Django 1.6.3 release notes

April 21, 2014

Django 1.6.3 fixes several bugs in 1.6.2, including three security issues, and makes one backwards-incompatible change:

Unexpected code execution using reverse()

Django’s URL handling is based on a mapping of regex patterns (representing the URLs) to callable views, and Django’s own processing consists of matching a requested URL against those patterns to determine the appropriate view to invoke.

Django also provides a convenience function – reverse() – which performs this process in the opposite direction. The reverse() function takes information about a view and returns a URL which would invoke that view. Use of reverse() is encouraged for application developers, as the output of reverse() is always based on the current URL patterns, meaning developers do not need to change other code when making changes to URLs.

One argument signature for reverse() is to pass a dotted Python path to the desired view. In this situation, Django will import the module indicated by that dotted path as part of generating the resulting URL. If such a module has import-time side effects, those side effects will occur.

Thus it is possible for an attacker to cause unexpected code execution, given the following conditions:

1. One or more views are present which construct a URL based on user input (commonly, a “next” parameter in a querystring indicating where to redirect upon successful completion of an action).
2. One or more modules are known to an attacker to exist on the server’s Python import path, which perform code execution with side effects on importing.

To remedy this, reverse() will now only accept and import dotted paths based on the view-containing modules listed in the project’s URL pattern configuration, so as to ensure that only modules the developer intended to be imported in this fashion can or will be imported.

Caching of anonymous pages could reveal CSRF token

Django includes both a caching framework and a system for preventing cross-site request forgery (CSRF) attacks. The CSRF-protection system is based on a random nonce sent to the client in a cookie which must be sent by the client on future requests and, in forms, a hidden value which must be submitted back with the form.

The caching framework includes an option to cache responses to anonymous (i.e., unauthenticated) clients.

When the first anonymous request to a given page is by a client which did not have a CSRF cookie, the cache framework will also cache the CSRF cookie and serve the same nonce to other anonymous clients who do not have a CSRF cookie. This can allow an attacker to obtain a valid CSRF cookie value and perform attacks which bypass the check for the cookie.

To remedy this, the caching framework will no longer cache such responses. The heuristic for this will be:

1. If the incoming request did not submit any cookies, and
2. If the response did send one or more cookies, and
3. If the Vary: Cookie header is set on the response, then the response will not be cached.

MySQL typecasting

The MySQL database is known to “typecast” on certain queries; for example, when querying a table which contains string values, but using a query which filters based on an integer value, MySQL will first silently coerce the strings to integers and return a result based on that.

If a query is performed without first converting values to the appropriate type, this can produce unexpected results, similar to what would occur if the query itself had been manipulated.

Django’s model field classes are aware of their own types and most such classes perform explicit conversion of query arguments to the correct database-level type before querying. However, three model field classes did not correctly convert their arguments:

• FilePathField
• GenericIPAddressField
• IPAddressField

These three fields have been updated to convert their arguments to the correct types before querying.

Additionally, developers of custom model fields are now warned via documentation to ensure their custom field classes will perform appropriate type conversions, and users of the raw() and extra() query methods – which allow the developer to supply raw SQL or SQL fragments – will be advised to ensure they perform appropriate manual type conversions prior to executing queries.

select_for_update() requires a transaction

Historically, queries that use select_for_update() could be executed in autocommit mode, outside of a transaction. Before Django 1.6, Django’s automatic transactions mode allowed this to be used to lock records until the next write operation. Django 1.6 introduced database-level autocommit; since then, execution in such a context voids the effect of select_for_update(). It is, therefore, assumed now to be an error and raises an exception.

This change was made because such errors can be caused by including an app which expects global transactions (e.g. ATOMIC_REQUESTS set to True), or Django’s old autocommit behavior, in a project which runs without them; and further, such errors may manifest as data-corruption bugs.

This change may cause test failures if you use select_for_update() in a test class which is a subclass of TransactionTestCase rather than TestCase.

Other bugfixes and changes

• Content retrieved from the GeoIP library is now properly decoded from its default iso-8859-1 encoding (#21996).
• Fixed AttributeError when using bulk_create() with ForeignObject (#21566).
• Fixed crash of QuerySets that use F() + timedelta() when their query was compiled more once (#21643).
• Prevented custom widget class attribute of IntegerField subclasses from being overwritten by the code in their __init__ method (#22245).
• Improved strip_tags() accuracy (but it still cannot guarantee an HTML-safe result, as stated in the documentation).
• Fixed a regression in the django.contrib.gis SQL compiler for non-concrete fields (#22250).
• Fixed ModelAdmin.preserve_filters when running a site with a URL prefix (#21795).
• Fixed a crash in the find_command management utility when the PATH environment variable wasn’t set (#22256).
• Fixed changepassword on Windows (#22364).
• Avoided shadowing deadlock exceptions on MySQL (#22291).
• Wrapped database exceptions in _set_autocommit (#22321).
• Fixed atomicity when closing a database connection or when the database server disconnects (#21239 and #21202)
• Fixed regression in prefetch_related that caused the related objects query to include an unnecessary join (#21760).

Additionally, Django’s vendored version of six, django.utils.six has been upgraded to the latest release (1.6.1).