dbcve.org
Mattermost ServerApplication · Mattermost

CVE-2018-21257

MEDIUM · 5.3 CVSS v3.1 Published 2020-06-19
Fix available
A fix is available. Upgrade to 5.1.0 or later.
See remediation →
62/100
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
An issue was discovered in Mattermost Server before 5.1. It allows attackers to bypass intended access restrictions (for setting a channel header) via the Channel header slash command API.

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 Server before version 5.1 contains an access control bypass vulnerability in the Channel header slash command API that allows attackers to set channel headers without proper authorization, circumventing intended access restrictions.

MitigationUpgrade to Mattermost Server 5.1 or later to obtain the patch that enforces proper access control checks on the Channel header slash command API endpoint.

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:< 5.1.0

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

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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. Identify installed Mattermost Server version
    Run 'grep -i version /path/to/mattermost/server/version.go' or check the system administration console under 'About' > 'Mattermost Version'. On Linux, also check '/opt/mattermost/version.go' or the package manager output 'dpkg -l mattermost-server' or 'rpm -qi mattermost-server'.
    Affected if The installed version is lower than 5.1.0 (for example 5.0.x, 4.x.x, etc.)
  2. Verify the Channel header slash command API endpoint is accessible
    Check if the API endpoint '/api/v4/channels/{channel_id}/headers' or similar slash command endpoints are exposed. Review Mattermost configuration file 'config.json' for API settings under 'ServiceSettings' > 'EnableAPIv3' or similar API-related flags.
    Affected if The API endpoint is enabled and exposed without additional authorization middleware above version 5.1.0 expectations
  3. Review API access logs for unauthorized channel header modifications
    Examine Mattermost server logs (typically in '/var/log/mattermost/') or database audit tables for POST/PUT requests to channel header API endpoints where the requesting user is not a channel member or has insufficient permissions.
    Affected if Logs show POST requests to channel header APIs succeeded for users without 'manage_channel' permissions for that specific channel
  4. Check user permissions on channel header operations
    Query the database or use Mattermost system console to verify role permissions. Look for users with 'manage_channel' role vs regular members. Test by attempting to modify a channel header as a non-admin, non-channel-member user via the API.
    Affected if Non-authorized users can successfully modify channel headers via the API when they should be denied
  5. Confirm if the server uses pre-5.1.0 API behavior
    Review the Mattermost changelog or the specific commit history if accessible. Check the 'mattermost-server' binary build date. Compare API response headers and behavior against documentation for version 5.1+ which includes the access control fix.
    Affected if The server exhibits the pre-5.1 behavior where channel header modification requests succeed without proper permission checks

A user is affected if their Mattermost Server version is below 5.1.0 and the Channel header slash command API is accessible to users who should not have permission to modify channel headers.

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.

From vendor data
Upgrade available Upgrade to 5.1.0 or later
Fixed in 5.1.0
Interim mitigation

Upgrade to Mattermost Server 5.1 or later to obtain the patch that enforces proper access control checks on the Channel header slash command API endpoint.

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2018-21257 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2018-21257 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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