Reason

Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers'

What went wrong?

The response to the CORS request that was sent by the server includes an Access-Control-Allow-Headers header which includes at least one invalid header name.

The Access-Control-Allow-Headers header is sent by the server in response to a preflight request; it lets the client know which HTTP headers are permitted in CORS requests. If the client user agent finds among the comma-delineated values provided by the header any header name it does not recognize, this error occurs.

This is a problem that most likely can only be fixed on the server side, by modifying the server's configuration to no longer send the invalid or unknown header name with the Access-Control-Allow-Headers header. It may also be worth checking to ensure that the user agent or HTTP library you're using on the client is up-to-date.

See also

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