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Concrete CmsApplication · Concretecms

CVE-2026-8135

HIGH · 7.2 CVSS v3.1 Published 2026-05-21
Fix available
A fix is available. Upgrade to after 9.5.0 or later.
See remediation →
78/100
Remediation priority · High
Remotely reachable Zero-click

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
Concrete CMS 9.5.0 and below is vulnerable to Remote Code Execution due to insecure deserialization occurring in the ExpressEntryList block controller. An rogue administrator with privileges to add blocks to an area can bypass the intended protection mechanism (_fromCIF === true), which normally restricts malicious inputs over form POST requests, by leveraging the REST API functionality. Because the REST API parses requests using json_decode(), the string "true" is evaluated as a strict PHP Boolean(true).  This bypass allows the attacker to inject a malicious serialized payload  into the block's filterFields database column. The payload will subsequently be executed when the block's data is viewed or edited by an administrator leading to complete server takeover (RCE).The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 8.9 with a vector of CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H.  Thanks Nguyễn Văn Thiện https://github.com/Thien225409  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 confidence

Concrete CMS versions 9.5.0 and below have an insecure deserialization vulnerability in the ExpressEntryList block controller. A privileged administrator can bypass the _fromCIF protection mechanism by using the REST API, which evaluates the string 'true' as a strict PHP Boolean. This allows injection of a malicious serialized payload into the filterFields database column, which executes when the block is viewed or edited, achieving RCE.

MitigationUpgrade Concrete CMS to the latest patched version. As a temporary workaround, restrict administrator privileges and monitor/block suspicious REST API requests targeting ExpressEntryList blocks.

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
Concrete CmsApplication
Affected:<= 9.5.0

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
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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify Concrete CMS version
    Check the version by viewing the source code version file (typically at /concrete/config/version.php or check the dashboard footer for version display), or query the database table App\\Version to retrieve the installed version number.
    Affected if The installed version is 9.5.0 or any version lower than 9.5.0.
  2. Verify REST API is enabled
    Log into the Concrete CMS dashboard and navigate to Dashboard > System & Settings > API > REST API, or inspect the configuration file /application/config/database.php or /application/config/rest_api.php to confirm whether REST API endpoints are enabled.
    Affected if The REST API is enabled and accessible to administrator accounts.
  3. Locate ExpressEntryList blocks
    Query the database Blocks table for blocks with btType or btID corresponding to ExpressEntryList, or inspect page areas through the dashboard for any ExpressEntryList block instances.
    Affected if Any ExpressEntryList blocks exist in the system.
  4. Inspect filterFields for serialized payloads
    Query the Blocks or BlockTypes database table for the filterFields column, specifically looking for serialized PHP objects or unexpected serialized data patterns (such as O: for objects, C: for custom, a: for arrays with suspicious class names). For example: SELECT * FROM Blocks WHERE btID = 'ExpressEntryList' AND filterFields LIKE '%O:%'
    Affected if The filterFields column contains any serialized PHP data, especially malicious serialized payloads injected by a rogue administrator.

A user is affected if they run Concrete CMS version 9.5.0 or lower AND have the REST API enabled AND possess ExpressEntryList blocks with suspicious serialized data in the filterFields column.

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 a release after 9.5.0
Interim mitigation

Upgrade Concrete CMS to the latest patched version. As a temporary workaround, restrict administrator privileges and monitor/block suspicious REST API requests targeting ExpressEntryList blocks.

Fix this in Concrete Cms Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.

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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-2026-8135 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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