diff --git a/system/Cache/Handlers/FileHandler.php b/system/Cache/Handlers/FileHandler.php
index 9c00998c1f85..c11f44a9224a 100644
--- a/system/Cache/Handlers/FileHandler.php
+++ b/system/Cache/Handlers/FileHandler.php
@@ -14,6 +14,7 @@
 namespace CodeIgniter\Cache\Handlers;
 
 use CodeIgniter\Cache\Exceptions\CacheException;
+use CodeIgniter\Exceptions\InvalidArgumentException;
 use CodeIgniter\I18n\Time;
 use Config\Cache;
 use FilesystemIterator;
@@ -76,6 +77,32 @@ public function initialize(): void
     {
     }
 
+    /**
+     * Validates a cache key, ensuring it contains no directory traversal sequences
+     * or path separators, regardless of the configured reservedCharacters setting.
+     *
+     * @param mixed  $key    The key to validate
+     * @param string $prefix Optional prefix to include in length calculations
+     *
+     * @throws InvalidArgumentException When $key is not valid or contains path separators
+     */
+    public static function validateKey($key, $prefix = ''): string
+    {
+        if (! is_string($key)) {
+            throw new InvalidArgumentException('Cache key must be a string');
+        }
+
+        if (strpbrk($key, "/\\\0") !== false || str_contains($key, '..')) {
+            throw new InvalidArgumentException('Cache key cannot contain directory separators or traversal sequences.');
+        }
+
+        if (strpbrk($prefix, "/\\\0") !== false || str_contains($prefix, '..')) {
+            throw new InvalidArgumentException('Cache prefix cannot contain directory separators or traversal sequences.');
+        }
+
+        return parent::validateKey($key, $prefix);
+    }
+
     public function get(string $key): mixed
     {
         $key  = static::validateKey($key, $this->prefix);
diff --git a/tests/system/Cache/Handlers/FileHandlerTest.php b/tests/system/Cache/Handlers/FileHandlerTest.php
index 310b5a7ebce7..a656b295fddf 100644
--- a/tests/system/Cache/Handlers/FileHandlerTest.php
+++ b/tests/system/Cache/Handlers/FileHandlerTest.php
@@ -16,6 +16,7 @@
 use CodeIgniter\Cache\CacheFactory;
 use CodeIgniter\Cache\Exceptions\CacheException;
 use CodeIgniter\CLI\CLI;
+use CodeIgniter\Exceptions\InvalidArgumentException;
 use CodeIgniter\I18n\Time;
 use Config\Cache;
 use PHPUnit\Framework\Attributes\DataProvider;
@@ -504,4 +505,55 @@ public function testReconnect(): void
     {
         $this->assertTrue($this->handler->reconnect());
     }
+
+    #[DataProvider('provideValidateKeyRejectsTraversalAndSeparatorsEvenWithEmptyReservedCharacters')]
+    public function testValidateKeyRejectsTraversalAndSeparatorsEvenWithEmptyReservedCharacters(string $key): void
+    {
+        $config                     = config('Cache');
+        $originalReservedCharacters = $config->reservedCharacters;
+
+        // Multi-store setups often empty this setting to share keys between
+        // handlers, which must not weaken the file handler's own validation.
+        $config->reservedCharacters = '';
+
+        try {
+            $this->expectException(InvalidArgumentException::class);
+            $this->expectExceptionMessage('Cache key cannot contain directory separators or traversal sequences.');
+
+            FileHandler::validateKey($key);
+        } finally {
+            $config->reservedCharacters = $originalReservedCharacters;
+        }
+    }
+
+    /**
+     * @return iterable<string, array{string}>
+     */
+    public static function provideValidateKeyRejectsTraversalAndSeparatorsEvenWithEmptyReservedCharacters(): iterable
+    {
+        yield 'parent directory' => ['../evil'];
+
+        yield 'nested traversal' => ['sub/../../evil'];
+
+        yield 'forward slash separator' => ['sub/key'];
+
+        yield 'backslash separator' => ['sub\\key'];
+
+        yield 'parent traversal only' => ['..'];
+
+        yield 'null byte' => ["sub\0key"];
+    }
+
+    public function testValidateKeyRejectsTraversalInPrefix(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Cache prefix cannot contain directory separators or traversal sequences.');
+
+        FileHandler::validateKey('valid_key', '../');
+    }
+
+    public function testValidateKeySuccess(): void
+    {
+        $this->assertSame('valid_key', FileHandler::validateKey('valid_key'));
+    }
 }