From 547a26b685d08cac0aa64e5e65f7867ac0ea9bc0 Mon Sep 17 00:00:00 2001
From: Aymeric Augustin <aymeric.augustin@m4x.org>
Date: Sun, 23 May 2021 18:51:27 +0200
Subject: [PATCH] Use constant-time comparison for passwords.

Backport of c91b4c2a and dfecbd03.
---
 docs/changelog.rst            |  6 ++++++
 src/websockets/legacy/auth.py | 28 +++++++++++++++-------------
 tests/legacy/test_auth.py     | 11 +++++++++--
 3 files changed, 30 insertions(+), 15 deletions(-)

diff --git a/docs/changelog.rst b/docs/changelog.rst
index 1064af736..f3e1acf08 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -30,6 +30,12 @@ They may change at any time.
 
 *In development*
 
+.. note::
+
+    **Version 9.1 fixes a security issue introduced in version 8.0.**
+
+    Version 8.0 was vulnerable to timing attacks on HTTP Basic Auth passwords.
+
 9.0.2
 .....
 
diff --git a/src/websockets/legacy/auth.py b/src/websockets/legacy/auth.py
index e0beede57..80ceff28d 100644
--- a/src/websockets/legacy/auth.py
+++ b/src/websockets/legacy/auth.py
@@ -6,6 +6,7 @@
 
 
 import functools
+import hmac
 import http
 from typing import Any, Awaitable, Callable, Iterable, Optional, Tuple, Union, cast
 
@@ -132,24 +133,23 @@ def basic_auth_protocol_factory(
 
     if credentials is not None:
         if is_credentials(credentials):
-
-            async def check_credentials(username: str, password: str) -> bool:
-                return (username, password) == credentials
-
+            credentials_list = [cast(Credentials, credentials)]
         elif isinstance(credentials, Iterable):
             credentials_list = list(credentials)
-            if all(is_credentials(item) for item in credentials_list):
-                credentials_dict = dict(credentials_list)
-
-                async def check_credentials(username: str, password: str) -> bool:
-                    return credentials_dict.get(username) == password
-
-            else:
+            if not all(is_credentials(item) for item in credentials_list):
                 raise TypeError(f"invalid credentials argument: {credentials}")
-
         else:
             raise TypeError(f"invalid credentials argument: {credentials}")
 
+        credentials_dict = dict(credentials_list)
+
+        async def check_credentials(username: str, password: str) -> bool:
+            try:
+                expected_password = credentials_dict[username]
+            except KeyError:
+                return False
+            return hmac.compare_digest(expected_password, password)
+
     if create_protocol is None:
         # Not sure why mypy cannot figure this out.
         create_protocol = cast(
@@ -158,5 +158,7 @@ async def check_credentials(username: str, password: str) -> bool:
         )
 
     return functools.partial(
-        create_protocol, realm=realm, check_credentials=check_credentials
+        create_protocol,
+        realm=realm,
+        check_credentials=check_credentials,
     )
diff --git a/tests/legacy/test_auth.py b/tests/legacy/test_auth.py
index bb8c6a6eb..3d8eb90d7 100644
--- a/tests/legacy/test_auth.py
+++ b/tests/legacy/test_auth.py
@@ -1,3 +1,4 @@
+import hmac
 import unittest
 import urllib.error
 
@@ -76,7 +77,7 @@ def test_basic_auth_bad_multiple_credentials(self):
         )
 
     async def check_credentials(username, password):
-        return password == "iloveyou"
+        return hmac.compare_digest(password, "iloveyou")
 
     create_protocol_check_credentials = basic_auth_protocol_factory(
         realm="auth-tests",
@@ -140,7 +141,13 @@ def test_basic_auth_unsupported_credentials_details(self):
         self.assertEqual(raised.exception.read().decode(), "Unsupported credentials\n")
 
     @with_server(create_protocol=create_protocol)
-    def test_basic_auth_invalid_credentials(self):
+    def test_basic_auth_invalid_username(self):
+        with self.assertRaises(InvalidStatusCode) as raised:
+            self.start_client(user_info=("goodbye", "iloveyou"))
+        self.assertEqual(raised.exception.status_code, 401)
+
+    @with_server(create_protocol=create_protocol)
+    def test_basic_auth_invalid_password(self):
         with self.assertRaises(InvalidStatusCode) as raised:
             self.start_client(user_info=("hello", "ihateyou"))
         self.assertEqual(raised.exception.status_code, 401)