Twig is a template language for PHP. From 3.9.0 until 3.26.0, template_from_string() compiles an inner template under a synthesized __string_template__<hash> name that can fall outside a SourcePolicyInterface sandbox decision, allowing a sandboxed template that can call template_from_string and include to render an inner template without security policy enforcement. This issue is fixed in version 3.26.0.

Affected: Packagist: twig/twig

### 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__<hash>`), 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 wou…

Affected: Ubuntu:Pro:20.04:LTS: php-twig, Ubuntu:Pro:22.04:LTS: php-twig, Ubuntu:Pro:24.04:LTS: php-twig, Ubuntu:25.10: php-twig, Ubuntu:Pro:26.04:LTS: php-twig

Twig is a template language for PHP. From 3.9.0 until 3.26.0, template_from_string() compiles an inner template under a synthesized __string_template__<hash> name that can fall outside a SourcePolicyInterface sandbox decision, allowing a sandboxed template that can call template_from_string and include to render an inner template without security policy enforcement. This issue is fixed in version 3.26.0.