Backport HMAC authentication for CI_Session

1 files changed, 34 insertions, 22 deletions

diff --git a/system/libraries/Session.php b/system/libraries/Session.php
index 53b914fbd..89c699765 100644
--- a/system/libraries/Session.php
+++ b/system/libraries/Session.php
@@ -144,24 +144,36 @@ class CI_Session {
 			return FALSE;
 		}
 
-		// Decrypt the cookie data
-		if ($this->sess_encrypt_cookie == TRUE)
+		// HMAC authentication
+		if (($len = strlen($session) - 40) <= 0)
 		{
-			$session = $this->CI->encrypt->decode($session);
+			log_message('error', 'Session: The session cookie was not signed.');
+			return FALSE;
 		}
-		else
+
+		// Check cookie authentication
+		$hmac = substr($session, $len);
+		$session = substr($session, 0, $len);
+
+		// Time-attack-safe comparison
+		$hmac_check = hash_hmac('sha1', $session, $this->encryption_key);
+		$diff = 0;
+		for ($i = 0; $i < 40; $i++)
 		{
-			// encryption was not used, so we need to check the md5 hash
-			$hash	 = substr($session, strlen($session)-32); // get last 32 chars
-			$session = substr($session, 0, strlen($session)-32);
+			$diff |= ord($hmac[$i]) ^ ord($hmac_check[$i]);
+		}
 
-			// Does the md5 hash match?  This is to prevent manipulation of session data in userspace
-			if ($hash !==  md5($session.$this->encryption_key))
-			{
-				log_message('error', 'The session cookie data did not match what was expected. This could be a possible hacking attempt.');
-				$this->sess_destroy();
-				return FALSE;
-			}
+		if ($diff !== 0)
+		{
+			log_message('error', 'Session: HMAC mismatch. The session cookie data did not match what was expected.');
+			$this->sess_destroy();
+			return FALSE;
+		}
+
+		// Decrypt the cookie data
+		if ($this->sess_encrypt_cookie == TRUE)
+		{
+			$session = $this->CI->encrypt->decode($session);
 		}
 
 		// Unserialize the session array
@@ -659,20 +671,20 @@ class CI_Session {
 		else
 		{
 			// if encryption is not used, we provide an md5 hash to prevent userside tampering
-			$cookie_data = $cookie_data.md5($cookie_data.$this->encryption_key);
+			$cookie_data .= hash_hmac('sha1', $cookie_data, $this->encryption_key);
 		}
 
 		$expire = ($this->sess_expire_on_close === TRUE) ? 0 : $this->sess_expiration + time();
 
 		// Set the cookie
 		setcookie(
-					$this->sess_cookie_name,
-					$cookie_data,
-					$expire,
-					$this->cookie_path,
-					$this->cookie_domain,
-					$this->cookie_secure
-				);
+			$this->sess_cookie_name,
+			$cookie_data,
+			$expire,
+			$this->cookie_path,
+			$this->cookie_domain,
+			$this->cookie_secure
+		);
 	}
 
 	// --------------------------------------------------------------------