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Mattermost ServerApplication · Mattermost

CVE-2026-4339

MEDIUM · 6.5 CVSS v3.1 Published 2026-06-26
Fix available
A fix is available. Upgrade to 10.11.19 / 11.5.7 or later.
See remediation →
67/100
Remediation priority · Elevated
Zero-click 8 weeks old

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 <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635

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

This is a Server-Side Request Forgery (SSRF) vulnerability in the Mattermost Agents plugin MCP server. The plugin fails to validate attachment URLs against internal or private IP ranges when processing file attachments in post creation requests. An attacker with access to the MCP server in stdio mode can supply internal URLs as file attachments to exfiltrate data from internal network services.

MitigationImplement URL validation in the MCP server to reject attachment URLs pointing to internal/private IP ranges (127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, etc.) before making requests. Limit MCP server stdio mode access to trusted users.

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, < 10.11.19>= 11.5.0, < 11.5.7>= 11.6.0, < 11.6.4

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
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/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 `mattermost version` or check the system console for the server version
    Affected if Version falls within 10.11.0 to 10.11.18, 11.5.0 to 11.5.6, or 11.6.0 to 11.6.3
  2. Verify Agents plugin is installed
    Navigate to System Console > Plugins > Plugin Management and check if the 'Mattermost Agents' plugin is listed and enabled
    Affected if The Agents plugin is installed and enabled
  3. Confirm MCP server stdio mode is in use
    Check the Agents plugin configuration for MCP server settings. Look for stdio mode configuration in the plugin's JSON config or environment settings
    Affected if MCP server is configured to use stdio mode, allowing external input
  4. Inspect attachment URL validation
    Examine the MCP server code or proxy logs to determine if attachment URLs are validated against private IP ranges (127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) before requests are made
    Affected if No validation against internal/private IP ranges is performed on attachment URLs

You are affected if you run a vulnerable Mattermost Server version with the Agents plugin enabled using stdio mode and the MCP server does not validate attachment URLs against private IP ranges.

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 10.11.19 / 11.5.7 / 11.6.4 or later
Fixed in 10.11.1911.5.711.6.4
Interim mitigation

Implement URL validation in the MCP server to reject attachment URLs pointing to internal/private IP ranges (127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, etc.) before making requests. Limit MCP server stdio mode access to trusted users.

Recommended fix High confidence

10.11.19 (for 10.11.x), 11.5.7 (for 11.5.x), or 11.6.4 (for 11.6.x) - or later versions in each respective branch

  1. Identify the currently running Mattermost Server version using the system console or mmctl command
  2. Based on your current version, plan upgrade to the minimum fixed release: If using 10.11.x, upgrade to 10.11.19 or later; If using 11.5.x, upgrade to 11.5.7 or later; If using 11.6.x, upgrade to 11.6.4 or later
  3. Review the Mattermost upgrade guide and release notes for your target version
  4. Back up your Mattermost database and configuration files before upgrading
  5. Follow standard Mattermost upgrade procedures for your deployment type (e.g., helm upgrade for Kubernetes, apt/yum for Linux packages, or manual upgrade)
  6. After upgrade, verify the server starts successfully and all plugins load without errors
  7. Confirm the upgrade by checking the version number in the system console
Caveat Review Mattermost release notes for your target version as minor releases may include breaking changes or require migration steps

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,176.

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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-2026-4339 in production — separate from our analysis above.

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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