Raise ScramException for malformed messages

scramp/core.py

@@ -414,11 +414,23 @@ def _get_client_first(username, c_nonce, channel_binding):
 
 
 def _set_client_first(client_first, s_nonce, channel_binding):
-    first_comma = client_first.index(",")
-    second_comma = client_first.index(",", first_comma + 1)
+    try:
+        first_comma = client_first.index(",")
+        second_comma = client_first.index(",", first_comma + 1)
+    except ValueError:
+        raise ScramException(
+            "The client sent a malformed first message.",
+            SERVER_ERROR_OTHER_ERROR,
+        )
     gs2_header = client_first[:second_comma].split(",")
-    gs2_cbind_flag = gs2_header[0]
-    gs2_char = gs2_cbind_flag[0]
+    try:
+        gs2_cbind_flag = gs2_header[0]
+        gs2_char = gs2_cbind_flag[0]
+    except IndexError:
+        raise ScramException(
+            "The client sent malformed gs2 data.",
+            SERVER_ERROR_OTHER_ERROR,
+        )
 
     if gs2_char == "y":
         if channel_binding is not None:
@@ -462,6 +474,14 @@ def _set_client_first(client_first, s_nonce, channel_binding):
 
     client_first_bare = client_first[second_comma + 1 :]
     msg = _parse_message(client_first_bare)
+
+    missing = [letter for letter in "rn" if letter not in msg]
+    if missing:
+        raise ScramException(
+            "The server returned a message without expected parameters. Missing: "
+            f"{', '.join(missing)}."
+        )
+
     c_nonce = msg["r"]
     nonce = c_nonce + s_nonce
     user = msg["n"]
@@ -479,6 +499,14 @@ def _set_server_first(server_first, c_nonce, client_first_bare, channel_binding)
     msg = _parse_message(server_first)
     if "e" in msg:
         raise ScramException(f"The server returned the error: {msg['e']}")
+
+    missing = [letter for letter in "rsi" if letter not in msg]
+    if missing:
+        raise ScramException(
+            "The server returned a message without expected parameters. Missing: "
+            f"{', '.join(missing)}."
+        )
+
     nonce = msg["r"]
     salt = msg["s"]
     iterations = int(msg["i"])
@@ -532,6 +560,17 @@ def _set_client_final(
     auth_msg = uenc(auth_msg_str)
 
     msg = _parse_message(client_final)
+
+    missing = [letter for letter in "rpc" if letter not in msg]
+    if missing:
+        raise ScramException(
+            (
+                "The client sent a message without expected parameters. "
+                f"Missing: {', '.join(missing)}."
+            ),
+            SERVER_ERROR_OTHER_ERROR,
+        )
+
     nonce = msg["r"]
     proof = msg["p"]
     channel_binding = msg["c"]
@@ -558,6 +597,10 @@ def _set_server_final(message, server_signature):
     msg = _parse_message(message)
     if "e" in msg:
         raise ScramException(f"The server returned the error: {msg['e']}")
+    if "v" not in msg:
+        raise ScramException(
+            f"The server returned a final message without the 'v' parameter."
+        )
 
     if server_signature != msg["v"]:
         raise ScramException(

test/test_scramp.py

@@ -345,6 +345,78 @@ def test_set_server_first_error():
         c.set_server_first("e=other-error")
 
 
+def test_set_server_first_missing_param():
+    c = ScramClient(["SCRAM-SHA-256"], "user", "pencil")
+    c.get_client_first()
+    with pytest.raises(
+        ScramException,
+        match="The server returned a message without expected parameters. Missing: r, s, i.",
+    ):
+        c.set_server_first("junk")
+
+
+def test_set_server_final_missing_param():
+    x = SCRAM_SHA_256_EXCHANGE
+    c = ScramClient(
+        ["SCRAM-SHA-256"],
+        USERNAME,
+        PASSWORD,
+        c_nonce=x["c_nonce"],
+    )
+    c.get_client_first()
+    c.set_server_first(x["sfirst"])
+    c.get_client_final()
+    with pytest.raises(
+        ScramException,
+        match="The server returned a final message without the 'v' parameter.",
+    ):
+        c.set_server_final("junk")
+
+
+def test_set_client_first_nonsense():
+    m = ScramMechanism(mechanism="SCRAM-SHA-256")
+    s = m.make_server(lambda x: None)
+    with pytest.raises(
+        ScramException, match="The client sent a malformed first message."
+    ):
+        s.set_client_first("junk")
+
+
+def test_set_client_first_missing_param():
+    m = ScramMechanism(mechanism="SCRAM-SHA-256")
+    s = m.make_server(lambda x: None)
+    with pytest.raises(
+        ScramException,
+        match="The server returned a message without expected parameters. Missing: r, n.",
+    ):
+        s.set_client_first("n,morejunk,bonusjunk")
+
+
+def test_set_client_final_missing_param():
+    x = SCRAM_SHA_256_EXCHANGE
+    m = ScramMechanism(mechanism="SCRAM-SHA-256")
+
+    def auth_fn(username):
+        lookup = {
+            USERNAME: m.make_auth_info(
+                PASSWORD, salt=b64dec(x["salt"]), iteration_count=x["iterations"]
+            )
+        }
+        return lookup[username]
+
+    s = m.make_server(
+        auth_fn, channel_binding=x["channel_binding"], s_nonce=x["s_nonce"]
+    )
+
+    s.set_client_first(x["cfirst"])
+    s.get_server_first()
+    with pytest.raises(
+        ScramException,
+        match="The client sent a message without expected parameters. Missing: r, p, c.",
+    ):
+        s.set_client_final("junk")
+
+
 def test_make_channel_binding_tls_server_end_point(mocker):
     ssl_socket = mocker.Mock()
     ssl_socket.getpeercert = mocker.Mock(return_value=b"cafe")