CVE-2021-40105
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 · uneditedAn issue was discovered in Concrete CMS through 8.5.5. There is XSS via Markdown Comments.
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 confidenceConcrete CMS through version 8.5.5 contains a stored Cross-Site Scripting (XSS) vulnerability in the Markdown Comments feature. Attackers can inject malicious JavaScript payload through markdown syntax in comments, which executes when other users view the comment content without proper sanitization or output encoding.
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<= 8.5.5CVSS 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 checksWork through these to decide whether this CVE applies to you.
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Determine Concrete CMS versionAccess the dashboard and navigate to Dashboard > System & Settings > About (or check the concrete/VERSION file in the installation root). Compare the version number to 8.5.5 or lower.Affected if The installed version is 8.5.5 or any earlier version (e.g., 8.5.4, 8.5.3, etc.).
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Verify Markdown Comments feature is in useCheck if the site has comment functionality enabled. Navigate to the page attributes or block settings where comments are configured, and confirm that Markdown parsing is enabled for comments.Affected if Markdown Comments feature is enabled and users can post comments that accept markdown syntax.
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Inspect comment database table for stored payloadsQuery the database (typically the Comments or CollectionVersionCommentComments table) to search for comment entries containing markdown syntax that may include script tags, javascript: URIs, or event handlers (onclick, onerror, etc.).Affected if Any comment entries contain raw HTML or javascript: links that could execute as XSS when rendered without encoding.
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Verify output encoding on comment renderingExamine the comment rendering template (usually in blocks/comment/templates or similar) to determine if output encoding is applied when displaying comment content.Affected if The comment display code does not apply proper output encoding/escaping to markdown-rendered content, allowing script execution.
A user is affected if their Concrete CMS installation is version 8.5.5 or lower AND the Markdown Comments feature is enabled, with comments containing unsanitized markdown/HTML that executes when viewed.
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.
From vendor dataUpgrade Concrete CMS to the patched version. As an interim measure, implement input validation and output encoding on comment rendering to neutralize malicious markdown/HTML payloads.
- Consultation2.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-40105 — 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-2021-40105 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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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.
- 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