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gh-87389: Fix an open redirection vulnerability in http.server. (GH-9…
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…3879) (GH-94094)

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 4abab6b)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
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miss-islington and gpshead authored Jun 22, 2022
1 parent 5776f72 commit 4dc2cae
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7 changes: 7 additions & 0 deletions Lib/http/server.py
Original file line number Diff line number Diff line change
Expand Up @@ -332,6 +332,13 @@ def parse_request(self):
return False
self.command, self.path = command, path

# 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,
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53 changes: 51 additions & 2 deletions Lib/test/test_httpservers.py
Original file line number Diff line number Diff line change
Expand Up @@ -330,7 +330,7 @@ class request_handler(NoLogRequestHandler, SimpleHTTPRequestHandler):
pass

def setUp(self):
BaseTestCase.setUp(self)
super().setUp()
self.cwd = os.getcwd()
basetempdir = tempfile.gettempdir()
os.chdir(basetempdir)
Expand Down Expand Up @@ -358,7 +358,7 @@ def tearDown(self):
except:
pass
finally:
BaseTestCase.tearDown(self)
super().tearDown()

def check_status_and_reason(self, response, status, data=None):
def close_conn():
Expand Down Expand Up @@ -414,6 +414,55 @@ def test_undecodable_filename(self):
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')
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Original file line number Diff line number Diff line change
@@ -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.

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