CVE-2026-18426 Missing authorization in Concrete CMS
Authenticated editors can modify unauthorized Express Forms due to missing block-level permission checks, potentially leading to stored XSS.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-18426?
CVE-2026-18426 is a low-severity missing authorization vulnerability in Concretecms Concrete Cms, published on 15 Sep 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link.
Official description
Concrete CMS 9.0.0 through 9.5.2 did not enforce a block-level edit-permission check on the Express Form block's control-management actions, which relied solely on CSRF token validation. Because the token is bound to the user and action rather than to a specific block, page, or form, an authenticated user with edit access to one Express Form could reuse a validly obtained token to add, modify, or delete controls on Express Forms they were not authorized to edit, including injecting a control whose value is later rendered as HTML to achieve stored XSS. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.0 with vector CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Yat Wu for reporting.
Weakness: CWE-862
Which versions are affected by CVE-2026-18426?
It affects 1 product.
Products include Concretecms Concrete Cms.
How do I fix CVE-2026-18426?
No fix has been published for CVE-2026-18426 yet. Limit exposure of the affected software until one is available.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-18426 being exploited?
There is no known exploitation of CVE-2026-18426 in the wild and no public exploit code.
Other vulnerabilities in Concretecms Concrete Cms
- CVE-2023-48648 Concrete CMS allows unauthorized access because directories can be created with insecure permissions
- CVE-2022-21829 Code injection in concrete CMS
- CVE-2021-22958 Server-Side Request Forgery vulnerability in concrete5
- CVE-2021-40098 Path traversal in n/a
- CVE-2026-8134 Concrete CMS Vulnerable to Relative Path Traversal
Sources
Also known as GHSA-g5mv-jcqh-ch66.