Read HTTP Header and Pass as CORS headers in WSO2 ESB

Are you facing CORS headers issue? If you want to understand it better, it has been defined well in https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS and https://en.wikipedia.org/wiki/Cross-origin_resource_sharing. You can also follow http://enable-cors.org/.

As you read, you would understand that you may have to pass the following response headers
Access-Control-Allow-Origin
Access-Control-Allow-Methods
Access-Control-Allow-Headers

It is straightforward to add the first two headers in the response path (i.e., Out Sequence). For third header i.e., Access-Control-Allow-Headers, you may have to use the value that has come in the request header "Access-Control-Request-Headers". If so, you can achieve this in two steps:

Step 1:
Get the value for header "Access-Control-Request-Headers" and store the value in synapse config scope. It means that the value will be available both in and out sequence.

Snippet that need to be added in "In Sequence":
<property name="Access-Control-Request-Headers" expression="get-property('transport', 'Access-Control-Request-Headers')"/>

Step 2:
Get the value from synapse config, and set it as response header

Snippet that need to be added in "Out Sequence":
<property name="Access-Control-Allow-Headers"
     expression="$ctx:Access-Control-Request-Headers"
     scope="transport"
     type="STRING"/>