From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001

From: "Miss Islington (bot)"

 <31488909+miss-islington@users.noreply.github.com>

Date: Wed, 22 Jun 2022 15:05:00 -0700

Subject: [PATCH] 00386: CVE-2021-28861

Fix an open redirection vulnerability in the `http.server` module when

an URI path starts with `//` that could produce a 301 Location header

with a misleading target.  Vulnerability discovered, and logic fix

proposed, by Hamza Avvan (@hamzaavvan).

Test and comments authored by Gregory P. Smith [Google].

(cherry picked from commit 4abab6b603dd38bec1168e9a37c40a48ec89508e)

Upstream: https://github.com/python/cpython/pull/93879

Tracking bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2120642

Co-authored-by: Gregory P. Smith <greg@krypto.org>

---

 Lib/http/server.py                            |  7 +++

 Lib/test/test_httpservers.py                  | 53 ++++++++++++++++++-

 ...2-06-15-20-09-23.gh-issue-87389.QVaC3f.rst |  3 ++

 3 files changed, 61 insertions(+), 2 deletions(-)

 create mode 100644 Misc/NEWS.d/next/Security/2022-06-15-20-09-23.gh-issue-87389.QVaC3f.rst

diff --git a/Lib/http/server.py b/Lib/http/server.py

index 60a4dadf03..ce05be13d3 100644

--- a/Lib/http/server.py

+++ b/Lib/http/server.py

@@ -323,6 +323,13 @@ class BaseHTTPRequestHandler(socketserver.StreamRequestHandler):

             return False

         self.command, self.path, self.request_version = command, path, version

+        # gh-87389: 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.

         try:

             self.headers = http.client.parse_headers(self.rfile,

diff --git a/Lib/test/test_httpservers.py b/Lib/test/test_httpservers.py

index 66e937e04b..5a0a7c3f74 100644

--- a/Lib/test/test_httpservers.py

+++ b/Lib/test/test_httpservers.py

@@ -324,7 +324,7 @@ class SimpleHTTPServerTestCase(BaseTestCase):

         pass

     def setUp(self):

-        BaseTestCase.setUp(self)

+        super().setUp()

         self.cwd = os.getcwd()

         basetempdir = tempfile.gettempdir()

         os.chdir(basetempdir)

@@ -343,7 +343,7 @@ class SimpleHTTPServerTestCase(BaseTestCase):

             except:

                 pass

         finally:

-            BaseTestCase.tearDown(self)

+            super().tearDown()

     def check_status_and_reason(self, response, status, data=None):

         def close_conn():

@@ -399,6 +399,55 @@ class SimpleHTTPServerTestCase(BaseTestCase):

         self.check_status_and_reason(response, HTTPStatus.OK,

                                      data=support.TESTFN_UNDECODABLE)

+    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 = f'/python.org/..%2f..%2f..%2f..%2f..%2f../%0a%0d/../{self.tempdir_name}/existing_directory'

+        expected_location = f'{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, HTTPStatus.MOVED_PERMANENTLY)

+        location = response.getheader('Location')

+        self.assertEqual(location, expected_location, msg='non-attack failed!')

+

+        # //python.org... multi-slash prefix, no trailing slash

+        attack_url = f'/{url}'

+        response = self.request(attack_url)

+        self.check_status_and_reason(response, HTTPStatus.MOVED_PERMANENTLY)

+        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 = f'//{url}'

+        response = self.request(attack3_url)

+        self.check_status_and_reason(response, HTTPStatus.MOVED_PERMANENTLY)

+        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 = f'https://pypi.org/{url}'

+        expected_scheme_netloc_location = f'{attack_scheme_netloc_2slash_url}/'

+        response = self.request(attack_scheme_netloc_2slash_url)

+        self.check_status_and_reason(response, HTTPStatus.MOVED_PERMANENTLY)

+        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)

+

     def test_get(self):

         #constructs the path relative to the root directory of the HTTPServer

         response = self.request(self.base_url + '/test')

diff --git a/Misc/NEWS.d/next/Security/2022-06-15-20-09-23.gh-issue-87389.QVaC3f.rst b/Misc/NEWS.d/next/Security/2022-06-15-20-09-23.gh-issue-87389.QVaC3f.rst

new file mode 100644

index 0000000000..029d437190

--- /dev/null

+++ b/Misc/NEWS.d/next/Security/2022-06-15-20-09-23.gh-issue-87389.QVaC3f.rst

@@ -0,0 +1,3 @@

+:mod:`http.server`: Fix an open redirection vulnerability in the HTTP server

+when an URI path starts with ``//``.  Vulnerability discovered, and initial

+fix proposed, by Hamza Avvan.