Twig: `template_from_string()` escapes a SourcePolicy-driven sandbox via synthesized template name

Description

When the sandbox is enabled selectively via SourcePolicyInterface (and not globally), a sandboxed template that is allowed to call template_from_string and include can render an arbitrary inner template with no security policy enforcement.

Environment::createTemplate() compiles the inner string under a synthesized name (__string_template__), so a name/path-based SourcePolicy returns false for it, and the inner template's checkSecurity() becomes a no-op. From a template the integrator believes is sandboxed, an attacker can use any tag/filter/function (including constant() to read secrets, or |map("system") to execute shell commands).

Resolution

This is a configuration trap rather than a code bug: there is no legitimate use case for exposing template_from_string to untrusted template authors, and propagating the parent sandbox state through template_from_string would require invasive changes to SourcePolicyInterface semantics with their own risks.

Starting with Twig 3.26.0, the documentation and the PHPDoc of StringLoaderExtension::templateFromString() explicitly warn against allowing template_from_string in a sandboxed environment (i.e. listing it in a SecurityPolicy allowed-functions list). Integrators using a SourcePolicyInterface MUST NOT allow template_from_string in their allowed functions; the safest option is not to register StringLoaderExtension at all when a sandbox is in use.

Credits

Twig would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue.