[1.5.x] Added a new required ALLOWED_HOSTS setting for HTTP host header validation.

This is a security fix; disclosure and advisory coming shortly.

docs/ref/settings.txt

@@ -68,6 +68,42 @@ of (Full name, email address). Example::
 Note that Django will email *all* of these people whenever an error happens.
 See :doc:`/howto/error-reporting` for more information.
 
+.. setting:: ALLOWED_HOSTS
+
+ALLOWED_HOSTS
+-------------
+
+Default: ``[]`` (Empty list)
+
+A list of strings representing the host/domain names that this Django site can
+serve. This is a security measure to prevent an attacker from poisoning caches
+and password reset emails with links to malicious hosts by submitting requests
+with a fake HTTP ``Host`` header, which is possible even under many
+seemingly-safe webserver configurations.
+
+Values in this list can be fully qualified names (e.g. ``'www.example.com'``),
+in which case they will be matched against the request's ``Host`` header
+exactly (case-insensitive, not including port). A value beginning with a period
+can be used as a subdomain wildcard: ``'.example.com'`` will match
+``example.com``, ``www.example.com``, and any other subdomain of
+``example.com``. A value of ``'*'`` will match anything; in this case you are
+responsible to provide your own validation of the ``Host`` header (perhaps in a
+middleware; if so this middleware must be listed first in
+:setting:`MIDDLEWARE_CLASSES`).
+
+If the ``Host`` header (or ``X-Forwarded-Host`` if
+:setting:`USE_X_FORWARDED_HOST` is enabled) does not match any value in this
+list, the :meth:`django.http.HttpRequest.get_host()` method will raise
+:exc:`~django.core.exceptions.SuspiciousOperation`.
+
+When :setting:`DEBUG` is ``True`` or when running tests, host validation is
+disabled; any host will be accepted. Thus it's usually only necessary to set it
+in production.
+
+This validation only applies via :meth:`~django.http.HttpRequest.get_host()`;
+if your code accesses the ``Host`` header directly from ``request.META`` you
+are bypassing this security protection.
+
 .. setting:: ALLOWED_INCLUDE_ROOTS
 
 ALLOWED_INCLUDE_ROOTS

docs/releases/1.5.txt

@@ -354,6 +354,16 @@ Backwards incompatible changes in 1.5
     deprecation timeline for a given feature, its removal may appear as a
     backwards incompatible change.
 
+``ALLOWED_HOSTS`` required in production
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The new :setting:`ALLOWED_HOSTS` setting validates the request's ``Host``
+header and protects against host-poisoning attacks. This setting is now
+required whenever :setting:`DEBUG` is ``False``, or else
+:meth:`django.http.HttpRequest.get_host()` will raise
+:exc:`~django.core.exceptions.SuspiciousOperation`. For more details see the
+:setting:`full documentation<ALLOWED_HOSTS>` for the new setting.
+
 Managers on abstract models
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

docs/topics/security.txt

@@ -167,47 +167,40 @@ server, there are some additional steps you may need:
 
 .. _host-headers-virtual-hosting:
 
-Host headers and virtual hosting
-================================
+Host header validation
+======================
 
-Django uses the ``Host`` header provided by the client to construct URLs
-in certain cases. While these values are sanitized to prevent Cross
-Site Scripting attacks, they can be used for Cross-Site Request
-Forgery and cache poisoning attacks in some circumstances. We
-recommend you ensure your Web server is configured such that:
+Django uses the ``Host`` header provided by the client to construct URLs in
+certain cases. While these values are sanitized to prevent Cross Site Scripting
+attacks, a fake ``Host`` value can be used for Cross-Site Request Forgery,
+cache poisoning attacks, and poisoning links in emails.
 
-* It always validates incoming HTTP ``Host`` headers against the expected
-  host name.
-* Disallows requests with no ``Host`` header.
-* Is *not* configured with a catch-all virtual host that forwards requests
-  to a Django application.
+Because even seemingly-secure webserver configurations are susceptible to fake
+``Host`` headers, Django validates ``Host`` headers against the
+:setting:`ALLOWED_HOSTS` setting in the
+:meth:`django.http.HttpRequest.get_host()` method.
+
+This validation only applies via :meth:`~django.http.HttpRequest.get_host()`;
+if your code accesses the ``Host`` header directly from ``request.META`` you
+are bypassing this security protection.
+
+For more details see the full :setting:`ALLOWED_HOSTS` documentation.
+
+.. warning::
+
+   Previous versions of this document recommended configuring your webserver to
+   ensure it validates incoming HTTP ``Host`` headers. While this is still
+   recommended, in many common webservers a configuration that seems to
+   validate the ``Host`` header may not in fact do so. For instance, even if
+   Apache is configured such that your Django site is served from a non-default
+   virtual host with the ``ServerName`` set, it is still possible for an HTTP
+   request to match this virtual host and supply a fake ``Host`` header. Thus,
+   Django now requires that you set :setting:`ALLOWED_HOSTS` explicitly rather
+   than relying on webserver configuration.
 
 Additionally, as of 1.3.1, Django requires you to explicitly enable support for
-the ``X-Forwarded-Host`` header if your configuration requires it.
-
-Configuration for Apache
-------------------------
-
-The easiest way to get the described behavior in Apache is as follows. Create
-a `virtual host`_ using the ServerName_ and ServerAlias_ directives to restrict
-the domains Apache reacts to. Please keep in mind that while the directives do
-support ports the match is only performed against the hostname. This means that
-the ``Host`` header could still contain a port pointing to another webserver on
-the same machine. The next step is to make sure that your newly created virtual
-host is not also the default virtual host. Apache uses the first virtual host
-found in the configuration file as default virtual host.  As such you have to
-ensure that you have another virtual host which will act as catch-all virtual
-host. Just add one if you do not have one already, there is nothing special
-about it aside from ensuring it is the first virtual host in the configuration
-file. Debian/Ubuntu users usually don't have to take any action, since Apache
-ships with a default virtual host in ``sites-available`` which is linked into
-``sites-enabled`` as ``000-default`` and included from ``apache2.conf``. Just
-make sure not to name your site ``000-abc``, since files are included in
-alphabetical order.
-
-.. _virtual host: http://httpd.apache.org/docs/2.2/vhosts/
-.. _ServerName: http://httpd.apache.org/docs/2.2/mod/core.html#servername
-.. _ServerAlias: http://httpd.apache.org/docs/2.2/mod/core.html#serveralias
+the ``X-Forwarded-Host`` header (via the :setting:`USE_X_FORWARDED_HOST`
+setting) if your configuration requires it.
 
 
 .. _additional-security-topics: