Preflight Blob Request


Updated: June 29, 2016

The Preflight Blob Request operation queries the Cross-Origin Resource Sharing (CORS) rules for the Blob service prior to sending the actual request. A web browser or another user agent sends a preflight request that includes the origin domain, method, and headers for the actual request that the agent wishes to make. If CORS is enabled for the Blob service, then the Blob service evaluates the preflight request against the CORS rules that the account owner has configured via Set Blob Service Properties, and accepts or rejects the request.

For more information about CORS and the preflight request, see the CORS specification and Cross-Origin Resource Sharing (CORS) Support for the Azure Storage Services

The Preflight Blob Request may be specified as follows. Replace <account-name> with the name of your storage account. Replace <blob-resource> with the container or blob resource that is to be the target of the actual request:


Request URI

HTTP Version




Note that the URI must always include the forward slash (/) to separate the host name from the path and query portions of the URI. In the case of this operation, the path portion of the URI can be empty, or can point to any container or blob resource. The resource may or may not exist at the time the preflight request is made; the preflight request is evaluated at the service level against the service's CORS rules, so the presence or absence of the resource name does not affect the success or failure of the operation.

The following table describes required and optional request headers.

Request Header



Required. Specifies the origin from which the actual request will be issued. The origin is checked against the service's CORS rules to determine the success or failure of the preflight request.


Required. Specifies the method (or HTTP verb) for the actual request. The method is checked against the service's CORS rules to determine the failure or success of the preflight request.


Optional. Specifies the headers for the actual request headers that will be sent, then the service assumes no headers are sent with the actual request.

The response includes an HTTP status code and a set of response headers.

A successful operation returns status code 200 (OK).

For information about status codes, see Status and Error Codes.

The response for this operation includes the following headers. The response may also include additional standard HTTP headers. All standard headers conform to the HTTP/1.1 protocol specification.

See the CORS specification for details about preflight request headers.

Response Header



The allowed origin, which matches the origin header in the request if the preflight request succeeded.


If the preflight requests succeeds, this header is set to the value or values specified for the request header Access-Control-Request-Method.


If the preflight request succeeds, this header is set to the value or values specified for the request header Access-Control-Request-Headers.


Specifies the length of time the user agent is allowed to cache the preflight request for future requests.


Indicates whether the actual request can be made using credentials. This header is always set to true.

The Preflight Blob Request operation authenticates any request that includes proper authentication headers or includes a shared access signature (SAS).

Anonymous non-authenticated requests will also be processed regardless of whether the resource exists, since OPTIONS preflight requests are evaluated against the account service settings.

The following example sends a preflight request for the origin, with the request method set to PUT and the request headers set to content-type and accept.

Accept: */*
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: content-type, accept
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)
Content-Length: 0

The response indicates that CORS is enabled for the service, and that a CORS rule matches the preflight request.

HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Length: 0
Content-Type: text/html; charset=UTF-8
Access-Control-Allow-Origin: *
Access-Control-Max-Age: 60
Access-Control-Allow-Methods: PUT
Access-Control-Allow-Headers: accept,content-type

If CORS is enabled for the service and there is a CORS rule that matches the preflight request, the service responds to the preflight request with status code 200 (OK). The response includes the required Access-Control headers. In this case, the request will be billed.

If CORS is not enabled or no CORS rule matches the preflight request, the service responds with status code 403 (Forbidden). In this case, the request will not be billed.

If the OPTIONS request is malformed, the service responds with status code 400 (Bad Request) and the request is not billed. An example of a malformed request is one that doesn’t contain the required Origin and Access-Control-Request-Method headers.

Note that the preflight request is a mechanism to query the CORS capability of a storage service associated with a certain storage account. The preflight request is not targeted to a specific resource.