Internet attack: BBC's Rory Cellan-Jones explains
27 March 2013 Last updated at 17:16 GMT
The internet around the world has been slowed down in what security experts are describing as the biggest cyber-attack of its kind in history.
A row between a spam-fighting group and hosting firm has sparked retaliation attacks affecting the wider internet.
David Emm from Kaspersky Lab and the BBC's Rory Cellan-Jones examine what the implications are for web users.