Rocket.Chat/app/lib/client/OAuthProxy.js

import _ from 'underscore';
import { OAuth } from 'meteor/oauth';

import { settings } from '../../settings';

OAuth.launchLogin = _.wrap(OAuth.launchLogin, function(func, options) {
	const proxy = settings.get('Accounts_OAuth_Proxy_services').replace(/\s/g, '').split(',');
	if (proxy.includes(options.loginService)) {
		const redirect_uri = options.loginUrl.match(/(&redirect_uri=)([^&]+|$)/)[2];
		options.loginUrl = options.loginUrl.replace(/(&redirect_uri=)([^&]+|$)/, `$1${ encodeURIComponent(settings.get('Accounts_OAuth_Proxy_host')) }/oauth_redirect`);
		options.loginUrl = options.loginUrl.replace(/(&state=)([^&]+|$)/, `$1${ redirect_uri }!$2`);
		options.loginUrl = `${ settings.get('Accounts_OAuth_Proxy_host') }/redirect/${ encodeURIComponent(options.loginUrl) }`;
	}

	return func(options);
});