Error Troubleshooting

Request header field dwt-md5 is not allowed by Access-Control-Allow-Headers in preflight response

Symptom

When you fail to upload images, you may get this error

Cause

dwt-md5 is a default built-in header in Dynamic Web TWAIN . It is used for each uploading process to test the integrity of data. Since this is not a standard header, the browser will send an OPTIONS preflight request before the original request is sent to verify that this header is allowed. If not, the browser will return the above error.

Solution

Update your server-side configuration file as per your environment. If you are using IIS, you can refer to the following configuration.

<system.webServer>
    <httpProtocol>
        <customHeaders>
            <add name="Access-Control-Allow-Origin" value="*" />
            <add name="Access-Control-Allow-Methods" value="OPTIONS, POST, GET, PUT" />
            <add name="Access-Control-Allow-Headers" value="x-requested-with, dwt-md5" />
            <add name="Access-Control-Allow-Credentials" value="true" />
        </customHeaders>
    </httpProtocol>
</system.webServer>

Note

After updating the server configuration file, you’ll need to restart the server (i.e. IIS).

Check out more info here