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>

Lib/test/test_urllib2.py

@@ -1446,40 +1446,64 @@ def test_osx_proxy_bypass(self):
         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()

Lib/urllib/request.py

@@ -937,8 +937,15 @@ class AbstractBasicAuthHandler:
 
     # allow for double- and single-quoted realm values
     # (single quotes are a violation of the RFC, but appear in the wild)
-    rx = re.compile('(?:.*,)*[ \t]*([^ \t]+)[ \t]+'
-                    'realm=(["\']?)([^"\']*)\\2', re.I)
+    rx = re.compile('(?:^|,)'   # start of the string or ','
+                    '[ \t]*'    # optional whitespaces
+                    '([^ \t]+)' # scheme like "Basic"
+                    '[ \t]+'    # mandatory whitespaces
+                    # realm=xxx
+                    # realm='xxx'
+                    # realm="xxx"
+                    'realm=(["\']?)([^"\']*)\\2',
+                    re.I)
 
     # XXX could pre-emptively send auth info already accepted (RFC 2617,
     # end of section 2, and section 1.2 immediately after "credentials"
@@ -950,27 +957,51 @@ def __init__(self, password_mgr=None):
         self.passwd = password_mgr
         self.add_password = self.passwd.add_password
 
+    def _parse_realm(self, header):
+        # parse WWW-Authenticate header: accept multiple challenges per header
+        found_challenge = False
+        for mo in AbstractBasicAuthHandler.rx.finditer(header):
+            scheme, quote, realm = mo.groups()
+            if quote not in ['"', "'"]:
+                warnings.warn("Basic Auth Realm was unquoted",
+                              UserWarning, 3)
+
+            yield (scheme, realm)
+
+            found_challenge = True
+
+        if not found_challenge:
+            if header:
+                scheme = header.split()[0]
+            else:
+                scheme = ''
+            yield (scheme, None)
+
     def http_error_auth_reqed(self, authreq, host, req, headers):
         # host may be an authority (without userinfo) or a URL with an
         # authority
-        # XXX could be multiple headers
-        authreq = headers.get(authreq, None)
+        headers = headers.get_all(authreq)
+        if not headers:
+            # no header found
+            return
 
-        if authreq:
-            scheme = authreq.split()[0]
-            if scheme.lower() != 'basic':
-                raise ValueError("AbstractBasicAuthHandler does not"
-                                 " support the following scheme: '%s'" %
-                                 scheme)
-            else:
-                mo = AbstractBasicAuthHandler.rx.search(authreq)
-                if mo:
-                    scheme, quote, realm = mo.groups()
-                    if quote not in ['"',"'"]:
-                        warnings.warn("Basic Auth Realm was unquoted",
-                                      UserWarning, 2)
-                    if scheme.lower() == 'basic':
-                        return self.retry_http_basic_auth(host, req, realm)
+        unsupported = None
+        for header in headers:
+            for scheme, realm in self._parse_realm(header):
+                if scheme.lower() != 'basic':
+                    unsupported = scheme
+                    continue
+
+                if realm is not None:
+                    # Use the first matching Basic challenge.
+                    # Ignore following challenges even if they use the Basic
+                    # scheme.
+                    return self.retry_http_basic_auth(host, req, realm)
+
+        if unsupported is not None:
+            raise ValueError("AbstractBasicAuthHandler does not "
+                             "support the following scheme: %r"
+                             % (scheme,))
 
     def retry_http_basic_auth(self, host, req, realm):
         user, pw = self.passwd.find_user_password(realm, host)

Misc/NEWS.d/next/Library/2020-03-25-16-02-16.bpo-39503.YmMbYn.rst

@@ -0,0 +1,3 @@
+:class:`~urllib.request.AbstractBasicAuthHandler` of :mod:`urllib.request`
+now parses all WWW-Authenticate HTTP headers and accepts multiple challenges
+per header: use the realm of the first Basic challenge.

Misc/NEWS.d/next/Security/2020-01-30-16-15-29.bpo-39503.B299Yq.rst

@@ -0,0 +1,5 @@
+CVE-2020-8492: The :class:`~urllib.request.AbstractBasicAuthHandler` class of the
+:mod:`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.