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Mattermost ServerApplication · Mattermost

CVE-2017-18879

MEDIUM · 6.1 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 →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

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. XSS could occur via the author_link field of a Slack attachment.

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

A stored XSS vulnerability in Mattermost Server allows injection of malicious JavaScript through the author_link field of Slack message attachments. When users view messages containing crafted Slack attachments, the payload executes in their browsers.

MitigationUpgrade Mattermost Server to version 4.3.0, 4.2.1, 4.1.2 or later. The fix requires proper sanitization of the author_link field when processing Slack attachments.

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
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/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. Determine installed Mattermost Server version
    Run 'mmctl version' or check the Mattermost config file (config.json) for the Version field, or look at the system console > About page
    Affected if The installed version is < 4.1.2, or >= 4.2.0 and < 4.2.1, or exactly 4.3.0
  2. Check if Slack integration is enabled
    Navigate to System Console > Integrations > Custom Integrations, or check the config.json file for 'EnableCustomGroups', 'EnableOAuth2Provider', and related integration settings
    Affected if Slack message attachments can be imported or posted to the server (the vulnerable code path processes Slack attachment data)
  3. Verify Slack import functionality status
    Check System Console > Import > Slack (or check config.json for 'ImportSettings' section) to see if Slack import is configured
    Affected if Slack import is enabled and messages with attachments can be imported, as this triggers the vulnerable code path

You are affected if your Mattermost Server version falls within the ranges < 4.1.2, >= 4.2.0 but < 4.2.1, or exactly 4.3.0, AND Slack message attachments can be posted or imported into your server.

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 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. The fix requires proper sanitization of the author_link field when processing Slack attachments.

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,696.

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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-2017-18879 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