Craft CMS versions >= 5.0.0-RC1 before 5.10.6 and >= 4.0.0-RC1 before 4.18.2 interpolate environment variables and secrets (via ${ENV_VAR} strings in the elementId parameter) into Twig templates before rendering, even when the Twig sandbox is enabled. An authenticated attacker with control panel access can render a malicious sandboxed Twig template and, using a blind error-based technique across many requests, incrementally leak arbitrary environment variables and secrets. These can be abused to forge sessions (via CRAFT_SECURITY_KEY), escalate privileges, and steal database, SMTP, API, or blob storage credentials. Fixed in 5.10.6 and 4.18.2.