Provide a leeway time in the verification of timestamps to account for clock skew

Author: Luis Miguel Cabral

Authentication/JWT.php

@@ -15,6 +15,12 @@
  */
 class JWT
 {
+
+    /**
+     * When cheking nbf, iat or expiration times, we want to provide some extra leeway time to account for clock skew.
+     */
+    const LEEWAYTIME = 60;
+
     public static $supported_algs = array(
         'HS256' => array('hash_hmac', 'SHA256'),
         'HS512' => array('hash_hmac', 'SHA512'),
@@ -80,7 +86,7 @@ public static function decode($jwt, $key = null, $allowed_algs = array())
 
             // Check if the nbf if it is defined. This is the time that the
             // token can actually be used. If it's not yet that time, abort.
-            if (isset($payload->nbf) && $payload->nbf > time()) {
+            if (isset($payload->nbf) && $payload->nbf > (time() + self::LEEWAYTIME)) {
                 throw new BeforeValidException(
                     'Cannot handle token prior to ' . date(DateTime::ISO8601, $payload->nbf)
                 );
@@ -89,14 +95,14 @@ public static function decode($jwt, $key = null, $allowed_algs = array())
             // Check that this token has been created before 'now'. This prevents
             // using tokens that have been created for later use (and haven't
             // correctly used the nbf claim).
-            if (isset($payload->iat) && $payload->iat > time()) {
+            if (isset($payload->iat) && $payload->iat > (time() + self::LEEWAYTIME)) {
                 throw new BeforeValidException(
                     'Cannot handle token prior to ' . date(DateTime::ISO8601, $payload->iat)
                 );
             }
 
             // Check if this token has expired.
-            if (isset($payload->exp) && time() >= $payload->exp) {
+            if (isset($payload->exp) && (time() - self::LEEWAYTIME) >= $payload->exp) {
                 throw new ExpiredException('Expired token');
             }
         }

tests/JWTTest.php

@@ -38,29 +38,35 @@ public function testMalformedJsonThrowsException()
     public function testExpiredToken()
     {
         $this->setExpectedException('ExpiredException');
+        $timeInPast = time() - JWT::LEEWAYTIME - 20;
         $payload = array(
             "message" => "abc",
-            "exp" => time() - 20); // time in the past
+            "exp" => $timeInPast // time in the past
+        );
         $encoded = JWT::encode($payload, 'my_key');
         JWT::decode($encoded, 'my_key', array('HS256'));
     }
 
     public function testBeforeValidTokenWithNbf()
     {
         $this->setExpectedException('BeforeValidException');
+        $timeInFuture = time() + JWT::LEEWAYTIME + 20;
         $payload = array(
             "message" => "abc",
-            "nbf" => time() + 20); // time in the future
+            "nbf" => $timeInFuture // time in the future
+        );
         $encoded = JWT::encode($payload, 'my_key');
         JWT::decode($encoded, 'my_key', array('HS256'));
     }
 
     public function testBeforeValidTokenWithIat()
     {
         $this->setExpectedException('BeforeValidException');
+        $timeInFuture = time() + JWT::LEEWAYTIME + 20;
         $payload = array(
             "message" => "abc",
-            "iat" => time() + 20); // time in the future
+            "iat" => $timeInFuture // time in the future
+        );
         $encoded = JWT::encode($payload, 'my_key');
         JWT::decode($encoded, 'my_key', array('HS256'));
     }