CVE-2021-40109
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 · uneditedA SSRF issue was discovered in Concrete CMS through 8.5.5. Users can access forbidden files on their local network. A user with permissions to upload files from external sites can upload a URL that redirects to an internal resource of any file type. The redirect is followed and loads the contents of the file from the redirected-to server. Files of disallowed types can be uploaded.
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 Server-Side Request Forgery (SSRF) vulnerability in Concrete CMS through version 8.5.5. An authenticated user with permissions to upload files from external sites can provide a URL that redirects to internal/local network resources, bypassing intended security restrictions. The CMS follows the redirect and loads the file contents, allowing upload of disallowed file types.
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< 8.5.6CVSS 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
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/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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Check Concrete CMS versionLog into the CMS dashboard and navigate to Dashboard > System Information > Basics, or check the version displayed in the footer of admin pages. Alternatively, check the file /application/config/version.php for the $APP_VERSION variable.Affected if The installed version is lower than 8.5.6 (e.g., 8.5.5, 8.5.4, etc.)
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Verify external URL file upload feature is enabledNavigate to Dashboard > System Settings > Files > File Upload Options, or check the CMS configuration for any setting that allows uploading files from external URLs. Inspect the database table FileSystemPermissions or similar permission configurations if direct database access is available.Affected if The feature allowing file uploads from external URLs is enabled or permitted in the configuration
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Confirm authenticated users have file upload permissionsNavigate to Dashboard > Members > Groups, or check the permissions assigned to user groups. Look for groups with 'Add File' or 'Upload Files' permissions enabled, particularly for users who are not administrators.Affected if Non-admin authenticated users or specific user groups possess permissions to upload files to the CMS
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Test redirect behavior with internal IP addressAs an authenticated user with upload permissions, attempt to use the external URL upload feature with a URL that redirects to a private/internal IP (e.g., http://127.0.0.1 or http://192.168.x.x). If the CMS follows the redirect and attempts to process the content, the vulnerability is present.Affected if The CMS follows redirects from external URLs to internal/private network addresses and processes the content without blocking it
The environment is affected if running Concrete CMS version 8.5.5 or earlier AND the external URL upload feature is enabled AND authenticated users have file upload permissions, allowing potential SSRF attacks to internal resources.
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 · scoped8.5.6
Update Concrete CMS to the patched version. Additionally, implement strict URL validation that rejects redirects to internal/private IP addresses (RFC 1918 addresses) and disallows following redirects when fetching external URLs for file uploads.
Concrete CMS 8.5.6
- Create a complete backup of your Concrete CMS files and database before proceeding
- Download Concrete CMS version 8.5.6 from the official concretecms.org website
- Extract the downloaded package
- Upload the new 8.5.6 files to your server, overwriting the existing installation
- Run the automatic update process by accessing your site URL (or use the concrete CLI if available): php concrete/bin/concrete5 cms:install or visit /index.php/tools/required/install
- Verify the update was successful by logging into the dashboard and checking the version number
- Confirm the SSRF vulnerability is patched by testing that URL redirects to internal resources (localhost, private IPs) are now blocked during external URL uploads
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation12.0 h
- Testing8.0 h
- Review / QA4.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-40109 — 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-2021-40109 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
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