CVE-2026-26067: October CMS Information Disclosure Flaw

CVE-2026-26067 Overview

A server-side information disclosure vulnerability has been identified in October CMS, a popular Content Management System and web platform. The flaw exists in the handling of CSS preprocessor files (.less, .sass, .scss), allowing backend users with Editor permissions to craft malicious files that leverage the compiler's import functionality to read arbitrary files from the server. Notably, this vulnerability bypasses the cms.safe_mode protection, making it exploitable even in hardened configurations.

Critical Impact

Authenticated backend users can exploit CSS preprocessor import functionality to access sensitive server files, potentially exposing configuration files, credentials, and other confidential data.

Affected Products

• October CMS versions prior to 3.7.14
• October CMS versions prior to 4.1.10

Discovery Timeline

• 2026-04-21 - CVE CVE-2026-26067 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-26067

Vulnerability Analysis

This vulnerability represents a classic information disclosure flaw through an unexpected attack surface—CSS preprocessor compilation. When backend users with Editor permissions upload or modify .less, .sass, or .scss files, the server-side compiler processes these files to generate standard CSS. The vulnerability arises because the import functionality of these preprocessors can be abused to read arbitrary files from the server's filesystem.

The weakness is classified under CWE-184 (Incomplete List of Disallowed Inputs), indicating that the input validation mechanism fails to properly restrict which files can be imported during CSS preprocessing. Even with cms.safe_mode enabled—a configuration intended to restrict potentially dangerous operations—the CSS preprocessor import functionality remains exploitable.

Root Cause

The root cause stems from insufficient input validation in the CSS preprocessor file handling mechanism. The application fails to properly sanitize or restrict file paths used in @import directives within .less, .sass, and .scss files. This incomplete blocklist approach allows attackers to craft import statements that reference files outside the intended asset directories, enabling arbitrary file read operations on the server.

Attack Vector

The attack requires network access and authenticated backend access with Editor permissions. An attacker would upload or modify a CSS preprocessor file containing a malicious import directive that references a sensitive file path (such as /etc/passwd, configuration files, or database credentials). When the CMS processes the file for compilation, the contents of the targeted file are read and may be exposed through error messages, compiled output, or other observable behaviors.

The vulnerability mechanism leverages CSS preprocessor import statements to reference arbitrary filesystem paths. When the preprocessor compiles the stylesheet, it attempts to read and include the contents of the referenced file. This allows extraction of sensitive data such as server configuration files, application secrets, or database credentials. For complete technical details, see the GitHub Security Advisory.

Detection Methods for CVE-2026-26067

Indicators of Compromise

• Unusual .less, .sass, or .scss files containing @import directives with absolute filesystem paths
• Backend audit logs showing Editor users uploading or modifying CSS preprocessor files with suspicious content
• CSS preprocessor compilation errors referencing unexpected file paths (e.g., /etc/passwd, config/, .env)
• Abnormal access patterns to backend theme or asset management features

Detection Strategies

• Monitor file upload activities for CSS preprocessor files containing path traversal sequences or absolute paths
• Implement file content scanning for @import directives referencing sensitive system locations
• Review backend user activity logs for repeated modifications to stylesheet assets
• Deploy web application firewall (WAF) rules to detect path traversal patterns in uploaded file content

Monitoring Recommendations

• Enable verbose logging for CSS preprocessor compilation operations
• Alert on compilation errors that reference files outside the expected asset directories
• Monitor for unusual file read operations during theme asset processing
• Implement integrity monitoring on sensitive configuration files that could be targeted

How to Mitigate CVE-2026-26067

Immediate Actions Required

• Upgrade October CMS to version 3.7.14 or 4.1.10 immediately
• Audit backend user accounts and review Editor permission assignments
• Review recently modified .less, .sass, and .scss files for suspicious import statements
• Temporarily restrict Editor permissions for untrusted backend users until patches are applied

Patch Information

October CMS has released security patches addressing this vulnerability. Users running version 3.x should upgrade to 3.7.14 or later. Users running version 4.x should upgrade to 4.1.10 or later. The fix implements proper input validation and sanitization for CSS preprocessor import directives, preventing arbitrary file read operations. For detailed patch information, see the GitHub Security Advisory.

Workarounds

• Restrict Editor permissions to only trusted backend users until the patch can be applied
• Disable CSS preprocessor compilation if not required for your deployment
• Implement additional file validation at the web server level to block path traversal in uploaded files
• Monitor and audit all CSS preprocessor file modifications through backend activity logs

# Update October CMS using Composer
composer update october/cms

# Verify the installed version
php artisan october:version