Description

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

Environment::createTemplate() compiles the inner string under a synthesized name (stringtemplate<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 templatefromstring to untrusted template authors, and propagating the parent sandbox state through templatefromstring 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 templatefromstring in a sandboxed environment (i.e. listing it in a SecurityPolicy allowed-functions list). Integrators using a SourcePolicyInterface MUST NOT allow templatefromstring 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.

Description

When using the sandbox with a SourcePolicyInterface, Twig does not always apply the sandbox restriction that forbids non-Closure callbacks for callback-accepting filters.

The issue affects the sort, filter, map, and reduce filters.

In the affected versions, the runtime check that rejects non-Closure callbacks in sandbox mode does not use the current template Source. As a result, when the sandbox is enabled through a source policy instead of being enabled globally, Twig can incorrectly treat the current execution as non-sandboxed for these callback checks.

This can allow user-controlled templates to pass arbitrary PHP callables to callback-accepting filters even though the template is being sandboxed through a source policy.

The issue happens when all these conditions are met:

- The sandbox is not enabled globally; - A SourcePolicyInterface enables the sandbox for the rendered template; - The template uses one of the sort, filter, map, or reduce filters; - The callback is not a Closure.

Resolution

The patch makes callback sandbox checks source-aware by propagating the current template Source to callback-accepting filters and using it when deciding whether sandbox restrictions apply.

Credits

We would like to thank XavLim and Wade Sparks for reporting the issue and Fabien Potencier for fixing the issue.