adding changelog and test cases

CHANGELOG.rst

@@ -25,6 +25,7 @@ Added
 - Add to_jwk static method to ECAlgorithm by @leonsmith in https://github.com/jpadilla/pyjwt/pull/732
 - Add ``get_algorithm_by_name`` as a method of ``PyJWS`` objects, and expose
   the global PyJWS method as part of the public API
+- Adding validation for `issued_at` when `iat > (now + leeway)` as `ImmatureSignatureError` by @sriharan16 in https://github.com/jpadilla/pyjwt/pull/794
 
 `v2.4.0 <https://github.com/jpadilla/pyjwt/compare/2.3.0...2.4.0>`__
 -----------------------------------------------------------------------

tests/test_api_jwt.py

@@ -219,6 +219,14 @@ def test_decode_raises_exception_if_iat_is_not_int(self, jwt):
         with pytest.raises(InvalidIssuedAtError):
             jwt.decode(example_jwt, "secret", algorithms=["HS256"])
 
+    def test_decode_raises_exception_if_iat_is_greater_than_now(self, jwt, payload):
+        payload["iat"] = utc_timestamp() + 10
+        secret = "secret"
+        jwt_message = jwt.encode(payload, secret)
+
+        with pytest.raises(ImmatureSignatureError):
+            jwt.decode(jwt_message, secret, algorithms=["HS256"])
+
     def test_decode_raises_exception_if_nbf_is_not_int(self, jwt):
         # >>> jwt.encode({'nbf': 'not-an-int'}, 'secret')
         example_jwt = (