========================
CORS support for aiohttp

aiohttp_cors library implements
Cross Origin Resource Sharing (CORS) <cors_>__
support for aiohttp <aiohttp_>__
asyncio-powered asynchronous HTTP server.

Jump directly to Usage_ part to see how to use aiohttp_cors.

Same-origin policy

Web security model is tightly connected to
Same-origin policy (SOP) <sop_>__.
In short: web pages cannot Read resources which origin
doesn't match origin of requested page, but can Embed (or Execute)
resources and have limited ability to Write resources.

Origin of a page is defined in the Standard <cors_>__ as tuple
(schema, host, port)
(there is a notable exception with Internet Explorer: it doesn't use port to
define origin, but uses it's own
Security Zones <https://msdn.microsoft.com/en-us/library/ms537183.aspx>__).

Can Embed means that resource from other origin can be embedded into
the page,
e.g. by using <script src="...">, <img src="...">,
<iframe src="...">.

Cannot Read means that resource from other origin source cannot be
obtained by page
(source — any information that would allow to reconstruct resource).
E.g. the page can Embed image with <img src="...">,
but it can't get information about specific pixels, so page can't reconstruct
original image
(though some information from the other resource may still be leaked:
e.g. the page can read embedded image dimensions).

Limited ability to Write means, that the page can send POST requests to
other origin with limited set of Content-Type values and headers.

Restriction to Read resource from other origin is related to authentication
mechanism that is used by browsers:
when browser reads (downloads) resource he automatically sends all security
credentials that user previously authorized for that resource
(e.g. cookies, HTTP Basic Authentication).

For example, if Read would be allowed and user is authenticated
in some internet banking,
malicious page would be able to embed internet banking page with iframe
(since authentication is done by the browser it may be embedded as if
user is directly navigated to internet banking page),
then read user private information by reading source of the embedded page
(which may be not only source code, but, for example,
screenshot of the embedded internet banking page).

Cross-origin resource sharing

Cross-origin Resource Sharing (CORS) <cors_>__ allows to override
SOP for specific resources.

In short, CORS works in the following way.

When page https://client.example.com request (Read) resource
https://server.example.com/resource that have other origin,
browser implicitly appends Origin: https://client.example.com header
to the HTTP request,
effectively requesting server to give read permission for
the resource to the https://client.example.com page::

GET /resource HTTP/1.1
Origin: https://client.example.com
Host: server.example.com

If server allows access from the page to the resource, it responds with
resource with Access-Control-Allow-Origin: https://client.example.com
HTTP header
(optionally allowing exposing custom server headers to the page and
enabling use of the user credentials on the server resource)::

Access-Control-Allow-Origin: https://client.example.com
Access-Control-Allow-Credentials: true
Access-Control-Expose-Headers: X-Server-Header

Browser checks, if server responded with proper
Access-Control-Allow-Origin header and accordingly allows or denies
access for the obtained resource to the page.

CORS specification designed in a way that servers that are not aware
of CORS will not expose any additional information, except allowed by the
SOP.

To request resources with custom headers or using custom HTTP methods
(e.g. PUT, DELETE) that are not allowed by SOP,
CORS-enabled browser first send preflight request to the
resource using OPTIONS method, in which he queries access to the resource
with specific method and headers::

OPTIONS / HTTP/1.1
Origin: https://client.example.com
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: X-Client-Header

CORS-enabled server responds is requested method is allowed and which of
the specified headers are allowed::

Access-Control-Allow-Origin: https://client.example.com
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: PUT
Access-Control-Allow-Headers: X-Client-Header
Access-Control-Max-Age: 3600

Browser checks response to preflight request, and, if actual request allowed,
does actual request.

Installation

You can install aiohttp_cors as a typical Python library from PyPI or
from git:

.. code-block:: bash

$ pip install aiohttp_cors

Usage

To use aiohttp_cors you need to configure the application and
enable CORS on
resources and routes <https://aiohttp.readthedocs.org/en/stable/web.html#resources-and-routes>__
that you want to expose:

.. code-block:: python

import asyncio
from aiohttp import web
import aiohttp_cors

@asyncio.coroutine
def handler(request):
    return web.Response(
        text="Hello!",
        headers={
            "X-Custom-Server-Header": "Custom data",
        })

app = web.Application()

# `aiohttp_cors.setup` returns `aiohttp_cors.CorsConfig` instance.
# The `cors` instance will store CORS configuration for the
# application.
cors = aiohttp_cors.setup(app)

# To enable CORS processing for specific route you need to add
# that route to the CORS configuration object and specify its
# CORS options.
resource = cors.add(app.router.add_resource("/hello"))
route = cors.add(
    resource.add_route("GET", handler), {
        "http://client.example.org": aiohttp_cors.ResourceOptions(
            allow_credentials=True,
            expose_headers=("X-Custom-Server-Header",),
            allow_headers=("X-Requested-With", "Content-Type"),
            max_age=3600,
        )
    })

Each route has it's own CORS configuration passed in CorsConfig.add()
method.

CORS configuration is a mapping from origins to options for that origins.

In the example above CORS is configured for the resource under path /hello
and HTTP method GET, and in the context of CORS:

This resource will be available using CORS only to
http://client.example.org origin.
Passing of credentials to this resource will be allowed.
The resource will expose to the client X-Custom-Server-Header
server header.
The client will be allowed to pass X-Requested-With and
Content-Type headers to the server.
Preflight requests will be allowed to be cached by client for 3600
seconds.

Resource will be available only to the explicitly specified origins.
You can specify "all other origins" using special * origin:

.. code-block:: python

cors.add(route, {
        "*":
            aiohttp_cors.ResourceOptions(allow_credentials=False),
        "http://client.example.org":
            aiohttp_cors.ResourceOptions(allow_credentials=True),
    })

Here the resource specified by route will be available to all origins with
disallowed credentials passing, and with allowed credentials passing only to
http://client.example.org.

By default ResourceOptions will be constructed without any allowed CORS
options.
This means, that resource will be available using CORS to specified origin,
but client will not be allowed to send either credentials,
or send non-simple headers, or read from server non-simple headers.

To enable sending or receiving all headers you can specify special value
* instead of sequence of headers:

.. code-block:: python

cors.add(route, {
        "http://client.example.org":
            aiohttp_cors.ResourceOptions(
                expose_headers="*",
                allow_headers="*"),
    })

You can specify default CORS-enabled resource options using
aiohttp_cors.setup()'s defaults argument:

.. code-block:: python

cors = aiohttp_cors.setup(app, defaults={
        # Allow all to read all CORS-enabled resources from
        # http://client.example.org.
        "http://client.example.org": aiohttp_cors.ResourceOptions(),
    })

# Enable CORS on routes.

# According to defaults POST and PUT will be available only to
# "http://client.example.org".
hello_resource = cors.add(app.router.add_resource("/hello"))
cors.add(hello_resource.add_route("POST", handler_post))
cors.add(hello_resource.add_route("PUT", handler_put))

# In addition to "http://client.example.org", GET request will be
# allowed from "http://other-client.example.org" origin.
cors.add(hello_resource.add_route("GET", handler), {
        "http://other-client.example.org":
            aiohttp_cors.ResourceOptions(),
    })

# CORS will be enabled only on the resources added to `CorsConfig`,
# so following resource will be NOT CORS-enabled.
app.router.add_route("GET", "/private", handler)

Also you can specify default options for resources:

.. code-block:: python

# Allow POST and PUT requests from "http://client.example.org" origin.
hello_resource = cors.add(app.router.add_resource("/hello"), {
        "http://client.example.org": aiohttp_cors.ResourceOptions(),
    })
cors.add(hello_resource.add_route("POST", handler_post))
cors.add(hello_resource.add_route("PUT", handler_put))

Resource CORS configuration allows to use allow_methods option that
explicitly specifies list of allowed HTTP methods for origin
(or * for all HTTP methods).
By using this option it is not required to add all resource routes to
CORS configuration object:

.. code-block:: python

# Allow POST and PUT requests from "http://client.example.org" origin.
hello_resource = cors.add(app.router.add_resource("/hello"), {
        "http://client.example.org":
            aiohttp_cors.ResourceOptions(allow_methods=["POST", "PUT"]),
    })
# No need to add POST and PUT routes into CORS configuration object.
hello_resource.add_route("POST", handler_post)
hello_resource.add_route("PUT", handler_put)
# Still you can add additional methods to CORS configuration object:
cors.add(hello_resource.add_route("DELETE", handler_delete))

Here is an example of how to enable CORS for all origins with all CORS
features:

.. code-block:: python

cors = aiohttp_cors.setup(app, defaults={
    "*": aiohttp_cors.ResourceOptions(
            allow_credentials=True,
            expose_headers="*",
            allow_headers="*",
        )
})

# Add all resources to `CorsConfig`.
resource = cors.add(app.router.add_resource("/hello"))
cors.add(resource.add_route("GET", handler_get))
cors.add(resource.add_route("PUT", handler_put))
cors.add(resource.add_route("POST", handler_put))
cors.add(resource.add_route("DELETE", handler_delete))

Old routes API is supported — you can use router.add_router and
router.register_route as before, though this usage is discouraged:

.. code-block:: python

cors.add(
    app.router.add_route("GET", "/hello", handler), {
        "http://client.example.org": aiohttp_cors.ResourceOptions(
            allow_credentials=True,
            expose_headers=("X-Custom-Server-Header",),
            allow_headers=("X-Requested-With", "Content-Type"),
            max_age=3600,
        )
    })

You can enable CORS for all added routes by accessing routes list
in the router:

.. code-block:: python

# Setup application routes.
app.router.add_route("GET", "/hello", handler_get)
app.router.add_route("PUT", "/hello", handler_put)
app.router.add_route("POST", "/hello", handler_put)
app.router.add_route("DELETE", "/hello", handler_delete)

# Configure default CORS settings.
cors = aiohttp_cors.setup(app, defaults={
    "*": aiohttp_cors.ResourceOptions(
            allow_credentials=True,
            expose_headers="*",
            allow_headers="*",
        )
})

# Configure CORS on all routes.
for route in list(app.router.routes()):
    cors.add(route)

You can also use CorsViewMixin on web.View:

.. code-block:: python

class CorsView(web.View, CorsViewMixin):

    cors_config = {
        "*": ResourceOption(
            allow_credentials=True,
            allow_headers="X-Request-ID",
        )
    }

    @asyncio.coroutine
    def get(self):
        return web.Response(text="Done")

    @custom_cors({
        "*": ResourceOption(
            allow_credentials=True,
            allow_headers="*",
        )
    })
    @asyncio.coroutine
    def post(self):
        return web.Response(text="Done")

cors = aiohttp_cors.setup(app, defaults={
    "*": aiohttp_cors.ResourceOptions(
            allow_credentials=True,
            expose_headers="*",
            allow_headers="*",
        )
})

cors.add(
    app.router.add_route("*", "/resource", CorsView),
    webview=True)

Security

TODO: fill this

Development

To setup development environment:

.. code-block:: bash

Clone sources repository:

git clone https://github.com/aio-libs/aiohttp_cors.git .

Create and activate virtual Python environment:

python3 -m venv env
source env/bin/activate

Install requirements and aiohttp_cors into virtual environment

pip install -r requirements-dev.txt

To run tests:

.. code-block:: bash

tox

To run only runtime tests in current environment:

.. code-block:: bash

py.test

To run only static code analysis checks:

.. code-block:: bash

tox -e check

Running Selenium tests

To run Selenium tests with Firefox web driver you need to install Firefox.

To run Selenium tests with Chromium web driver you need to:

Install Chrome driver. On Ubuntu 14.04 it's in chromium-chromedriver
package.
Either add chromedriver to PATH or set WEBDRIVER_CHROMEDRIVER_PATH
environment variable to chromedriver, e.g. on Ubuntu 14.04
WEBDRIVER_CHROMEDRIVER_PATH=/usr/lib/chromium-browser/chromedriver.

Release process

To release version vA.B.C from the current version of master branch
you need to:

Create local branch vA.B.C.
In CHANGES.rst set release date to today.
In aiohttp_cors/__about__.py change version from A.B.Ca0 to
A.B.C.
Create pull request with vA.B.C branch, wait for all checks to
successfully finish (Travis and Appveyor).
Merge pull request to master.
Update and checkout master branch.
Create and push tag for release version to GitHub:

.. code-block:: bash

git tag vA.B.C
git push --tags

Now Travis should ran tests again, and build and deploy wheel on PyPI.

If Travis release doesn't work for some reason, use following steps
for manual release upload.
1. Install fresh versions of setuptools and pip.
  Install wheel for building wheels.
  Install twine for uploading to PyPI.
  
  .. code-block:: bash
  
  pip install -U pip setuptools twine wheel
2. Configure PyPI credentials in ~/.pypirc.
3. Build distribution:
  
  .. code-block:: bash
  
  rm -rf build dist; python setup.py sdist bdist_wheel
4. Upload new release to PyPI:
  
  .. code-block:: bash
  
  twine upload dist/*
Edit release description on GitHub if needed.
Announce new release on the aio-libs mailing list:
https://groups.google.com/forum/#!forum/aio-libs.

Post release steps:

In CHANGES.rst add template for the next release.
In aiohttp_cors/__about__.py change version from A.B.C to
A.(B + 1).0a0.

Bugs

Please report bugs, issues, feature requests, etc. on
GitHub <https://github.com/aio-libs/aiohttp_cors/issues>__.

License

Copyright 2015 Vladimir Rutsky vladimir@rutsky.org.

Licensed under the
Apache License, Version 2.0 <https://www.apache.org/licenses/LICENSE-2.0>__,
see LICENSE file for details.

.. _cors: http://www.w3.org/TR/cors/
.. _aiohttp: https://github.com/KeepSafe/aiohttp/
.. _sop: https://en.wikipedia.org/wiki/Same-origin_policy

=========
CHANGES

0.8.1 (2025-03-31)

Fix packaging to not install on Python 3.8.

0.8.0 (2025-03-11)

Make the library compatible with aiohttp 3.9+ and Python 3.9+

0.7.0 (2018-03-05)

Make web view check implicit and type based (#159)
Disable Python 3.4 support (#156)
Support aiohttp 3.0+ (#155)

0.6.0 (2017-12-21)

Support aiohttp views by CorsViewMixin (#145)

0.5.3 (2017-04-21)

Fix typing being installed on Python 3.6.

0.5.2 (2017-03-28)

Fix tests compatibility with aiohttp 2.0.
This release and release v0.5.0 should work on aiohttp 2.0.

0.5.1 (2017-03-23)

Enforce aiohttp version to be less than 2.0.
Newer aiohttp releases will be supported in the next release.

0.5.0 (2016-11-18)

Fix compatibility with aiohttp 1.1

0.4.0 (2016-04-04)

Fixed support with new Resources objects introduced in aiohttp 0.21.0.
Minimum supported version of aiohttp is 0.21.4 now.

New Resources objects are supported.
You can specify default configuration for a Resource and use
allow_methods to explicitly list allowed methods (or * for all
HTTP methods):

.. code-block:: python

  # Allow POST and PUT requests from "http://client.example.org" origin.
  hello_resource = cors.add(app.router.add_resource("/hello"), {
          "http://client.example.org":
              aiohttp_cors.ResourceOptions(
                  allow_methods=["POST", "PUT"]),
      })
  # No need to add POST and PUT routes into CORS configuration object.
  hello_resource.add_route("POST", handler_post)
  hello_resource.add_route("PUT", handler_put)
  # Still you can add additional methods to CORS configuration object:
  cors.add(hello_resource.add_route("DELETE", handler_delete))

AbstractRouterAdapter was completely rewritten to be more Router
agnostic.

0.3.0 (2016-02-06)

Rename UrlDistatcherRouterAdapter to UrlDispatcherRouterAdapter.
Set maximum supported aiohttp version to 0.20.2, see bug #30 for
details.

0.2.0 (2015-11-30)

Move ABCs from aiohttp_cors.router_adapter to aiohttp_cors.abc.
Rename RouterAdapter to AbstractRouterAdapter.
Fix bug with configuring CORS for named routes.

0.1.0 (2015-11-05)

Initial release.

Package profile

aiohttp-cors

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