Fix sandbox __toString bypass via Stringable + Traversable containers

CHANGELOG

@@ -7,7 +7,7 @@
  * Escape root profile name in `HtmlDumper`
  * Deprecate the `Twig\Sandbox\SourcePolicyInterface` interface with no replacement
  * Fix sandbox bypass in the "column" filter when sandboxing is enabled via `SourcePolicyInterface`
- * Fix sandbox `__toString` bypass via `Traversable` arguments to the `join` and `replace` filters
+ * Fix sandbox `__toString` bypass via `Traversable` arguments to the `join` and `replace` filters (also covers containers that implement both `Stringable` and `Traversable`)
  * Fix sandbox `__toString` bypass via the `in` and `not in` operators
 
 # 3.26.0 (2026-05-20)

src/Extension/SandboxExtension.php

@@ -132,17 +132,11 @@ final class SandboxExtension extends AbstractExtension
             return $obj;
         }
 
-        // A non-Stringable Traversable would later be materialised (e.g. by filters such as
-        // `join` or `replace`) and its elements coerced to string by PHP itself, bypassing
-        // the policy. Materialise it now and recursively check the contents.
-        if ($obj instanceof \Traversable && !$obj instanceof \Stringable && $this->isSandboxed($source)) {
-            $obj = iterator_to_array($obj);
-            $this->ensureToStringAllowedForArray($obj, $lineno, $source);
-
+        if (!$this->isSandboxed($source)) {
             return $obj;
         }
 
-        if ($obj instanceof \Stringable && $this->isSandboxed($source)) {
+        if ($obj instanceof \Stringable) {
             try {
                 $this->policy->checkMethodAllowed($obj, '__toString');
             } catch (SecurityNotAllowedMethodError $e) {
@@ -153,6 +147,25 @@ final class SandboxExtension extends AbstractExtension
             }
         }
 
+        // A Traversable would later be materialised (e.g. by filters such as `join`
+        // or `replace`) and its elements coerced to string by PHP itself, bypassing
+        // the policy. Materialise it now and recursively check the contents. This
+        // also applies to objects that implement both `Stringable` and `Traversable`:
+        // the `__toString` check above only validates the container's own coercion,
+        // not the elements yielded by `getIterator()`.
+        if ($obj instanceof \Traversable) {
+            $array = iterator_to_array($obj);
+            $this->ensureToStringAllowedForArray($array, $lineno, $source);
+
+            // Return the materialised array only when the object is not also
+            // Stringable, so that callers that rely on `__toString` (e.g. `{{ obj }}`)
+            // keep working. Plain consumers of iterables (join, replace, ...) call
+            // `iterator_to_array()` again, so the extra materialisation is benign.
+            if (!$obj instanceof \Stringable) {
+                return $array;
+            }
+        }
+
         return $obj;
     }
 

tests/Extension/SandboxTest.php

@@ -64,6 +64,8 @@ class SandboxTest extends TestCase
             'iterator' => new \ArrayIterator(['a', new FooObject()]),
             'iterator_map' => new \ArrayIterator(['__toString' => new FooObject()]),
             'iterator_nested' => new \ArrayIterator(['a', new \ArrayIterator(['b', new FooObject()])]),
+            'stringable_iterator' => new StringableTraversableObject(['a', new FooObject()]),
+            'stringable_iterator_map' => new StringableTraversableObject(['__toString' => new FooObject()]),
         ];
         self::$params['recursion'][] = &self::$params['recursion'];
         self::$params['recursion'][] = new FooObject();
@@ -1176,6 +1178,88 @@ EOF
         }
     }
 
+    /**
+     * @dataProvider getStringableTraversableBypassTemplates
+     */
+    public function testSandboxBlocksToStringInStringableTraversable(string $template)
+    {
+        $twig = $this->getEnvironment(
+            true,
+            [],
+            ['index' => $template],
+            [],
+            ['join', 'replace'],
+            ['Twig\Tests\Extension\StringableTraversableObject' => ['__tostring']],
+        );
+
+        try {
+            $twig->load('index')->render(self::$params);
+            $this->fail('Sandbox should block __toString on objects yielded by a Stringable+Traversable container, even when the container\'s own __toString is allowed.');
+        } catch (SecurityNotAllowedMethodError $e) {
+            $this->assertSame('Twig\Tests\Extension\FooObject', $e->getClassName());
+            $this->assertSame('__tostring', $e->getMethodName());
+        }
+    }
+
+    public static function getStringableTraversableBypassTemplates(): iterable
+    {
+        yield 'join' => ['{{ stringable_iterator|join(", ") }}'];
+        yield 'replace' => ['{{ "__toString"|replace(stringable_iterator_map) }}'];
+    }
+
+    /**
+     * @dataProvider getStringableTraversableBypassTemplates
+     */
+    public function testSourcePolicySandboxBlocksToStringInStringableTraversable(string $template)
+    {
+        $sourcePolicy = new class implements SourcePolicyInterface {
+            public function enableSandbox(Source $source): bool
+            {
+                return true;
+            }
+        };
+
+        $twig = $this->getEnvironment(
+            false,
+            [],
+            ['index' => $template],
+            [],
+            ['join', 'replace'],
+            ['Twig\Tests\Extension\StringableTraversableObject' => ['__tostring']],
+            [],
+            [],
+            $sourcePolicy,
+        );
+
+        try {
+            $twig->load('index')->render(self::$params);
+            $this->fail('Sandbox should block __toString on objects yielded by a Stringable+Traversable container under a SourcePolicyInterface-only sandbox.');
+        } catch (SecurityNotAllowedMethodError $e) {
+            $this->assertSame('Twig\Tests\Extension\FooObject', $e->getClassName());
+            $this->assertSame('__tostring', $e->getMethodName());
+        }
+    }
+
+    public function testSandboxAllowsPrintingStringableTraversableWhenToStringAllowed()
+    {
+        // Printing the container itself yields its `__toString()` value. The
+        // sandbox materialises the iterable to also policy-check the elements
+        // (some consumers like `join`/`replace` would coerce them too), so the
+        // inner items must not contain anything that violates the policy.
+        $twig = $this->getEnvironment(
+            true,
+            ['autoescape' => 'html'],
+            ['index' => '{{ obj }}'],
+            [],
+            ['escape'],
+            ['Twig\Tests\Extension\StringableTraversableObject' => ['__tostring']],
+        );
+
+        $params = ['obj' => new StringableTraversableObject(['a', 'b'])];
+
+        $this->assertSame('stringable-traversable', $twig->load('index')->render($params));
+    }
+
     public function testSourcePolicySandboxBlocksToStringInTraversableJoin()
     {
         $sourcePolicy = new class implements SourcePolicyInterface {
@@ -1722,3 +1806,24 @@ class ColumnObject
 {
     public $bar = 'bar';
 }
+
+// Implements both Stringable and Traversable: a sandbox policy may legitimately
+// allow the container's own `__toString`, but the elements yielded by
+// `getIterator()` must still be policy-checked when consumers (`join`, `replace`,
+// ...) materialise the iterable and coerce its contents to string.
+class StringableTraversableObject implements \IteratorAggregate, \Stringable
+{
+    public function __construct(private array $items)
+    {
+    }
+
+    public function __toString(): string
+    {
+        return 'stringable-traversable';
+    }
+
+    public function getIterator(): \Traversable
+    {
+        yield from $this->items;
+    }
+}