Unsafe usage of urljoin() within FileStorageSystem

[Aman Ali]

It may be possible to override the hostname when displaying a link to a static/media file with an attacker-controlled filename. The url() method in FileStorageSystem uses urljoin(base,url) to combine the base_url (typically STATIC_URL or MEDIA_URL) to the filename. The urljoin function has an edge case where if the url parameter starts with "", the base_url value is overwritten. Thus, if an attacker can set the name variable of a FileStorageSystem instance to "www.evil.com", any url method call on that instance will return an external link pointing to the attacker's site. Creating a file of the name "
www.evil.com" is also acceptable as the filepath_to_uri function (that is called on the filename before the urljoin call) converts all backslashes to forward-slashes. The latter example works better as it is a completely valid Linux file name.

This issue can't be exploited using framework-provided upload techniques (FileFields, ImageFields, etc) as they properly escape the filename. However, an application may directly initialize a FileStorageSystem using user-controlled data or allow users to modify the name attribute of an existing FileStorageSystem instance. In such cases, an attacker could convert the expected relative paths into absolute external URLs.

This issue can simply be patched by modifying the line found here​https://github.com/django/django/blob/master/django/core/files/storage.py#L302 as follows:

return urljoin(self.base_url, filepath_to_uri(get_valid_filename(name)))

This change filters the filename provided through the get_valid_filename function from django.utils.text. This function does a sufficient of eliminating the ability to override the base_url.

Note: This issue was initially disclosed to the Django security team and was decided not to be treated as a security issue, but instead a bug.