Chapter 19 – Django Middleware

Middleware is a framework of hooks into Djangos request/response processing. Its a light, low-level plugin system for globally altering Djangos input or output.

Each middleware component is responsible for doing some specific function. For example, Django includes a middleware component, AuthenticationMiddleware, that associates users with requests using sessions.

This document explains how middleware works, how you activate middleware, and how to write your own middleware. Django ships with some built-in middleware you can use right out of the box. See Available Middleware later in this chapter.

Activating middleware

To activate a middleware component, add it to the MIDDLEWARE_CLASSES list in your Django settings.

In MIDDLEWARE_CLASSES, each middleware component is represented by a string: the full Python path to the middlewares class name. For example, heres the default value created by django-admin startproject:

MIDDLEWARE_CLASSES = [
    "django.contrib.sessions.middleware.SessionMiddleware",
    "django.middleware.common.CommonMiddleware",
    "django.middleware.csrf.CsrfViewMiddleware",
    "django.contrib.auth.middleware.AuthenticationMiddleware",
    "django.contrib.messages.middleware.MessageMiddleware",
    "django.middleware.clickjacking.XFrameOptionsMiddleware",
]

A Django installation doesnt require any middleware  MIDDLEWARE_CLASSES can be empty, if youd like but its strongly suggested that you at least use CommonMiddleware.

The order in MIDDLEWARE_CLASSES matters because a middleware can depend on other middleware. For instance, AuthenticationMiddleware stores the authenticated user in the session; therefore, it must run afterSessionMiddleware. See middleware-ordering for some common hints about ordering of Django middleware classes.

Hooks and application order

During the request phase, before calling the view, Django applies middleware in the order its defined inMIDDLEWARE_CLASSES, top-down. Two hooks are available:

• process_request()
• process_view()

During the response phase, after calling the view, middleware are applied in reverse order, from the bottom up. Three hooks are available:

• process_exception() (only if the view raised an exception)
• process_template_response() (only for template responses)
• process_response()

If you prefer, you can also think of it like an onion: each middleware class is a layer that wraps the view.

The behavior of each hook is described below.

Writing your own middleware

Writing your own middleware is easy. Each middleware component is a single Python class that defines one or more of the following methods:

process_request

process_request(request)

request is an HttpRequest object.

process_request() is called on each request, before Django decides which view to execute.

It should return either None or an HttpResponse object. If it returns None, Django will continue processing this request, executing any other process_request() middleware, then, process_view() middleware, and finally, the appropriate view. If it returns an HttpResponse object, Django wont bother calling any other request, view or exception middleware, or the appropriate view; itll apply response middleware to thatHttpResponse, and return the result.

process_view

process_view(request, view_func, view_args, view_kwargs)

request is an HttpRequest object. view_func is the Python function that Django is about to use. (Its the actual function object, not the name of the function as a string.) view_args is a list of positional arguments that will be passed to the view, and view_kwargs is a dictionary of keyword arguments that will be passed to the view. Neither view_args nor view_kwargs include the first view argument (request).

process_view() is called just before Django calls the view.

It should return either None or an HttpResponse object. If it returns None, Django will continue processing this request, executing any other process_view() middleware and, then, the appropriate view. If it returns anHttpResponse object, Django wont bother calling any other view or exception middleware, or the appropriate view; itll apply response middleware to that HttpResponse, and return the result.

Note

Accessing request.POST inside middleware from process_request or process_view will prevent any view running after the middleware from being able to modify the upload handlers for the request, and should normally be avoided.

The CsrfViewMiddleware class can be considered an exception, as it provides the csrf_exempt() andcsrf_protect() decorators which allow views to explicitly control at what point the CSRF validation should occur.

process_template_response

process_template_response(request, response)

request is an HttpRequest object. response is the TemplateResponse object (or equivalent) returned by a Django view or by a middleware.

process_template_response() is called just after the view has finished executing, if the response instance has a render() method, indicating that it is a TemplateResponse or equivalent.

It must return a response object that implements a render method. It could alter the given response by changing response.template_name and response.context_data, or it could create and return a brand-newTemplateResponse or equivalent.

You dont need to explicitly render responses responses will be automatically rendered once all template response middleware has been called.

Middleware are run in reverse order during the response phase, which includesprocess_template_response().

process_response

process_response(request, response)

request is an HttpRequest object. response is the HttpResponse or StreamingHttpResponse object returned by a Django view or by a middleware.

process_response() is called on all responses before theyre returned to the browser.

It must return an HttpResponse or StreamingHttpResponse object. It could alter the given response, or it could create and return a brand-new HttpResponse or StreamingHttpResponse.

Unlike the process_request() and process_view() methods, the process_response() method is always called, even if the process_request() and process_view() methods of the same middleware class were skipped (because an earlier middleware method returned an HttpResponse). In particular, this means that yourprocess_response() method cannot rely on setup done in process_request().

Finally, remember that during the response phase, middleware are applied in reverse order, from the bottom up. This means classes defined at the end of MIDDLEWARE_CLASSES will be run first.

DEALING WITH STREAMING RESPONSES

Unlike HttpResponse, StreamingHttpResponse does not have a content attribute. As a result, middleware can no longer assume that all responses will have a content attribute. If they need access to the content, they must test for streaming responses and adjust their behavior accordingly:

if response.streaming:
    response.streaming_content = wrap_streaming_content(response.streaming_content)
else:
    response.content = alter_content(response.content)

Note

streaming_content should be assumed to be too large to hold in memory. Response middleware may wrap it in a new generator, but must not consume it. Wrapping is typically implemented as follows:

def wrap_streaming_content(content):
    for chunk in content:
        yield alter_content(chunk)

process_exception

process_exception(request, exception)

request is an HttpRequest object. exception is an Exception object raised by the view function.

Django calls process_exception() when a view raises an exception. process_exception() should return eitherNone or an HttpResponse object. If it returns an HttpResponse object, the template response and response middleware will be applied, and the resulting response returned to the browser. Otherwise, default exception handling kicks in.

Again, middleware are run in reverse order during the response phase, which includes process_exception. If an exception middleware returns a response, the middleware classes above that middleware will not be called at all.

__init__

Most middleware classes wont need an initializer since middleware classes are essentially placeholders for the process_* methods. If you do need some global state you may use __init__ to set up. However, keep in mind a couple of caveats:

• Django initializes your middleware without any arguments, so you cant define __init__ as requiring any arguments.
• Unlike the process_* methods which get called once per request, __init__ gets called only once, when the Web server responds to the first request.

MARKING MIDDLEWARE AS UNUSED

Its sometimes useful to determine at run-time whether a piece of middleware should be used. In these cases, your middlewares __init__ method may raise django.core.exceptions.MiddlewareNotUsed. Django will then remove that piece of middleware from the middleware process and a debug message will be logged to the django.request logger when DEBUG is set to True.

Guidelines

• Middleware classes dont have to subclass anything.
• The middleware class can live anywhere on your Python path. All Django cares about is that theMIDDLEWARE_CLASSES setting includes the path to it.
• Feel free to look at Djangos available middleware for examples.
• If you write a middleware component that you think would be useful to other people, contribute to the community! Let us know and well consider adding it to Django.

Available middleware

Cache middleware

class django.middleware.cache.UpdateCacheMiddleware

class 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. See the cache documentation .

Common middleware

class 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 doesnt 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 dont have a valid URL pattern forfoo.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 its 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 itll take care of sending Not Modified responses, if appropriate.

CommonMiddleware.response_redirect_class

Defaults to HttpResponsePermanentRedirect. Subclass CommonMiddleware and override the attribute to customize the redirects issued by the middleware.

class django.middleware.common.BrokenLinkEmailsMiddleware

• Sends broken link notification emails to MANAGERS

GZip middleware

class django.middleware.gzip.GZipMiddleware

Warning

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, Djangos CSRF protection. Before using GZipMiddleware on your site, you should consider very carefully whether you are subject to these attacks. If youre in any doubt about whether youre affected, you should avoid using GZipMiddleware. For more details, see the the BREACH paper (PDF) and breachattack.com.

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.

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) hasnt sent an Accept-Encoding header containing gzip.

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

Conditional GET middleware

class 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

class django.middleware.locale.LocaleMiddleware

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

LocaleMiddleware.response_redirect_class

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

Message middleware

class django.contrib.messages.middleware.MessageMiddleware

Enables cookie- and session-based message support. See the messages documentation .

Security middleware

Warning

If your deployment situation allows, its 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 arent served by Django (such as static media or user-uploaded files), they will have the same protections as requests to your Django application.

class django.middleware.security.SecurityMiddleware

The django.middleware.security.SecurityMiddleware provides several security enhancements to the request/response cycle. Each one can be independently enabled or disabled with a setting.

• SECURE_BROWSER_XSS_FILTER
• SECURE_CONTENT_TYPE_NOSNIFF
• SECURE_HSTS_INCLUDE_SUBDOMAINS
• SECURE_HSTS_SECONDS
• SECURE_REDIRECT_EXEMPT
• SECURE_SSL_HOST
• SECURE_SSL_REDIRECT

HTTP STRICT TRANSPORT SECURITY

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_SECONDSsetting to a non-zero integer value.

When enabling HSTS, its a good idea to first use a small value for testing, for example, SECURE_HSTS_SECONDS= 3600<SECURE_HSTS_SECONDS> 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 didnt break anything), its 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 theincludeSubDomains tag to the Strict-Transport-Security header. This is recommended (assuming all subdomains are served exclusively using HTTPS), otherwise your site may still be vulnerable via an insecure connection to a subdomain.

Warning

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!

Note

If you are deployed behind a load-balancer or reverse-proxy server, and the Strict-Transport-Securityheader is not being added to your responses, it may be because Django doesnt realize that its on a secure connection; you may need to set the SECURE_PROXY_SSL_HEADER setting.

X-CONTENT-TYPE-OPTIONS: NOSNIFF

Some browsers will try to guess the content types of the assets that they fetch, overriding the Content-Typeheader. 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 learn more about this header and how the browser treats it, you can read about it on the IE Security Blog.

To prevent the browser from guessing the content type and force it to always use the type provided in theContent-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 isnt involved in serving user-uploaded files, this setting wont help you. For example, if your MEDIA_URL is served directly by your front-end Web server (nginx, Apache, etc.) then youd 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: 1; MODE=BLOCK

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 servers 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 theSECURE_BROWSER_XSS_FILTER setting is True.

Warning

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

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.

Note

For performance reasons, its 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 isnt 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.

Note

If you are deployed behind a load-balancer or reverse-proxy server and Django cant seem to tell when a request actually is already secure, you may need to set the SECURE_PROXY_SSL_HEADER setting.

Session middleware

class django.contrib.sessions.middleware.SessionMiddleware

Enables session support. See the session documentation.

Site middleware

class django.contrib.sites.middleware.CurrentSiteMiddleware

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

Authentication middleware

class django.contrib.auth.middleware.AuthenticationMiddleware

Adds the user attribute, representing the currently-logged-in user, to every incoming HttpRequest object. See Authentication in Web requests.

class django.contrib.auth.middleware.RemoteUserMiddleware

Middleware for utilizing Web server provided authentication. See auth-remote-user for usage details.

class django.contrib.auth.middleware.SessionAuthenticationMiddleware

Allows a users sessions to be invalidated when their password changes. See session-invalidation-on-password-change for details. This middleware must appear afterdjango.contrib.auth.middleware.AuthenticationMiddleware in MIDDLEWARE_CLASSES.

CSRF protection middleware

class django.middleware.csrf.CsrfViewMiddleware

Adds protection against Cross Site Request Forgeries by adding hidden form fields to POST forms and checking requests for the correct value. See the Cross Site Request Forgery protection documentation .

X-Frame-Options middleware

class django.middleware.clickjacking.XFrameOptionsMiddleware

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

Middleware ordering

Here are some hints about the ordering of various Django middleware classes:

1. UpdateCacheMiddleware

   Before those that modify the Vary header (SessionMiddleware, GZipMiddleware, LocaleMiddleware).

2. GZipMiddleware

   Before any middleware that may change or use the response body.

   After UpdateCacheMiddleware: Modifies Vary header.

3. ConditionalGetMiddleware

   Before CommonMiddleware: uses its Etag header when USE_ETAGS = True.

4. SessionMiddleware

   After UpdateCacheMiddleware: Modifies Vary header.

5. LocaleMiddleware

   One of the topmost, after SessionMiddleware (uses session data) and CacheMiddleware (modifies Vary header).

6. CommonMiddleware

   Before any middleware that may change the response (it calculates ETags).

   After GZipMiddleware so it wont calculate an ETag header on gzipped contents.

   Close to the top: it redirects when APPEND_SLASH or PREPEND_WWW are set to True.

7. CsrfViewMiddleware

   Before any view middleware that assumes that CSRF attacks have been dealt with.

8. AuthenticationMiddleware

   After SessionMiddleware: uses session storage.

9. MessageMiddleware

   After SessionMiddleware: can use session-based storage.

10. FetchFromCacheMiddleware

After any middleware that modifies the `Vary` header: that header is used to pick a value for the cache hash-key.

1. FlatpageFallbackMiddleware

Should be near the bottom as its a last-resort type of middleware.

1. RedirectFallbackMiddleware

Should be near the bottom as its a last-resort type of middleware.