Using local HTTPS

Due to security considerations, Service Workers can only be registered over a secure connection (exception being the local development on localhost). In some project setups, you may require to run your local application over HTTPS and will shortly encouter the following error when starting MSW’s worker:

SecurityError: Failed to register a ServiceWorker:
  An SSL certificate error occurred when fetching the script.

This error occurs because your browser will not trust your self-signed certificate. As a result of it, the browser will block the Service Worker registration attempt and MSW will not work.

To use the library with a local HTTPS setup you must make your browser trust your self-signed certificate. This can be achieve in multiple ways described below.

Trust certificate on system level

You can add your self-signed certificate to the system’s trusted certificates. Please refer to the respective resources online on how to do so based on your operating system.

Trust certificate on browser level

Chrome / Brave

  1. Open chrome://flags (or brave://flags if using Brave);
  2. Search for unsafely-treat-insecure-origin-as-secure flag in the list;
  3. Select the “Enabled” option next to the flag.
  4. Enter your local host into the field.

If encountering certificate fetching/validation issues, set the ignore-certificate-errors flag to “Enabled”.


  1. Open your local application served by HTTPS;
  2. On the “Connection is not secure” screen click the “Advanced” button;
  3. In the section below, click the “Add Exception” button.
  4. Specify your application’s URL as the “Location” input value;
  5. Click the “Confirm Security Exception” button.


We are not aware of any settings to make Safari trust self-seigned certificates. Consider enabling System-wide trust on MacOS instead.