Concrete CMS Vulnerable to Deserialization of Untrusted Data

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.9 · 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 · score source

EPSS

• CVE-2026-8135: 0.47% estimated exploitation activity in the next 30 days; percentile 40.74 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 jsondecode(), 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). Concrete CMS thanks Nguyễn Văn Thiện https://github.com/Thien225409 for reporting this issue.

Published by the source

21 May 2026, 21:30 UTC

Last modified by the source

23 Jun 2026, 22:58 UTC

Source last checked

26 Oct 2026, 14:49 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-pv2v-6w2v-97x6

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# Patch Next advisory brief: CVE-2026-8135

Advisory: Concrete CMS Vulnerable to Deserialization of Untrusted Data
IDs: CVE-2026-8135, GHSA-pv2v-6w2v-97x6
Record: https://patchnext.com/advisories/cve-2026-8135 (revision 1, source checked 26 Oct 2026, 14:49 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: concrete5/concrete5 9.5.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist concrete5/concrete5: affected < 9.5.1; fixed in 9.5.1

## Suggested inspection (read-only)
- Packagist concrete5/concrete5: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on concrete5/concrete5.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-pv2v-6w2v-97x6 (GitHub-reviewed advisory GHSA-pv2v-6w2v-97x6 lists affected ranges: Packagist concrete5/concrete5 >=0 <9.5.1)

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