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

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• 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.

From the GitHub advisory

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. Concrete CMS thanks Yonatan Drori (Tenzai) for reporting this issue.

Weakness type: CWE-98, CWE-434, CWE-23.

Product: concrete5/concrete5 (Composer (PHP)) · Assigned by: ConcreteCMS · Published: 2026-05-21

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: high (admin)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• 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.

Timeline

• 2026-05-21 — CVE record published by ConcreteCMS
• 2026-05-21 — GitHub-reviewed advisory GHSA-645j-cm4x-3xvw published (critical)
• 2026-05-22 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. documentation.concretecms.org documentation.concretecms.org

Vulnerability records

1. GitHub Advisory GHSA-645j-cm4x-3xvw github.com
2. CVE.org record CVE-2026-8134 cve.org
3. NVD entry CVE-2026-8134 nvd.nist.gov

Critical Composer (PHP) advisories · Today’s Security Digest