CVE-2026-6850
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 · uneditedMattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658
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 confidenceThis is a denial of service vulnerability in Mattermost caused by missing input validation on message attachment field values. An authenticated attacker can send specially crafted markdown in attachment fields that triggers catastrophic backtracking (ReDoS) in the client-side markdown parser, causing all users viewing the channel to experience DoS.
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>= 10.11.0, < 10.11.20>= 11.6.0, < 11.6.5>= 11.7.0, < 11.7.3CVSS 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 checksWork through these to decide whether this CVE applies to you.
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Check Mattermost Server versionRun 'mattermost version' or check the /api/v4/system endpoint for version informationAffected if The installed version falls within >= 10.11.0, < 10.11.20; >= 11.6.0, < 11.6.5; or >= 11.7.0, < 11.7.3
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Verify markdown rendering is enabled for message attachmentsCheck the System Console under Site Posts > Enable Markdown or query the configuration setting for post.markdown_enabledAffected if Markdown rendering is enabled and the server version is in the affected range (both conditions must be true)
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Inspect channel messages for attachment fields containing complex markdown patternsQuery the database or use the API to retrieve recent messages with attachments; look for attachment field values containing nested or repetitive markdown syntax such as **bold**, *italic*, [links](url), or regex-like patternsAffected if Messages contain attachment field values with crafted markdown that could trigger catastrophic backtracking in the client-side parser
You are affected if your Mattermost Server version is in the vulnerable range AND markdown rendering in message attachments is enabled, allowing specially crafted attachment field markdown to cause client-side ReDoS when users view the channel.
Generated from the published advisory. Verify against your own configuration.
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 · scoped10.11.2011.6.511.7.3
Update Mattermost to a patched version once released, or implement server-side input validation/length limits on attachment field values before rendering.
10.11.20 (for 10.11.x) / 11.6.5 (for 11.6.x) / 11.7.3 (for 11.7.x)
- Identify the currently running Mattermost Server version using system console or mmctl command
- Determine which version branch your current installation falls into (10.11.x, 11.6.x, or 11.7.x)
- For 10.11.x branch: Upgrade to version 10.11.20 or later
- For 11.6.x branch: Upgrade to version 11.6.5 or later
- For 11.7.x branch: Upgrade to version 11.7.3 or later
- Follow Mattermost upgrade documentation to perform the upgrade (backup database, stop server, install new version, verify startup, test functionality)
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-6850 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-6850 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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