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Mattermost ServerApplication · Mattermost

CVE-2017-18883

CRITICAL · 9.1 CVSS v3.1 Published 2020-06-19
Fix available
A fix is available. Upgrade to 4.1.2 / 4.2.1 or later.
See remediation →
100/100
Remediation priority · Urgent
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 4.3.0, 4.2.1, and 4.1.2, when serving as an OAuth 2.0 Service Provider. There is low entropy for authorization data.

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 versions before 4.3.0, 4.2.1, and 4.1.2 have insufficient entropy in OAuth 2.0 authorization codes and state parameters when acting as a Service Provider. This allows attackers to potentially predict or brute-force authorization tokens due to the low randomness, enabling session hijacking or unauthorized OAuth token issuance.

MitigationUpgrade Mattermost Server to version 4.3.0, 4.2.1, 4.1.2 or later. Alternatively, implement cryptographically secure random number generation (minimum 128-bit entropy) for all OAuth authorization data including state parameters and authorization codes.

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:< 4.1.2>= 4.2.0, < 4.2.1= 4.3.0

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
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 Mattermost Server installation
    Look for Mattermost processes running on the system (e.g., `ps aux | grep mattermost`) or check for Mattermost configuration directories (typically at `/opt/mattermost` or `/var/opt/mattermost`).
    Affected if Mattermost Server is not installed or not in use.
  2. Determine installed Mattermost Server version
    Check the version by accessing the system console About section in the web UI, or by examining the version file in the Mattermost installation directory, or by running `mattermost --version` if the CLI is available.
    Affected if The installed version cannot be determined or is not a Mattermost Server version.
  3. Compare version against affected ranges
    Verify if the installed version falls within: < 4.1.2, OR >= 4.2.0 and < 4.2.1, OR exactly 4.3.0. These ranges indicate vulnerability to insufficient OAuth entropy.
    Affected if The version matches one of the affected ranges listed above.
  4. Check if OAuth 2.0 Service Provider is configured
    Examine the Mattermost configuration file (config.json) for the OAuth settings section. Look for `ServiceSettings.EnableOAuthServiceProvider` set to true, and verify OAuth providers are configured under `OAuthSettings`.
    Affected if OAuth 2.0 Service Provider is enabled and clients are configured.

A user is affected if they run a vulnerable Mattermost Server version (before 4.1.2, 4.2.0-4.2.0, or 4.3.0) AND have OAuth 2.0 Service Provider enabled in their configuration.

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 4.1.2 / 4.2.1 or later
Fixed in 4.1.24.2.1
Interim mitigation

Upgrade Mattermost Server to version 4.3.0, 4.2.1, 4.1.2 or later. Alternatively, implement cryptographically secure random number generation (minimum 128-bit entropy) for all OAuth authorization data including state parameters and authorization codes.

Recommended fix High confidence

Mattermost Server 4.3.0

  1. 1. Back up your Mattermost Server database and configuration files.
  2. 2. Download Mattermost Server version 4.3.0 (or 4.2.1 / 4.1.2 as alternative fixed versions).
  3. 3. Stop the Mattermost Server service.
  4. 4. Install the updated Mattermost Server package for your platform.
  5. 5. Verify the service starts successfully.
  6. 6. Test OAuth 2.0 authentication flows to confirm functionality works correctly.

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
  • Testing8.0 h
  • Review / QA4.0 h
24.0 hours of engineering $4,160
Get the upgrade done

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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-2017-18883 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