Control panel only API response emulation replaces "securityKey" and reaches server RCE

Overview

The app/process-api-response-headers action accepts fake Craft API response headers from any authenticated control panel user.

A non-admin user with only accessCp can place newline characters in X-Craft-Plugin-Licenses. Craft passes this value to its .env writer without removing the newlines.

This lets the user insert a new attacker-known CRAFT_SECURITY_KEY into .env. After Craft loads that key, the user can forge a signed login redirect containing a Twig system() callback.

This is different from the previous report. That report reads the existing key through a Twig sandbox bypass. This report replaces the key through fake API headers and CRLF injection. Fixing either source does not fix the other.

Affected

• craftcms/cms >= 5.0.0 < 5.11.2
• craftcms/cms >= 4.0.0 < 4.18.9

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.