Origin: upstream
Last-Update: 2013-08-08
Subject: Possible XSS via ``is_safe_url``

A common pattern in Django applications is for a view to accept, via
querystring parameter, a URL to redirect to upon successful completion
of the view's processing. This pattern is used in code bundled with
Django itself; for example, the ``login`` view in
``django.contrib.auth.views``, which accepts such a parameter to
determine where to send a user following successful login.

A utility function -- ``django.utils.http.is_safe_url()`` -- is
provided and used to validate that this URL is on the current host
(either via fully-qualified or relative URL), so as to avoid
potentially dangerous redirects from maliciously-constructed
querystrings.

The ``is_safe_url()`` function works as intended for HTTP and HTTPS
URLs, but due to the manner in which it parses the URL, will permit
redirects to other schemes, such as ``javascript:``. While the Django
project is unaware of any demonstrated ability to perform cross-site
scripting attacks via this mechanism, the potential for such is
sufficient to trigger a security response.

To remedy this issue, the ``is_safe_url()`` function will be modified
to properly recognize and reject URLs which specify a scheme other
than HTTP or HTTPS.

--- a/django/contrib/auth/tests/views.py
+++ b/django/contrib/auth/tests/views.py
@@ -309,7 +309,8 @@
         for bad_url in ('http://example.com',
                         'https://example.com',
                         'ftp://exampel.com',
-                        '//example.com'):
+                        '//example.com',
+                        'javascript:alert("XSS")'):
 
             nasty_url = '%(url)s?%(next)s=%(bad_url)s' % {
                 'url': login_url,
@@ -330,6 +331,7 @@
                          '/view?param=ftp://exampel.com',
                          'view/?param=//example.com',
                          'https:///',
+                         'HTTPS:///',
                          '//testserver/',
                          '/url%20with%20spaces/'):  # see ticket #12534
             safe_url = '%(url)s?%(next)s=%(good_url)s' % {
@@ -467,7 +469,8 @@
         for bad_url in ('http://example.com',
                         'https://example.com',
                         'ftp://exampel.com',
-                        '//example.com'):
+                        '//example.com',
+                        'javascript:alert("XSS")'):
             nasty_url = '%(url)s?%(next)s=%(bad_url)s' % {
                 'url': logout_url,
                 'next': REDIRECT_FIELD_NAME,
@@ -486,6 +489,7 @@
                          '/view?param=ftp://exampel.com',
                          'view/?param=//example.com',
                          'https:///',
+                         'HTTPS:///',
                          '//testserver/',
                          '/url%20with%20spaces/'):  # see ticket #12534
             safe_url = '%(url)s?%(next)s=%(good_url)s' % {
--- a/django/utils/http.py
+++ b/django/utils/http.py
@@ -228,11 +228,12 @@
 def is_safe_url(url, host=None):
     """
     Return ``True`` if the url is a safe redirection (i.e. it doesn't point to
-    a different host).
+    a different host and uses a safe scheme).
 
     Always returns ``False`` on an empty url.
     """
     if not url:
         return False
-    netloc = urlparse.urlparse(url)[1]
-    return not netloc or netloc == host
+    url_info = urlparse.urlparse(url)
+    return (not url_info[1] or url_info[1] == host) and \
+        (not url_info[0] or url_info[0] in ['http', 'https'])