The bug
When the mini-browser is used to sign in, an old version of Chrome is used (v61). This was released in 2017 and likely does not contain the latest security patches. I'm assuming something like sign-in shouldn't be left this vulnerable.
Steps to reproduce
Go to Settings > Accounts > Add Microsoft account
Navigate to a website such as https://amiunique.org to verify the browser version
This is by design, for compatibility purposes with macOS 10.9. We are currently looking into updating this dependency. Thanks for taking the time to report!