bpo-39503: CVE-2020-8492: Fix AbstractBasicAuthHandler (GH-18284)

The AbstractBasicAuthHandler class of the urllib.request module uses
an inefficient regular expression which can be exploited by an
attacker to cause a denial of service. Fix the regex to prevent the
catastrophic backtracking. Vulnerability reported by Ben Caller
and Matt Schwager.

AbstractBasicAuthHandler of urllib.request now parses all
WWW-Authenticate HTTP headers and accepts multiple challenges per
header: use the realm of the first Basic challenge.

Co-Authored-By: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 57 insertions, 33 deletions

diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index 8abedaa..e69ac3e 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -1446,40 +1446,64 @@ class HandlerTests(unittest.TestCase):
         bypass = {'exclude_simple': True, 'exceptions': []}
         self.assertTrue(_proxy_bypass_macosx_sysconf('test', bypass))
 
-    def test_basic_auth(self, quote_char='"'):
-        opener = OpenerDirector()
-        password_manager = MockPasswordManager()
-        auth_handler = urllib.request.HTTPBasicAuthHandler(password_manager)
-        realm = "ACME Widget Store"
-        http_handler = MockHTTPHandler(
-            401, 'WWW-Authenticate: Basic realm=%s%s%s\r\n\r\n' %
-            (quote_char, realm, quote_char))
-        opener.add_handler(auth_handler)
-        opener.add_handler(http_handler)
-        self._test_basic_auth(opener, auth_handler, "Authorization",
-                              realm, http_handler, password_manager,
-                              "http://acme.example.com/protected",
-                              "http://acme.example.com/protected",
-                              )
-
-    def test_basic_auth_with_single_quoted_realm(self):
-        self.test_basic_auth(quote_char="'")
-
-    def test_basic_auth_with_unquoted_realm(self):
-        opener = OpenerDirector()
-        password_manager = MockPasswordManager()
-        auth_handler = urllib.request.HTTPBasicAuthHandler(password_manager)
-        realm = "ACME Widget Store"
-        http_handler = MockHTTPHandler(
-            401, 'WWW-Authenticate: Basic realm=%s\r\n\r\n' % realm)
-        opener.add_handler(auth_handler)
-        opener.add_handler(http_handler)
-        with self.assertWarns(UserWarning):
+    def check_basic_auth(self, headers, realm):
+        with self.subTest(realm=realm, headers=headers):
+            opener = OpenerDirector()
+            password_manager = MockPasswordManager()
+            auth_handler = urllib.request.HTTPBasicAuthHandler(password_manager)
+            body = '\r\n'.join(headers) + '\r\n\r\n'
+            http_handler = MockHTTPHandler(401, body)
+            opener.add_handler(auth_handler)
+            opener.add_handler(http_handler)
             self._test_basic_auth(opener, auth_handler, "Authorization",
-                                realm, http_handler, password_manager,
-                                "http://acme.example.com/protected",
-                                "http://acme.example.com/protected",
-                                )
+                                  realm, http_handler, password_manager,
+                                  "http://acme.example.com/protected",
+                                  "http://acme.example.com/protected")
+
+    def test_basic_auth(self):
+        realm = "realm2@example.com"
+        realm2 = "realm2@example.com"
+        basic = f'Basic realm="{realm}"'
+        basic2 = f'Basic realm="{realm2}"'
+        other_no_realm = 'Otherscheme xxx'
+        digest = (f'Digest realm="{realm2}", '
+                  f'qop="auth, auth-int", '
+                  f'nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", '
+                  f'opaque="5ccc069c403ebaf9f0171e9517f40e41"')
+        for realm_str in (
+            # test "quote" and 'quote'
+            f'Basic realm="{realm}"',
+            f"Basic realm='{realm}'",
+
+            # charset is ignored
+            f'Basic realm="{realm}", charset="UTF-8"',
+
+            # Multiple challenges per header
+            f'{basic}, {basic2}',
+            f'{basic}, {other_no_realm}',
+            f'{other_no_realm}, {basic}',
+            f'{basic}, {digest}',
+            f'{digest}, {basic}',
+        ):
+            headers = [f'WWW-Authenticate: {realm_str}']
+            self.check_basic_auth(headers, realm)
+
+        # no quote: expect a warning
+        with support.check_warnings(("Basic Auth Realm was unquoted",
+                                     UserWarning)):
+            headers = [f'WWW-Authenticate: Basic realm={realm}']
+            self.check_basic_auth(headers, realm)
+
+        # Multiple headers: one challenge per header.
+        # Use the first Basic realm.
+        for challenges in (
+            [basic,  basic2],
+            [basic,  digest],
+            [digest, basic],
+        ):
+            headers = [f'WWW-Authenticate: {challenge}'
+                       for challenge in challenges]
+            self.check_basic_auth(headers, realm)
 
     def test_proxy_basic_auth(self):
         opener = OpenerDirector()