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Django app for handling the server headers required for Cross-Origin Resource Sharing (CORS)
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README.rst

django-cors-headers

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A Django App that adds Cross-Origin Resource Sharing (CORS) headers to responses. This allows in-browser requests to your Django application from other origins.

About CORS

Adding CORS headers allows your resources to be accessed on other domains. It's important you understand the implications before adding the headers, since you could be unintentionally opening up your site's private data to others.

Some good resources to read on the subject are:

Requirements

Python 3.5-3.7 supported.

Django 1.11-3.0 suppported.

Setup

Install from pip:

pip install django-cors-headers

and then add it to your installed apps:

INSTALLED_APPS = [
    ...
    'corsheaders',
    ...
]

You will also need to add a middleware class to listen in on responses:

MIDDLEWARE = [  # Or MIDDLEWARE_CLASSES on Django < 1.10
    ...
    'corsheaders.middleware.CorsMiddleware',
    'django.middleware.common.CommonMiddleware',
    ...
]

CorsMiddleware should be placed as high as possible, especially before any middleware that can generate responses such as Django's CommonMiddleware or Whitenoise's WhiteNoiseMiddleware. If it is not before, it will not be able to add the CORS headers to these responses.

Also if you are using CORS_REPLACE_HTTPS_REFERER it should be placed before Django's CsrfViewMiddleware (see more below).

About

django-cors-headers was created in January 2013 by Otto Yiu. It went unmaintained from August 2015 and was forked in January 2016 to the package django-cors-middleware by Laville Augustin at Zeste de Savoir. In September 2016, Adam Johnson, Ed Morley, and others gained maintenance responsibility for django-cors-headers (Issue 110) from Otto Yiu. Basically all of the changes in the forked django-cors-middleware were merged back, or re-implemented in a different way, so it should be possible to switch back. If there's a feature that hasn't been merged, please open an issue about it.

django-cors-headers has had 40+ contributors in its time; thanks to every one of them.

Configuration

Configure the middleware's behaviour in your Django settings. You must add the hosts that are allowed to do cross-site requests to CORS_ORIGIN_WHITELIST, or set CORS_ORIGIN_ALLOW_ALL to True to allow all hosts.

CORS_ORIGIN_ALLOW_ALL

If True, the whitelist will not be used and all origins will be accepted. Defaults to False.

CORS_ORIGIN_WHITELIST

A list of origins that are authorized to make cross-site HTTP requests. Defaults to [].

An Origin is defined by the CORS RFC Section 3.2 as a URI scheme + hostname + port, or one of the special values 'null' or 'file://'. Default ports (HTTPS = 443, HTTP = 80) are optional here.

The special value null is sent by the browser in "privacy-sensitive contexts", such as when the client is running from a file:// domain. The special value file:// is sent accidentally by some versions of Chrome on Android as per this bug.

Example:

CORS_ORIGIN_WHITELIST = [
    "https://example.com",
    "https://sub.example.com",
    "http://localhost:8080",
    "http://127.0.0.1:9000"
]

CORS_ORIGIN_REGEX_WHITELIST

A list of strings representing regexes that match Origins that are authorized to make cross-site HTTP requests. Defaults to []. Useful when CORS_ORIGIN_WHITELIST is impractical, such as when you have a large number of subdomains.

Example:

CORS_ORIGIN_REGEX_WHITELIST = [
    r"^https://\w+\.example\.com$",
]

The following are optional settings, for which the defaults probably suffice.

CORS_URLS_REGEX

A regex which restricts the URL's for which the CORS headers will be sent. Defaults to r'^.*$', i.e. match all URL's. Useful when you only need CORS on a part of your site, e.g. an API at /api/.

Example:

CORS_URLS_REGEX = r'^/api/.*$'

CORS_ALLOW_METHODS

A list of HTTP verbs that are allowed for the actual request. Defaults to:

CORS_ALLOW_METHODS = [
    'DELETE',
    'GET',
    'OPTIONS',
    'PATCH',
    'POST',
    'PUT',
]

The default can be imported as corsheaders.defaults.default_methods so you can just extend it with your custom methods. This allows you to keep up to date with any future changes. For example:

from corsheaders.defaults import default_methods

CORS_ALLOW_METHODS = list(default_methods) + [
    'POKE',
]

CORS_ALLOW_HEADERS

The list of non-standard HTTP headers that can be used when making the actual request. Defaults to:

CORS_ALLOW_HEADERS = [
    'accept',
    'accept-encoding',
    'authorization',
    'content-type',
    'dnt',
    'origin',
    'user-agent',
    'x-csrftoken',
    'x-requested-with',
]

The default can be imported as corsheaders.defaults.default_headers so you can extend it with your custom headers. This allows you to keep up to date with any future changes. For example:

from corsheaders.defaults import default_headers

CORS_ALLOW_HEADERS = list(default_headers) + [
    'my-custom-header',
]

CORS_EXPOSE_HEADERS

The list of HTTP headers that are to be exposed to the browser. Defaults to [].

CORS_PREFLIGHT_MAX_AGE

The number of seconds a client/browser can cache the preflight response. If this is 0 (or any falsey value), no max age header will be sent. Defaults to 86400 (one day).

Note: A preflight request is an extra request that is made when making a "not-so-simple" request (e.g. Content-Type is not application/x-www-form-urlencoded) to determine what requests the server actually accepts. Read more about it in the CORS MDN article.

CORS_ALLOW_CREDENTIALS

If True, cookies will be allowed to be included in cross-site HTTP requests. Defaults to False.

Note: in Django 2.1 the SESSION_COOKIE_SAMESITE setting was added, set to 'Lax' by default, which will prevent Django's session cookie being sent cross-domain. Change it to None to bypass this security restriction.

CSRF Integration

Most sites will need to take advantage of the Cross-Site Request Forgery protection that Django offers. CORS and CSRF are separate, and Django has no way of using your CORS configuration to exempt sites from the Referer checking that it does on secure requests. The way to do that is with its CSRF_TRUSTED_ORIGINS setting. For example:

CORS_ORIGIN_WHITELIST = [
    'http://read.only.com',
    'http://change.allowed.com',
]

CSRF_TRUSTED_ORIGINS = [
    'change.allowed.com',
]

CORS_REPLACE_HTTPS_REFERER

CSRF_TRUSTED_ORIGINS was introduced in Django 1.9, so users of earlier versions will need an alternate solution. If CORS_REPLACE_HTTPS_REFERER is True, CorsMiddleware will change the Referer header to something that will pass Django's CSRF checks whenever the CORS checks pass. Defaults to False.

Note that unlike CSRF_TRUSTED_ORIGINS, this setting does not allow you to distinguish between domains that are trusted to read resources by CORS and domains that are trusted to change resources by avoiding CSRF protection.

With this feature enabled you should also add corsheaders.middleware.CorsPostCsrfMiddleware after django.middleware.csrf.CsrfViewMiddleware in your MIDDLEWARE_CLASSES to undo the Referer replacement:

MIDDLEWARE_CLASSES = [
    ...
    'corsheaders.middleware.CorsMiddleware',
    ...
    'django.middleware.csrf.CsrfViewMiddleware',
    'corsheaders.middleware.CorsPostCsrfMiddleware',
    ...
]

Signals

If you have a use case that requires more than just the above configuration, you can attach code to check if a given request should be allowed. For example, this can be used to read the list of origins you allow from a model. Attach any number of handlers to the check_request_enabled Django signal, which provides the request argument (use **kwargs in your handler to protect against any future arguments being added). If any handler attached to the signal returns a truthy value, the request will be allowed.

For example you might define a handler like this:

# myapp/handlers.py
from corsheaders.signals import check_request_enabled

from myapp.models import MySite

def cors_allow_mysites(sender, request, **kwargs):
    return MySite.objects.filter(host=request.host).exists()

check_request_enabled.connect(cors_allow_mysites)

Then connect it at app ready time using a Django AppConfig:

# myapp/__init__.py

default_app_config = 'myapp.apps.MyAppConfig'
# myapp/apps.py

from django.apps import AppConfig

class MyAppConfig(AppConfig):
    name = 'myapp'

    def ready(self):
        # Makes sure all signal handlers are connected
        from myapp import handlers  # noqa

A common use case for the signal is to allow all origins to access a subset of URL's, whilst allowing a normal set of origins to access all URL's. This isn't possible using just the normal configuration, but it can be achieved with a signal handler.

First set CORS_ORIGIN_WHITELIST to the list of trusted origins that are allowed to access every URL, and then add a handler to check_request_enabled to allow CORS regardless of the origin for the unrestricted URL's. For example:

# myapp/handlers.py
from corsheaders.signals import check_request_enabled

def cors_allow_api_to_everyone(sender, request, **kwargs):
    return request.path.startswith('/api/')

check_request_enabled.connect(cors_allow_api_to_everyone)
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