CVE-2026-81910
Received Received - Intake

Server-Side Template Injection in Concrete CMS

Publication date: 2026-09-11

Last updated on: 2026-09-11

Assigner: ConcreteCMS

Description
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.

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Meta Information

Published
2026-09-11
Last Modified
2026-09-11
Generated
2026-09-12
AI Q&A
2026-09-11
EPSS Evaluated
N/A
NVD
CVE-2026-81910
EUVD
EUVD-2026-76193

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
concrete_cms concrete_cms From 9 (inc) to 9.5.2 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
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.

Attack-Flow Graph

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Executive Summary

Concrete CMS versions 9 through 9.5.2 are vulnerable to Server-Side Template Injection (SSTI) in the Theme Customizer. Attackers with Theme Customization permission can inject malicious LESS directives via unvalidated style values like color channels. This allows reading arbitrary files on the server and accessing internal network resources through PHP stream wrappers.

Detection Guidance

Check Concrete CMS versions 9 through 9.5.2 for Theme Customizer usage. Inspect server logs for unusual LESS or CSS file generation patterns. Look for unauthorized file reads or internal network requests in access logs.

Impact Analysis

An attacker could exploit this to read sensitive files such as database credentials, private keys, and other application secrets. The compiled malicious CSS is stored in a publicly accessible cache, exposing this data. It may also enable server-side request forgery attacks.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, potentially violating GDPR (data protection) and HIPAA (health information privacy) by exposing personal or protected health information. Compliance may be compromised due to data breaches.

Mitigation Strategies

Upgrade Concrete CMS to a version beyond 9.5.2 immediately. Disable Theme Customizer permissions for untrusted users. Review and restrict file access permissions on the server. Monitor CSS cache files for unexpected content.

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