diff --git a/system/core/compat/hash.php b/system/core/compat/hash.php
index 477535d..1595455 100644
--- a/system/core/compat/hash.php
+++ b/system/core/compat/hash.php
@@ -174,9 +174,56 @@
 		}
 
 		$hash_length = strlen(hash($algo, NULL, TRUE));
-		if (empty($length))
+		empty($length) && $length = $hash_length;
+
+		// Pre-hash password inputs longer than the algorithm's block size
+		// (i.e. prepare HMAC key) to mitigate potential DoS attacks.
+		static $block_sizes;
+		empty($block_sizes) && $block_sizes = array(
+			'gost' => 32,
+			'haval128,3' => 128,
+			'haval160,3' => 128,
+			'haval192,3' => 128,
+			'haval224,3' => 128,
+			'haval256,3' => 128,
+			'haval128,4' => 128,
+			'haval160,4' => 128,
+			'haval192,4' => 128,
+			'haval224,4' => 128,
+			'haval256,4' => 128,
+			'haval128,5' => 128,
+			'haval160,5' => 128,
+			'haval192,5' => 128,
+			'haval224,5' => 128,
+			'haval256,5' => 128,
+			'md2' => 16,
+			'md4' => 64,
+			'md5' => 64,
+			'ripemd128' => 64,
+			'ripemd160' => 64,
+			'ripemd256' => 64,
+			'ripemd320' => 64,
+			'salsa10' => 64,
+			'salsa20' => 64,
+			'sha1' => 64,
+			'sha224' => 64,
+			'sha256' => 64,
+			'sha384' => 128,
+			'sha512' => 128,
+			'snefru' => 32,
+			'snefru256' => 32,
+			'tiger128,3' => 64,
+			'tiger160,3' => 64,
+			'tiger192,3' => 64,
+			'tiger128,4' => 64,
+			'tiger160,4' => 64,
+			'tiger192,4' => 64,
+			'whirlpool' => 64
+		);
+
+		if (isset($block_sizes[$algo]) && strlen($password) > $block_sizes[$algo])
 		{
-			$length = $hash_length;
+			$password = hash($algo, $password, TRUE);
 		}
 
 		$hash = '';