Twig Sandbox

Rendering Untrusted Templates

The recommended way to render untrusted templates is the Twig\Sandbox\Sandbox class, which implements SandboxInterface. Type-hint SandboxInterface when injecting a sandbox into an application service. A Sandbox takes ownership of an environment crafted specifically for it and renders everything through it in sandbox mode. Being in full control of that environment, you decide exactly what untrusted templates can reach: its loader defines which templates exist, the extensions, filters, functions, tests, and globals you register on it define which capabilities exist, and the security policy defines what is allowed to execute:

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use Twig\Environment;
use Twig\Extra\Intl\IntlExtension;
use Twig\Loader\ArrayLoader;
use Twig\Sandbox\Sandbox;
use Twig\Sandbox\SecurityPolicy;

// craft an environment dedicated to untrusted templates
$env = new Environment(new ArrayLoader($untrustedTemplates), [
    'cache' => '/path/to/sandbox/cache',
]);
// register the capabilities untrusted templates may use
$env->addExtension(new IntlExtension());

$policy = new SecurityPolicy(
    allowedTags: ['if'],
    allowedFilters: ['upper', 'escape'],
);
$policy->setStrict(true);

$sandbox = new Sandbox($env, $policy);

// render a template known to the environment loader
echo $sandbox->render('newsletter.twig', ['name' => 'Fabien']);

// render an untrusted template held as a string
echo $sandbox->createTemplate($userTemplate)->render(['name' => 'Fabien']);

The environment must be dedicated to the sandbox: build a fresh environment and pass it before its first use (the constructor throws a LogicException otherwise). In particular, never pass your main application environment: all your application templates would suddenly be rendered in sandbox mode. Keeping the two environments separate also guarantees isolation in both directions: the sandbox cannot load or affect application templates, and application renders happening while a sandboxed render is in flight are not sandboxed.

A SecurityPolicy passed to the sandbox must be strict (call setStrict(true)) so it behaves the same way in Twig 3.x and 4.0; the constructor throws a LogicException otherwise.

Everything rendered through a Sandbox is sandboxed: render(), display(), and stream() render a template from the environment loader by name; renderBlock(), displayBlock(), and streamBlock() render a single block of such a template; createTemplate() turns a string into a sandboxed template. Templates included by a sandboxed template are sandboxed as well.

Data is passed through the render context (or registered as globals on the environment you crafted); the policy governs any method or property access on those values either way.

Rendering From a Trusted Template

To render an untrusted template from a trusted template, use the render_sandboxed() function.

Using the Sandbox Extension Directly

Before the Sandbox class existed, sandboxing was configured by registering the SandboxExtension on the environment via the addExtension() method:

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

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

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

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

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

Configuring the Sandbox Policy

The security policy is enforced the same way whether templates are rendered through a Sandbox or through the SandboxExtension directly.

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:

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

With the above configuration, the security policy will only allow usage of the if tag, the upper filter, and the my_test test (on top of the built-in tests that are always allowed, see below). 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.

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$methods = [
    'Article' => ['slug'],
];

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$policy->setStrict(true);

Marking Filters, Functions, Tests, and Tags as Always Allowed

Some filters, functions, tests, and tags are inherently safe and should always be usable in sandboxed templates without forcing every policy to allow-list them. Mark such callables by setting the always_allowed_in_sandbox option to true:

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$twig->addFilter(new \Twig\TwigFilter('upper', 'strtoupper', [
    'always_allowed_in_sandbox' => true,
]));

$twig->addFunction(new \Twig\TwigFunction('max', 'max', [
    'always_allowed_in_sandbox' => true,
]));

$twig->addTest(new \Twig\TwigTest('even', null, [
    'always_allowed_in_sandbox' => true,
]));

For tags, override isAlwaysAllowedInSandbox() on your token parser to return true:

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final class MyTagTokenParser extends \Twig\TokenParser\AbstractTokenParser
{
    public function isAlwaysAllowedInSandbox(): bool
    {
        return true;
    }

    // ...
}

Marked filters, functions, tests, and tags are skipped by the sandbox security check entirely, so they incur no runtime overhead, and they do not need to be listed in the SecurityPolicy allow-lists.

The sandbox assumes that attackers control template source, not the Twig environment, registered extensions, runtime configuration, security policy, custom escaping strategies, or context values passed by the application. Treat those application-provided pieces as trusted. If a callable or a value is not safe for untrusted template authors, don't register or expose it in the sandboxed environment.

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(); allowing url_encode on arrays may expose public object properties through PHP's query-string serialization; allowing max or min may compare objects by their public properties. 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():

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$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:

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$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.