If the referer is your own site, check for malicious injected scripts that generate the URL. If the referer is an external unknown site, that site may be compromised.
Or
If the answer to any of these is "yes," close the browser tab immediately. Do not interact with the pop-up. Do not click "Cancel," as that can sometimes trigger the download. Instead, force-quit your browser (Ctrl+Alt+Del on Windows, Option+Command+Esc on Mac). httpswwwbuumalcom upd