Cross-origin resource sharing¶
Note
This is a new feature in OpenStack Liberty.
OpenStack supports Cross-Origin Resource Sharing (CORS), a W3C specification defining a contract by which the single-origin policy of a user agent (usually a browser) may be relaxed. It permits the javascript engine to access an API that does not reside on the same domain, protocol, or port.
This feature is most useful to organizations which maintain one or more custom user interfaces for OpenStack, as it permits those interfaces to access the services directly, rather than requiring an intermediate proxy server. It can, however, also be misused by malicious actors; please review the security advisory below for more information.
Enabling CORS with configuration¶
In most cases, CORS support is built directly into the service itself. To enable it, simply follow the configuration options exposed in the default configuration file, or add it yourself according to the pattern below.
[cors]
allowed_origin = https://first_ui.example.com
max_age = 3600
allow_methods = GET,POST,PUT,DELETE
allow_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
expose_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
Additional origins can be explicitly added. To express this in
your configuration file, first begin with a [cors] group as above,
into which you place your default configuration values. Then, add as many
additional configuration groups as necessary, naming them
[cors.{something}] (each name must be unique). The purpose of the
suffix to cors. is legibility, we recommend using a reasonable
human-readable string:
[cors.ironic_webclient]
# CORS Configuration for a hypothetical ironic webclient, which overrides
# authentication
allowed_origin = https://ironic.example.com:443
allow_credentials = True
[cors.horizon]
# CORS Configuration for horizon, which uses global options.
allowed_origin = https://horizon.example.com:443
[cors.wildcard]
# CORS Configuration for the CORS specified domain wildcard, which only
# permits HTTP GET requests.
allowed_origin = *
allow_methods = GET
For more information about CORS configuration, see cross-origin resource sharing in OpenStack Configuration Reference.
Enabling CORS with PasteDeploy¶
CORS can also be configured using PasteDeploy. First of all, ensure that
OpenStack’s oslo_middleware package (version 2.4.0 or later) is
available in the Python environment that is running the service. Then,
add the following configuration block to your paste.ini file.
[filter:cors]
paste.filter_factory = oslo_middleware.cors:filter_factory
allowed_origin = https://website.example.com:443
max_age = 3600
allow_methods = GET,POST,PUT,DELETE
allow_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
expose_headers = Content-Type,Cache-Control,Content-Language,Expires,Last-Modified,Pragma,X-Custom-Header
Note
To add an additional domain in oslo_middleware v2.4.0, add
another filter. In v3.0.0 and after, you may add multiple domains
in the above allowed_origin field, separated by commas.
Security concerns¶
CORS specifies a wildcard character *, which permits access to all user
agents, regardless of domain, protocol, or host. While there are valid use
cases for this approach, it also permits a malicious actor to create a
convincing facsimile of a user interface, and trick users into revealing
authentication credentials. Please carefully evaluate your use case and the
relevant documentation for any risk to your organization.
Note
The CORS specification does not support using this wildcard as
a part of a URI. Setting allowed_origin to * would work, while
*.openstack.org would not.
Troubleshooting¶
CORS is very easy to get wrong, as even one incorrect property will violate the prescribed contract. Here are some steps you can take to troubleshoot your configuration.
Check the service log¶
The CORS middleware used by OpenStack provides verbose debug logging that should reveal most configuration problems. Here are some example log messages, and how to resolve them.
Problem¶
CORS request from origin 'http://example.com' not permitted.
Solution¶
A request was received from the origin http://example.com, however this
origin was not found in the permitted list. The cause may be a superfluous
port notation (ports 80 and 443 do not need to be specified). To correct,
ensure that the configuration property for this host is identical to the
host indicated in the log message.
Problem¶
Request method 'DELETE' not in permitted list: GET,PUT,POST
Solution¶
A user agent has requested permission to perform a DELETE request, however
the CORS configuration for the domain does not permit this. To correct, add
this method to the allow_methods configuration property.
Problem¶
Request header 'X-Custom-Header' not in permitted list: X-Other-Header
Solution¶
A request was received with the header X-Custom-Header, which is not
permitted. Add this header to the allow_headers configuration
property.
Open your browser’s console log¶
Most browsers provide helpful debug output when a CORS request is rejected. Usually this happens when a request was successful, but the return headers on the response do not permit access to a property which the browser is trying to access.
Manually construct a CORS request¶
By using curl or a similar tool, you can trigger a CORS response with a
properly constructed HTTP request. An example request and response might look
like this.
Request example:
$ curl -I -X OPTIONS https://api.example.com/api -H "Origin: https://ui.example.com"
Response example:
HTTP/1.1 204 No Content
Content-Length: 0
Access-Control-Allow-Origin: https://ui.example.com
Access-Control-Allow-Methods: GET,POST,PUT,DELETE
Access-Control-Expose-Headers: origin,authorization,accept,x-total,x-limit,x-marker,x-client,content-type
Access-Control-Allow-Headers: origin,authorization,accept,x-total,x-limit,x-marker,x-client,content-type
Access-Control-Max-Age: 3600
If the service does not return any access control headers, check the service
log, such as /var/log/upstart/ironic-api.log for an indication on what
went wrong.
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