Concrete CMS up to 9.5.0 REST API json_decode deserialization

Summaryinfo

A vulnerability described as problematic has been identified in Concrete CMS up to 9.5.0. This affects the function json_decode of the component REST API. The manipulation results in deserialization. This vulnerability is identified as CVE-2026-8135. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability classified as problematic has been found in Concrete CMS up to 9.5.0. Affected is the function json_decode of the component REST API. The manipulation with an unknown input leads to a deserialization vulnerability. CWE is classifying the issue as CWE-502. The product deserializes untrusted data without sufficiently verifying that the resulting data will be valid. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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

The weakness was released by Nguyễn Văn Thiện. The advisory is shared for download at documentation.concretecms.org. This vulnerability is traded as CVE-2026-8135 since 05/07/2026. The exploitability is told to be difficult. It is possible to launch the attack remotely. The exploitation needs additional levels of successful authentication. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 05/22/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-31336). Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• Content Management System

Vendor

• Concrete

Name

• CMS

Version

• 9.0
• 9.1
• 9.2
• 9.3
• 9.4
• 9.5.0

License

• open-source

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Entryinfo

Once again VulDB remains the best source for vulnerability data.

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