Twig Sandbox

The sandbox extension can be used to evaluate untrusted code.

Registering the Sandbox

$twig->addExtension(new \Twig\Extension\SandboxExtension($policy));

Configuring the Sandbox Policy

The sandbox security is managed by a policy instance, which must be passed to the SandboxExtension constructor.

By default, Twig comes with one policy class: \Twig\Sandbox\SecurityPolicy. This class allows you to allow-list some tags, filters, functions, and properties and methods on objects::

$tags = ['if'];
$filters = ['upper'];
$methods = [
    'Article' => ['getTitle', 'getBody'],
];
$properties = [
    'Article' => ['title', 'body'],
];
$functions = ['range'];
$policy = new \Twig\Sandbox\SecurityPolicy($tags, $filters, $methods, $properties, $functions);

With the above configuration, the security policy will only allow usage of the if tag, and the upper filter. Moreover, the templates will only be able to call the getTitle() and getBody() methods on Article objects, and the title and body public properties. Everything else won't be allowed and will generate a \Twig\Sandbox\SecurityError exception.

.. note::

As of Twig 3.14.1 (and on Twig 3.11.2), if the ``Article`` class implements
the ``ArrayAccess`` interface, the templates will only be able to access
the ``title`` and ``body`` attributes.

Note that native array-like classes (like ``ArrayObject``) are always
allowed, you don't need to configure them.

.. caution::

The ``extends`` and ``use`` tags, as well as the ``parent``, ``block``, and
``attribute`` functions are always allowed in a sandboxed template. That
behavior will change in 4.0 where they will need to be explicitly allowed
like any other tag or function. To opt-in to the 4.0 behavior now (so they
need to be allow-listed or get rejected), enable strict mode on the
security policy::

    $policy->setStrict(true);

Enabling the Sandbox

By default, the sandbox mode is disabled and should be enabled when including untrusted template code by using the sandboxed option of the include function:

.. code-block:: twig

{{ include('user.html.twig', sandboxed: true) }}

You can sandbox all templates by passing true as the second argument of the extension constructor::

$twig->addExtension(new \Twig\Extension\SandboxExtension($policy, true));

Allowed Operations Apply Transitively to Their Arguments

The method and property allow-lists only restrict attribute access written explicitly in the template (obj.foo and obj.foo()). Once an object is passed as an argument to an allowed tag, filter, function, or test, that operation can interact with it in any way PHP allows, without going through the sandbox allow-list.

This is especially easy to miss for implicit calls made through PHP interfaces. For example, allowing json_encode may expose public object properties and call JsonSerializable::jsonSerialize(); allowing sequence operations such as for, keys, slice, random, or join may call IteratorAggregate::getIterator(), Iterator methods, or Countable::count(); allowing cycle with an ArrayAccess value may call offsetGet(). None of these calls appear in the template source.

Only allow operations whose behavior is safe for the objects you expose to sandboxed templates. If this is not guaranteed, convert objects to plain arrays or scalars before passing them in.

Limiting Resource Usage

The sandbox prevents untrusted templates from reaching code, data, methods, or properties they shouldn't. It does not prevent a template from consuming CPU, memory, or wall-clock time, even under the strictest allow-list.

This is by design: any limit baked into Twig itself would be both arbitrary and trivial to work around, since there are many ways a template can burn resources (large ranges, nested loops, large string operations, recursive macros, expensive filters, deeply nested includes, and so on).

If you render untrusted templates, you should contain them at the process level rather than at the template engine level.

Accepting Callables Arguments

The Twig sandbox allows you to configure which functions, filters, tests and dot operations are allowed. Many of these calls can accept arguments. As these arguments are not validated by the sandbox, you must be very careful.

For instance, accepting a PHP callable as an argument is dangerous as it allows end user to call any PHP function (by passing a string) or any static methods (by passing an array). For instance, it would accept any PHP built-in functions like system() or exec()::

$twig->addFilter(new \Twig\TwigFilter('custom', function (callable $callable) {
    // ...
    $callable();
    // ...
}));

To avoid this security issue, don't type-hint such arguments with callable but use \Closure instead (not using a type-hint would also be problematic). This restricts the allowed callables to PHP closures only, which is enough to accept Twig arrow functions::

$twig->addFilter(new \Twig\TwigFilter('custom', function (\Closure $callable) {
    // ...
    $callable();
    // ...
}));

{{ people|custom(p => p.username|join(', ') }}

Any PHP callable can easily be converted to a closure by using the first-class callable syntax_.

.. _first-class callable syntax: https://www.php.net/manual/en/functions.first_class_callable_syntax.php