[1.6.x] Made is_safe_url() reject URLs that start with control characters.

This is a security fix; disclosure to follow shortly.

django/utils/http.py

@@ -5,7 +5,7 @@ import calendar
 import datetime
 import re
 import sys
-
+import unicodedata
 from binascii import Error as BinasciiError
 from email.utils import formatdate
 
@@ -254,9 +254,10 @@ def is_safe_url(url, host=None):
 
     Always returns ``False`` on an empty url.
     """
+    if url is not None:
+        url = url.strip()
     if not url:
         return False
-    url = url.strip()
     # Chrome treats \ completely as /
     url = url.replace('\\', '/')
     # Chrome considers any URL with more than two slashes to be absolute, but
@@ -270,5 +271,10 @@ def is_safe_url(url, host=None):
     # allow this syntax.
     if not url_info.netloc and url_info.scheme:
         return False
-    return (not url_info.netloc or url_info.netloc == host) and \
-        (not url_info.scheme or url_info.scheme in ['http', 'https'])
+    # Forbid URLs that start with control characters. Some browsers (like
+    # Chrome) ignore quite a few control characters at the start of a
+    # URL and might consider the URL as scheme relative.
+    if unicodedata.category(url[0])[0] == 'C':
+        return False
+    return ((not url_info.netloc or url_info.netloc == host) and
+            (not url_info.scheme or url_info.scheme in ['http', 'https']))

docs/releases/1.4.20.txt

@@ -5,3 +5,22 @@ Django 1.4.20 release notes
 *March 18, 2015*
 
 Django 1.4.20 fixes one security issue in 1.4.19.
+
+Mitigated possible XSS attack via user-supplied redirect URLs
+=============================================================
+
+Django relies on user input in some cases (e.g.
+:func:`django.contrib.auth.views.login` and :doc:`i18n </topics/i18n/index>`)
+to redirect the user to an "on success" URL. The security checks for these
+redirects (namely ``django.utils.http.is_safe_url()``) accepted URLs with
+leading control characters and so considered URLs like ``\x08javascript:...``
+safe. This issue doesn't affect Django currently, since we only put this URL
+into the ``Location`` response header and browsers seem to ignore JavaScript
+there. Browsers we tested also treat URLs prefixed with control characters such
+as ``%08//example.com`` as relative paths so redirection to an unsafe target
+isn't a problem either.
+
+However, if a developer relies on ``is_safe_url()`` to
+provide safe redirect targets and puts such a URL into a link, they could
+suffer from an XSS attack as some browsers such as Google Chrome ignore control
+characters at the start of a URL in an anchor ``href``.

docs/releases/1.6.11.txt

@@ -22,3 +22,22 @@ it detects the length of the string it's processing increases. Remember that
 absolutely NO guarantee is provided about the results of ``strip_tags()`` being
 HTML safe. So NEVER mark safe the result of a ``strip_tags()`` call without
 escaping it first, for example with :func:`~django.utils.html.escape`.
+
+Mitigated possible XSS attack via user-supplied redirect URLs
+=============================================================
+
+Django relies on user input in some cases (e.g.
+:func:`django.contrib.auth.views.login` and :doc:`i18n </topics/i18n/index>`)
+to redirect the user to an "on success" URL. The security checks for these
+redirects (namely ``django.utils.http.is_safe_url()``) accepted URLs with
+leading control characters and so considered URLs like ``\x08javascript:...``
+safe. This issue doesn't affect Django currently, since we only put this URL
+into the ``Location`` response header and browsers seem to ignore JavaScript
+there. Browsers we tested also treat URLs prefixed with control characters such
+as ``%08//example.com`` as relative paths so redirection to an unsafe target
+isn't a problem either.
+
+However, if a developer relies on ``is_safe_url()`` to
+provide safe redirect targets and puts such a URL into a link, they could
+suffer from an XSS attack as some browsers such as Google Chrome ignore control
+characters at the start of a URL in an anchor ``href``.

tests/utils_tests/test_http.py

@@ -110,7 +110,9 @@ class TestUtilsHttp(unittest.TestCase):
                         'http:\/example.com',
                         'http:/\example.com',
                         'javascript:alert("XSS")',
-                        '\njavascript:alert(x)'):
+                        '\njavascript:alert(x)',
+                        '\x08//example.com',
+                        '\n'):
             self.assertFalse(http.is_safe_url(bad_url, host='testserver'), "%s should be blocked" % bad_url)
         for good_url in ('/view/?param=http://example.com',
                      '/view/?param=https://example.com',