Internet users around the globe were facing slowed down service thanks to what's being called the biggest cyber attack in history.
The prolonged denial-of-service assault is targeting The Spamhaus Project, a European spam-fighting group that has gone after a data-storage company that offers to host any content "except child porn and anything related to terrorism."
The organization has been in a long-running feud with CyberBunker, which it says spammers use as a host to spray junk mail across the Web. Web security firm CloudFlare said Spamhaus contacted it last week, saying it had been hit with an attack big enough to knock its site offline.
Security experts say the attack uses more sophisticated techniques than most DDOS (distributed denial of service) attacks and targets the Web's infrastructure, which has led to other sites performing slowly.
"These things are essentially like nuclear bombs," Matthew Prince, CloudFlare's CEO, told the New York Times. "It's so easy to cause so much damage."
The Spamhaus Project is a nonprofit organization that patrols the Internet for spammers and publishes a list of Web servers those spammers use. According to Prince, the group may be responsible for up to 80% of all spam that gets blocked. This month, the group added CyberBunker to its blacklist.
"While we don't know who was behind this attack, Spamhaus has made plenty of enemies over the years," Prince wrote in a blog post. "Spammers aren't always the most lovable of individuals and Spamhaus has been threatened, sued, and DDoSed regularly."
In a DDoS attack, computers flood a website with requests, overwhelming its servers and causing it to crash or become inaccessible for many users.
For their part, CyberBunker isn't taking credit for the attack. But the the Dutch company, housed in a former NATO nuclear bunker, isn't shying away, either.
"This here is the internet community puking out SpamHaus," CyberBunker founder Sven Olaf Kamphuis told CNN. "We've had it with the guys ... . What we see right here is the internet puking out a cancer."
He said the owners of various websites got together on a Skype chat and hatched the plans for the attack.
He says the group organized to support CyberBunker in the dispute -- StopHaus -- ceased the attack after three days but that other hackers and activists kept it up after that.
He and other critics say that Spamhaus oversteps its bounds and has essentially destroyed innocent websites in its spam-fighting efforts.
"Spamhaus itself is a more urgent danger" than spam, he said. "Pointing at websites and saying they want it shut down and then they get it shut down without any court order. That is a significantly larger threat to internet and freedom of speech and net neutrality than anything else."
CNN's Atika Shubert contributed to this report.
Doug Gross | CNN