0 through 10.11.14, 11.4.0 through 11.4.4, 11.5.0 through 11.5.3, or 11.6.0 through 11.6.0 (the first release in that branch)
  2. Verify outgoing webhooks are enabled
    Check the System Console under Integrations > Integration Management, or inspect the config.json file for the setting `ServiceSettings.EnableOutgoingWebhooks` set to true
    Affected if Outgoing webhooks are enabled in the Mattermost configuration
  3. Identify users with webhook creation permissions
    Check which users or groups have the ability to create or modify outgoing webhooks by reviewing roles in System Console under Users and Teams section, or by checking if the system has any custom roles granting incoming/webhook permissions
    Affected if There are users with permission to create or modify outgoing webhooks, particularly untrusted or external users
  4. Check for recent webhook callback failures
    Review Mattermost server logs for entries containing keywords such as 'nil pointer', 'panic', 'webhook', 'attachment', or crash reports around webhook processing timestamps
    Affected if The server has recently crashed or produced panic logs related to webhook attachment processing

You are affected if your Mattermost Server version is within any of the affected ranges AND outgoing webhooks are enabled with at least one user who can create or modify webhooks.

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 10.11.15 / 11.4.5 / 11.5.4 or later
Fixed in 10.11.1511.4.511.5.4
Interim mitigation

Upgrade to Mattermost versions 11.6.1, 11.5.4, 11.4.5, and 10.11.15 or later which include proper nil element filtering in webhook attachment processing.

Recommended fix High confidence

Upgrade to 10.11.15+ (for 10.11.x), 11.4.5+ (for 11.4.x), 11.5.4+ (for 11.5.x), or 11.6.1+ (for 11.6.x)

  1. Identify the currently installed Mattermost Server version using 'mattermost version' or system logs
  2. Determine which affected version branch your server is running (10.11.x, 11.4.x, 11.5.x, or 11.6.x)
  3. Back up the Mattermost database and configuration files before proceeding
  4. For 10.11.x: Upgrade to version 10.11.15 or later
  5. For 11.4.x: Upgrade to version 11.4.5 or later
  6. For 11.5.x: Upgrade to version 11.5.4 or later
  7. For 11.6.x: Upgrade to version 11.6.1 or later
  8. Download the appropriate version from https://mattermost.com/download/
Caveat Standard Mattermost minor version upgrades typically have minimal breaking changes but review release notes for any behavior changes

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
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,224.

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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-2026-4915 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