# Twig Sandbox

The `sandbox` extension can be used to evaluate untrusted code.

## Registering the Sandbox

Register the `SandboxExtension` extension via the `addExtension()` method:

    $twig->addExtension(new \Twig\Extension\SandboxExtension($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'];
    $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.

Note

The `allowedTests` argument is available since Twig 3.28 (in earlier versions all tests were always allowed). Most built-in tests (`empty`, `defined`, `even`, `same as`, `iterable`, etc.) are always allowed and do not need to be listed. Only custom tests and the built-in `constant` test must be allow-listed like filters and functions.

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, the `parent`, `block`, and `attribute` functions, the `constant` test, and any custom test 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, filter, function, or test. 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);

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

Added in version 3.28: The `always_allowed_in_sandbox` option for filters, functions, and tests, and the `isAlwaysAllowedInSandbox()` method for token parsers, were added in Twig 3.28.

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`:

    $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`:

    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.

### Criteria for Marking an Item as Always Allowed

Only mark a callable or tag as always allowed when all the following conditions hold:

- No new capability. The item must not expose anything beyond what the sandbox already accepts. Pure value predicates (`is even`, `is empty`), pure value transformations (`upper`, `trim`, `abs`), and pure control flow (`if`, `for`, `set`) qualify.

- No PHP runtime access. The item must not read arbitrary PHP constants, call arbitrary classes or functions, instantiate objects from user-controlled names, or otherwise reach into the PHP runtime. This rules out `constant`, `enum`, `invoke`, and similar.

- No callable arguments. The item must not accept a callable parameter it dispatches to. This rules out higher-order operations like `map`, `filter`, `reduce`, `find`, `sort`, and `column`: applications may have deliberate reasons to forbid those, and they need the policy gate to do so.

- No cross-template resolution. The item must not resolve template names at runtime or pivot through the loader. This rules out `include`, `extends`, `embed`, `use`, `import`, `from`, `source`, and `template_from_string`.

- No output-safety bypass. The item must not let the template declare its own output safe. This rules out `raw`.

- No dedicated introspection or debugging surface. The item must not be intended to dump arbitrary object internals or call user-defined serialization hooks. This rules out `json_encode` and `dump`.

- No side effects on the PHP environment. The item must not flush output buffers, trigger deprecations, or otherwise affect global state. This rules out `flush` and `deprecated`.

- Deterministic output. The item must return the same value for the same arguments across renders. Applications that rely on sandboxed templates being reproducible (for caching, content hashing, golden-output tests, or audit comparisons) lose that property if a template can pull from the PHP random number generator without the policy opting in. This rules out `random` and `shuffle`: applications that want them can still allow-list them explicitly.

Note that several allowed items will still interact with PHP interfaces on objects passed as arguments (`Countable::count()`, `IteratorAggregate::getIterator()`, `Stringable::__toString()` on iterated items). That transitive behavior is documented separately under Allowed Operations Apply Transitively to Their Arguments and is considered an accepted property of the sandbox model. The criteria above are about what the item itself exposes, not about how its arguments behave.

### Built-ins That Will Be Always Allowed in 4.0

The following Twig built-ins meet the criteria above and will have the `always_allowed_in_sandbox` flag set in Twig 4.0. They still need to be explicitly allow-listed in 3.x.

- Tags: `apply`, `block`, `do`, `for`, `guard`, `if`, `macro`, `set`, `types`, `with`.

- Filters: `abs`, `batch`, `capitalize`, `convert_encoding`, `default`, `e`, `escape`, `first`, `format`, `join`, `keys`, `last`, `length`, `lower`, `merge`, `nl2br`, `number_format`, `replace`, `reverse`, `round`, `slice`, `split`, `striptags`, `title`, `trim`, `upper`, `url_encode`.

- Functions: `cycle`, `max`, `min`.

When upgrading to 4.0, you can drop these names from your `SecurityPolicy` allow-lists. Leaving them in is harmless: listing a name that is always allowed has no effect.

The corresponding built-in tests (`defined`, `divisible by`, `empty`, `even`, `iterable`, `mapping`, `none`, `null`, `odd`, `same as`, `sequence`, `true`) are already flagged as always allowed since Twig 3.28, so they never need to be allow-listed. This is safe because tests were never enforced by the sandbox before 3.28: flagging them keeps existing templates working unchanged. The `constant` test is the exception: it reaches into the PHP runtime, so it is not always allowed and must be allow-listed (it is still implicitly allowed in 3.x with a deprecation, and rejected in 4.0).

## 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:

    {{ 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()`; 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()`:

    $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.