I know thats not 100% backdoor but if you have some old expired password for
sexyhub or fakehub sites you can enjoy many videos in 720p or 1080p resolution.
That HD links you can found under 'Viev page source' option of your browser.
Search for 'mp4' and first found will be 1080p video link.
You must edit that link b4 use ... very easy, delete 5 '\' and your link is ready
If thats help you please Karma me thanks!