Fast flux is a DNS technique used by botnets to hide phishing and malware delivery sites behind an ever-changing network of compromised hosts acting as proxies. It can also refer to the combination of peer-to-peer networking, distributed command and control, web-based load balancing and proxy redirection used to make malware networks more resistant to discovery and counter-measures. The Storm Worm is one of the recent malware variants to make use of this technique.

Internet users may see fast flux used in phishing attacks linked to criminal organizations, including attacks on MySpace.

While security researchers have been aware of the technique since at least November 2006, the technique has only received wider attention in the security trade press starting from July 2007.