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Origin: upstream
Last-Update: 2013-09-08
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
@@ -231,7 +231,8 @@
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,
--- a/django/utils/http.py
+++ b/django/utils/http.py
@@ -126,12 +126,13 @@
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'])
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