Concrete CMS up to 9.5.2 Theme Customizer special elements in template engine

Summaryinfo

A vulnerability marked as problematic has been reported in Concrete CMS up to 9.5.2. Affected is an unknown function of the component Theme Customizer. This manipulation causes special elements in template engine. This vulnerability is handled as CVE-2026-81910. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability has been found in Concrete CMS up to 9.5.2 and classified as problematic. Affected by this vulnerability is an unknown code of the component Theme Customizer. The manipulation with an unknown input leads to a special elements in template engine vulnerability. The CWE definition for the vulnerability is CWE-1336. The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine. As an impact it is known to affect confidentiality. The summary by CVE is:

Concrete CMS 9 through 9.5.2 is vulnerable to Server-Side Template Injection (SSTI) in Theme Customizer via Unvalidated Style Values. Values submitted through the customizer (color channels and other style properties handled by ColorStyle and sibling Style classes such as FontFamilyStyle and ImageStyle) are interpolated into server-compiled LESS source without neutralization of LESS syntax, allowing a user with the Theme Customization permission to inject arbitrary LESS directives. By injecting the @import (inline) directive, an attacker can read arbitrary files on the server and reach internal network resources through PHP stream wrappers. The compiled output, including any disclosed file contents, is written to the site's publicly served CSS cache, exposing database credentials, private keys, and other application secrets, and enabling server-side request forgery. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 5.9 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Yonatan Drori from Tenzai for reporting.

It is possible to read the advisory at documentation.concretecms.org. This vulnerability is known as CVE-2026-81910 since 08/27/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation needs additional levels of successful authentication. The technical details are unknown and an exploit is not publicly available. The attack technique deployed by this issue is T1221 according to MITRE ATT&CK.

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

Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Type

• Content Management System

Name

• Concrete CMS

Version

• 9.5.0
• 9.5.1
• 9.5.2

License

• open-source

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

Be aware that VulDB is the high quality source for vulnerability data.

Discussion