Reason: missing token 'xyz' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel

The Access-Control-Allow-Headers header is sent by the server to let the client know which headers it supports for CORS requests. The value of Access-Control-Allow-Headers should be a comma-delineated list of header names, such as "X-Custom-Information" or any of the standard but non-basic header names (which are always allowed).

This error occurs when attempting to preflight a header that is not expressly allowed (that is, it's not included in the list specified by the Access-Control-Allow-Headers header sent by the server). To fix this, the server needs to be updated so that it allows the indicated header, or you need to avoid using that header.

• CORS errors
• Glossary: CORS
• CORS introduction
• HTTP headers