Index: Python-2.7.18/Lib/BaseHTTPServer.py
===================================================================
--- Python-2.7.18.orig/Lib/BaseHTTPServer.py
+++ Python-2.7.18/Lib/BaseHTTPServer.py
@@ -287,6 +287,14 @@ class BaseHTTPRequestHandler(SocketServe
             return False
         self.command, self.path, self.request_version = command, path, version
 
+        # CVE-2021-28861: The purpose of replacing '//' with '/' is to
+        # protect against open redirect attacks possibly triggered if the
+        # path starts with '//' because http clients treat //path as an
+        # absolute URI without scheme (similar to http://path) rather than
+        # a path.
+        if self.path.startswith('//'):
+            self.path = '/' + self.path.lstrip('/')  # Reduce to a single /
+
         # Examine the headers and look for a Connection directive
         self.headers = self.MessageClass(self.rfile, 0)
 
Index: Python-2.7.18/Lib/test/test_httpservers.py
===================================================================
--- Python-2.7.18.orig/Lib/test/test_httpservers.py
+++ Python-2.7.18/Lib/test/test_httpservers.py
@@ -417,6 +417,52 @@ class SimpleHTTPServerTestCase(BaseTestC
         self.assertEqual(response.getheader("Location"),
                          self.tempdir_name + "/?hi=1")
 
+    def test_get_dir_redirect_location_domain_injection_bug(self):
+        """Ensure //evil.co/..%2f../../X does not put //evil.co/ in Location.
+        //netloc/ in a Location header is a redirect to a new host.
+        https://github.com/python/cpython/issues/87389
+        This checks that a path resolving to a directory on our server cannot
+        resolve into a redirect to another server.
+        """
+        os.mkdir(os.path.join(self.tempdir, 'existing_directory'))
+        url = '/python.org/..%2f..%2f..%2f..%2f..%2f../%0a%0d/../' + self.tempdir_name + '/existing_directory'
+        expected_location = url + '/'  # /python.org.../ single slash single prefix, trailing slash
+        # Canonicalizes to /tmp/tempdir_name/existing_directory which does
+        # exist and is a dir, triggering the 301 redirect logic.
+        response = self.request(url)
+        self.check_status_and_reason(response, 301)
+        location = response.getheader('Location')
+        self.assertEqual(location, expected_location, msg='non-attack failed!')
+
+        # //python.org... multi-slash prefix, no trailing slash
+        attack_url = '/' + url
+        response = self.request(attack_url)
+        self.check_status_and_reason(response, 301)
+        location = response.getheader('Location')
+        self.assertFalse(location.startswith('//'), msg=location)
+        self.assertEqual(location, expected_location,
+                msg='Expected Location header to start with a single / and '
+                'end with a / as this is a directory redirect.')
+        # ///python.org... triple-slash prefix, no trailing slash
+        attack3_url = '//' + url
+        response = self.request(attack3_url)
+        self.check_status_and_reason(response, 301)
+        self.assertEqual(response.getheader('Location'), expected_location)
+
+        # If the second word in the http request (Request-URI for the http
+        # method) is a full URI, we don't worry about it, as that'll be parsed
+        # and reassembled as a full URI within BaseHTTPRequestHandler.send_head
+        # so no errant scheme-less //netloc//evil.co/ domain mixup can happen.
+        attack_scheme_netloc_2slash_url = 'https://pypi.org/' + url
+        expected_scheme_netloc_location = attack_scheme_netloc_2slash_url + '/'
+        response = self.request(attack_scheme_netloc_2slash_url)
+        self.check_status_and_reason(response, 301)
+        location = response.getheader('Location')
+        # We're just ensuring that the scheme and domain make it through, if
+        # there are or aren't multiple slashes at the start of the path that
+        # follows that isn't important in this Location: header.
+        self.assertTrue(location.startswith('https://pypi.org/'), msg=location)
+
 
 cgi_file1 = """\
 #!%s