Built-in Middleware

Django comes with some built-in middleware to deal with common problems, which we discuss in the sections that follow.

Cache Middleware

django.middleware.cache.UpdateCacheMiddleware; and

django.middleware.cache.FetchFromCacheMiddleware

Enable the site-wide cache. If these are enabled, each Django-powered page will be cached for as long as the CACHE_MIDDLEWARE_SECONDS setting defines.

Common Middleware

django.middleware.common.CommonMiddleware

 

Adds a few conveniences for perfectionists:

• Forbids access to user agents in the DISALLOWED_USER_AGENTS setting, which should be a list of compiled regular expression objects.
• Performs URL rewriting based on the APPEND_SLASH and PREPEND_WWW settings.If APPEND_SLASH is True and the initial URL doesn’t end with a slash, and it is not found in the URLconf, then a new URL is formed by appending a slash at the end. If this new URL is found in the URLconf, then Django redirects the request to this new URL. Otherwise, the initial URL is processed as usual.For example, foo.com/bar will be redirected to foo.com/bar/ if you don’t have a valid URL pattern for foo.com/bar but do have a valid pattern for foo.com/bar/.

If PREPEND_WWW is True, URLs that lack a leading www. will be redirected to the same URL with a leading www.

 

Both of these options are meant to normalize URLs. The philosophy is that each URL should exist in one, and only one, place. Technically a URL foo.com/bar is distinct from foo.com/bar/ – a search-engine indexer would treat them as separate URLs – so it’s best practice to normalize URLs.

• Handles ETags based on the USE_ETAGS setting. If USE_ETAGS is set to True, Django will calculate an ETag for each request by MD5-hashing the page content, and it’ll take care of sending Not Modified responses, if appropriate.
• response_redirect_class. Defaults to HttpResponsePermanentRedirect. Subclass CommonMiddleware and override the attribute to customize the redirects issued by the middleware.
• middleware.common.BrokenLinkEmailsMiddleware. Sends broken link notification emails to MANAGERS.

GZip Middleware

django.middleware.gzip.GZipMiddleware

Compresses content for browsers that understand GZip compression (all modern browsers). This middleware should be placed before any other middleware that need to read or write the response body so that compression happens afterward. Security researchers recently revealed that when compression techniques (including GZipMiddleware) are used on a website, the site becomes exposed to a number of possible attacks. These approaches can be used to compromise, among other things, Django’s CSRF protection. Before using GZipMiddleware on your site, you should consider very carefully whether you are subject to these attacks. If you’re in any doubt about whether you’re affected, you should avoid using GZipMiddleware.

It will NOT compress content if any of the following are true:

• The content body is less than 200 bytes long.
• The response has already set the Content-Encoding header.
• The request (the browser) hasn’t sent an Accept-Encoding header containing gzip.

 

You can apply GZip compression to individual views using the gzip_page() decorator.

Conditional GET Middleware

django.middleware.http.ConditionalGetMiddleware

Handles conditional GET operations. If the response has a ETag or Last-Modified header, and the request has If-None-Match or If-Modified-Since, the response is replaced by an HttpResponseNotModified. Also sets the Date and Content-Length response-headers.

Locale Middleware

django.middleware.locale.LocaleMiddleware

Enables language selection based on data from the request. It customizes content for each user.

LocaleMiddleware.response_redirect_class defaults to HttpResponseRedirect. Subclass LocaleMiddleware and override the attribute to customize the redirects issued by the middleware.

Message Middleware

django.contrib.messages.middleware.MessageMiddleware

Enables cookie– and session-based message support.

Security Middleware

If your deployment situation allows, it’s usually a good idea to have your front-end Web server perform the functionality provided by the SecurityMiddleware. That way, if there are requests that aren’t served by Django (such as static media or user-uploaded files), they will have the same protections as requests to your Django application.

The django.middleware.security.SecurityMiddleware provides several security enhancements to the request/response cycle. The SecurityMiddleware achieves this by passing special headers to the browser. Each one can be independently enabled or disabled with a setting.

HTTP Strict Transport Security

Settings:

• SECURE_HSTS_INCLUDE_SUBDOMAINS
• SECURE_HSTS_SECONDS

For sites that should only be accessed over HTTPS, you can instruct modern browsers to refuse to connect to your domain name via an insecure connection (for a given period of time) by setting the Strict-Transport-Security header. This reduces your exposure to some SSL-stripping man-in-the-middle (MITM) attacks.

SecurityMiddleware will set this header for you on all HTTPS responses if you set the SECURE_HSTS_SECONDS setting to a non-zero integer value.

When enabling HSTS, it’s a good idea to first use a small value for testing, for example, SECURE_HSTS_SECONDS = 3600 for one hour. Each time a Web browser sees the HSTS header from your site, it will refuse to communicate non-securely (using HTTP) with your domain for the given period of time.

Once you confirm that all assets are served securely on your site (i.e. HSTS didn’t break anything), it’s a good idea to increase this value so that infrequent visitors will be protected (31536000 seconds, i.e. 1 year, is common).

Additionally, if you set the SECURE_HSTS_INCLUDE_SUBDOMAINS setting to True, SecurityMiddleware will add the includeSubDomains tag to the Strict-Transport-Security header. This is recommended (assuming all sub-domains are served exclusively using HTTPS), otherwise your site may still be vulnerable via an insecure connection to a sub-domain.

The HSTS policy applies to your entire domain, not just the URL of the response that you set the header on. Therefore, you should only use it if your entire domain is served via HTTPS only.Browsers properly respecting the HSTS header will refuse to allow users to bypass warnings and connect to a site with an expired, self-signed, or otherwise invalid SSL certificate. If you use HSTS, make sure your certificates are in good shape and stay that way!

X-Content-Type-Options: nosniff

Setting:

• SECURE_CONTENT_TYPE_NOSNIFF

Some browsers will try to guess the content types of the assets that they fetch, overriding the Content-Type header. While this can help display sites with improperly configured servers, it can also pose a security risk.

If your site serves user-uploaded files, a malicious user could upload a specially-crafted file that would be interpreted as HTML or Javascript by the browser when you expected it to be something harmless.

To prevent the browser from guessing the content type and force it to always use the type provided in the Content-Type header, you can pass the X-Content-Type-Options: nosniff header. SecurityMiddleware will do this for all responses if the SECURE_CONTENT_TYPE_NOSNIFF setting is True.

Note that in most deployment situations where Django isn’t involved in serving user-uploaded files, this setting won’t help you. For example, if your MEDIA_URL is served directly by your front-end Web server (nginx, Apache, etc.) then you’d want to set this header there.

On the other hand, if you are using Django to do something like require authorization in order to download files and you cannot set the header using your Web server, this setting will be useful.

X-XSS-Protection

Setting:

• SECURE_BROWSER_XSS_FILTER

Some browsers have the ability to block content that appears to be an XSS attack. They work by looking for Javascript content in the GET or POST parameters of a page. If the Javascript is replayed in the server’s response, the page is blocked from rendering and an error page is shown instead.

The X-XSS-Protection header is used to control the operation of the XSS filter.

To enable the XSS filter in the browser, and force it to always block suspected XSS attacks, you can pass the X-XSS-Protection: 1; mode=block header. SecurityMiddleware will do this for all responses if the SECURE_BROWSER_XSS_FILTER setting is True.

The browser XSS filter is a useful defense measure, but must not be relied upon exclusively. It cannot detect all XSS attacks and not all browsers support the header. Ensure you are still validating and all input to prevent XSS attacks.

SSL Redirect

Settings:

• SECURE_REDIRECT_EXEMPT
• SECURE_SSL_HOST
• SECURE_SSL_REDIRECT

If your site offers both HTTP and HTTPS connections, most users will end up with an unsecured connection by default. For best security, you should redirect all HTTP connections to HTTPS.

If you set the SECURE_SSL_REDIRECT setting to True, SecurityMiddleware will permanently (HTTP 301) redirect all HTTP connections to HTTPS.

For performance reasons, it’s preferable to do these redirects outside of Django, in a front-end load balancer or reverse-proxy server such as nginx. SECURE_SSL_REDIRECT is intended for the deployment situations where this isn’t an option.

If the SECURE_SSL_HOST setting has a value, all redirects will be sent to that host instead of the originally-requested host.

If there are a few pages on your site that should be available over HTTP, and not redirected to HTTPS, you can list regular expressions to match those URLs in the SECURE_REDIRECT_EXEMPT setting.

If you are deployed behind a load-balancer or reverse-proxy server and Django can’t seem to tell when a request actually is already secure, you may need to set the SECURE_PROXY_SSL_HEADER setting.

Session Middleware

django.contrib.sessions.middleware.SessionMiddleware

Enables session support.

Site Middleware

django.contrib.sites.middleware.CurrentSiteMiddleware

Adds the site attribute representing the current site to every incoming HttpRequest object.

Authentication Middleware

django.contrib.auth.middleware provides three middlewares for use in authentication:

 

• *.AuthenticationMiddleware. Adds the user attribute, representing the currently-logged-in user, to every incoming HttpRequest object.
• *.RemoteUserMiddleware. Middleware for utilizing Web server provided authentication.
• *.SessionAuthenticationMiddleware. Allows a user’s sessions to be invalidated when their password changes. This middleware must appear after *.AuthenticationMiddleware in MIDDLEWARE_CLASSES.

CSRF Protection Middleware

django.middleware.csrf.CsrfViewMiddleware

Adds protection against Cross Site Request Forgeries (CSRF) by adding hidden form fields to POST forms and checking requests for the correct value.

X-Frame-Options Middleware

django.middleware.clickjacking.XFrameOptionsMiddleware

Simple clickjacking protection via the X-Frame-Options header.

Middleware Ordering

The below table provides some hints about the ordering of various Django middleware classes:

 

Class	Notes
UpdateCacheMiddleware	Before those that modify the Vary header (SessionMiddleware, GZipMiddleware, LocaleMiddleware).
GZipMiddleware	Before any middleware that may change or use the response body. After UpdateCacheMiddleware: Modifies Vary header.
ConditionalGetMiddleware	Before CommonMiddleware: uses its Etag header when USE_ETAGS = True.
SessionMiddleware	After UpdateCacheMiddleware: Modifies Vary header.
LocaleMiddleware	One of the topmost, after SessionMiddleware (uses session data) and CacheMiddleware (modifies Vary header).
CommonMiddleware	Before any middleware that may change the response (it calculates ETags). After GZipMiddleware so it won’t calculate an ETag header on gzipped contents. Close to the top: it redirects when APPEND_SLASH or PREPEND_WWW are set to True.
CsrfViewMiddleware	Before any view middleware that assumes that CSRF attacks have been dealt with.
AuthenticationMiddleware	After SessionMiddleware: uses session storage.
MessageMiddleware	After SessionMiddleware: can use session-based storage.
FetchFromCacheMiddleware	After any middleware that modifies the Vary header: that header is used to pick a value for the cache hash-key.
FlatpageFallbackMiddleware	Should be near the bottom as it’s a last-resort type of middleware.
RedirectFallbackMiddleware	Should be near the bottom as it’s a last-resort type of middleware.

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