---
title: "CVE-2026-8134: Concrete CMS Vulnerable to Relative Path Traversal"
url: https://erroneo.us/security/cve/CVE-2026-8134/
description: "CVE-2026-8134 (concrete5/concrete5 (Composer (PHP))): Concrete CMS 9.5.0 and below fails to sanitize path traversal sequences in the…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-05-21
date_modified: 2026-06-23
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-8134"
kev: false
cvss: 9.4
epss: 0.01113
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-8134: Concrete CMS Vulnerable to Relative Path Traversal

**Signal:** Critical, preconditions apply. Severe if exploited, but exploitation needs conditions that may not apply to you. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 1.1% , percentile 66: FIRST’s model estimates a 1.1% chance of exploitation activity in the next 30 days, higher than 66% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Needs an account (high (admin)). Exposure depends on who can log in.
- Only applies if your dependency tree includes concrete5/concrete5 (Composer (PHP)) < 9.5.1, fixed in 9.5.1 . Check your lockfile, including transitive dependencies.

## What it is

Concrete 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.

## Affected packages

- Composer (PHP) `concrete5/concrete5` < 9.5.1 → fixed: 9.5.1

## Affected products (CVE record)

- Concrete CMS Concrete CMS: 5.0 <= 9.5.0

## Severity

- ConcreteCMS CVSS 4.0: 9.4 `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`
- GitHub CVSS 4.0: 9.4 `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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 1.11% probability of exploitation activity in the next 30 days; percentile 66.

## Sources

- [documentation.concretecms.org](https://documentation.concretecms.org/9-x/developers/introduction/version-history/951-release-notes)
- [GitHub Advisory GHSA-645j-cm4x-3xvw](https://github.com/advisories/GHSA-645j-cm4x-3xvw)
- [CVE.org record CVE-2026-8134](https://www.cve.org/CVERecord?id=CVE-2026-8134)
- [NVD entry CVE-2026-8134](https://nvd.nist.gov/vuln/detail/CVE-2026-8134)

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Source page: https://erroneo.us/security/cve/CVE-2026-8134/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt