Simon Willison’s Weblog

I stumbled across a nasty XSS hole involving DNS A records. Found out today that an old subdomain that I had assigned an IP address to via a DNS A record was serving unexpected content—turned out I’d shut down the associated VPS and the IP had been recycled to someone else, so their content was now appearing under my domain. It strikes me that if you got really unlucky this could turn into an XSS hole—and that new server could even use Let’s Encrypt to obtain an HTTPS certificate for your subdomain.

I’ve added “audit your A records” to my personal security checklist.