CVE-2026-8135
HighCVSS 7.2Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
Concrete CMS version 9.5.0 and below is vulnerable to Remote Code Execution due to insecure deserialization in the ExpressEntryList block controller. A rogue administrator with privileges to add blocks can bypass the protection mechanism (_fromCIF === true) by leveraging the REST API, which parses requests using json_decode(), where the string "true" is evaluated as a strict PHP Boolean(true). This allows injection of a malicious serialized payload into the block's filterFields column, which is executed when the block data is viewed or edited, leading to full server takeover.
Risk Assessment
An attacker with administrator privileges could gain full control over the server, potentially leading to data theft, content modification, or further attacks on the infrastructure.
Recommendation
Immediately update Concrete CMS to version 9.5.1 or later, which contains a fix for this vulnerability.
Other vulnerabilities in Concrete CMS
See all- CVE-2026-10721High
Vulnerability in Concrete CMS below version 9.5.2 allows an unauthenticated attacker to perform PHP object injection via unserialize() calls in Permission, Cache, and Search components. The attack requires a malicious payload to be placed in the database.
- CVE-2026-7888High
Concrete CMS below version 9.5.2 is vulnerable to PHP Object Injection via unserialize() calls in the Workflow, Form block, and File/Set components that lack the allowed_classes restriction. An unauthenticated attacker may trigger arbitrary PHP object instantiation if a malicious serialized payload has been placed in the database.
- CVE-2026-8353Medium
Concrete CMS versions 9.0 to 9.5.0 are vulnerable to Stored XSS via page name in the Atomik theme. A rogue editor can inject arbitrary JavaScript that executes in the context of any authenticated user visiting the affected account pages.
- CVE-2026-8347Medium
Concrete CMS 9.5.0 and below is vulnerable to IDOR + wrong-authorization-level in the Express association Reorder dialog. This can cause Cross-entity state tampering with view-only permission on one entry. To be affected, a website has to be using express and relying on express entity ordering.
- CVE-2026-8340Medium
Concrete CMS version 9.5.0 and below is vulnerable to CSRF via Backend\File::approveVersion. A victim with edit_file_contents permission can be tricked into publishing an attacker-chosen previously-uploaded file version (downgrade or activation of an unpublished version).
- CVE-2026-8435Medium
Concrete CMS 9 before version 9.5.0 is vulnerable to Cross-Site Request Forgery (CSRF) in the approveVersion() controller. An attacker can exploit this vulnerability to perform unauthorized actions on behalf of an administrator.
- CVE-2026-8434High
Concrete CMS 9 before 9.5.0 is vulnerable to Cross-Site Request Forgery (CSRF) at concrete/controllers/backend/file rescanMultiple(). An attacker can trick an administrator into performing unwanted file operations.
- CVE-2026-8433High
Concrete CMS 9 before 9.5.0 is vulnerable to Cross-Site Request Forgery (CSRF) at concrete/controllers/backend/file rescan(). An attacker can trick an administrator into rescanning a file.
- CVE-2026-8432High
Concrete CMS 9 before 9.5.0 is vulnerable to Cross-Site Request Forgery (CSRF) at concrete/controllers/backend/file star(). An attacker can trick an administrator into starring a file.
- CVE-2026-8427High
Concrete CMS 9 before 9.5.0 is vulnerable to CSRF in the removeFavoriteFolder($id) endpoint. An attacker can exploit this to perform unauthorized actions on behalf of the victim.
Original NVD description (English source)
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