CVE-2026-8134
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 · uneditedConcrete CMS 9.5.0 and below fails to sanitize path traversal sequences in the ptComposerFormLayoutSetControlCustomTemplate field when saving page type composer form layouts. An authenticated rogue administrator with composer form editing rights can exploit this to include arbitrary readable files on the server. Combined with the file uploader's extension-only validation (which permits PHP code in files saved with image extensions like .png), this can result in authenticated remote code execution. The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 9.4 with vector CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H Thanks Yonatan Drori (Tenzai) for reporting.
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 confidencePath traversal vulnerability in Concrete CMS's composer form layout save function allows an authenticated administrator to read arbitrary server files via the ptComposerFormLayoutSetControlCustomTemplate field. When chained with the file uploader's weak extension-only validation that permits PHP code in .png files, this enables authenticated remote code execution.
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<= 9.5.0CVSS 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
- High
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/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 installed Concrete CMS versionLocate the version number in the application dashboard (typically under Dashboard > System & Settings > About Concrete CMS) or check the composer.json file in the installation root directoryAffected if The installed version is 9.5.0 or any version lower than 9.5.0
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Identify administrators with composer form editing rightsReview user groups and permissions in Dashboard > Members > Groups, specifically checking for access to page type composer form layout editing capabilitiesAffected if Any authenticated administrator account exists with permissions to edit composer form layouts for page types
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Inspect file uploader extension validationCheck the file uploader configuration or validation logic to determine which file extensions are permitted for upload (particularly image extensions like .png, .jpg, .gif)Affected if The file uploader only validates file extensions without performing proper content-type or MIME type validation, allowing files with image extensions to contain PHP code
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Verify ptComposerFormLayoutSetControlCustomTemplate field usageExamine the page type composer form layout saving mechanism for the ptComposerFormLayoutSetControlCustomTemplate field, typically found in the core file handling or page type composer codeAffected if The ptComposerFormLayoutSetControlCustomTemplate field exists and accepts user input without sanitizing path traversal sequences (../ or similar)
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Check for presence of sensitive files in accessible pathsReview server file permissions and verify if the web server user has read access to sensitive files such as configuration files containing database credentials, environment files, or other system filesAffected if The application server process can read configuration or sensitive files outside the web root directory using path traversal
A user is affected if their Concrete CMS installation is version 9.5.0 or lower AND they have administrators with composer form editing permissions AND the file uploader permits image extensions without proper content validation.
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 to Concrete CMS version above 9.5.0. Additionally, implement strict server-side file type validation using magic byte inspection rather than extension-only checks, and apply the principle of least privilege to composer form editing permissions.
- Consultation4.0 h
- Implementation12.0 h
- Testing8.0 h
- Review / QA4.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-8134 — 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-2026-8134 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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