Spam crusaders The Spamhaus Project have been battling massive distributed denial of service (DDoS) attacks that have reportedly resulted in a slowdown of the entire Web.

Spamhaus tracks the Internet's spam operations and sources, and maintains real-time, spam-blocking databases that help Internet networks weed out bogus email. Trouble started, however, when Spamhaus added a firm known as Cyberbunker to its blacklist, the BBC said.

Spamhaus said last week that it experienced a "large-scale DDoS attack" over the course of several days, knocking out its website and mail systems. By Friday, March 22, Spamhaus said all its systems were "green" and back online.

That was due, in part, to CloudFlare, which published a blog post that went into greater detail about the attack. "Spamhaus signed up for CloudFlare ... and we immediately mitigated the attack, making the site once again reachable," the company said.

CloudFlare, however, said it logged attacks that topped 75 Gbps, while the New York Times and the BBC said those attacks actually reached 300 Gbps. For comparison, a "normal" DDoS attack that might take down a bank website, for example, is around 50 Gbps, the Times said.

"It was sufficiently large to fully saturate [Spamhaus's] connection to the rest of the Internet and knock their site offline," CloudFlare said.

"These very large attacks, which are known as Layer 3 attacks, are difficult to stop with any on-premise solution," CloudFlare continued. "Put simply: if you have a router with a 10Gbps port, and someone sends you 11Gbps of traffic, it doesn't matter what intelligent software you have to stop the attack because your network link is completely saturated."

This makes it "one of the largest computer attacks on the Internet," the Times said; the BBC echoed that sentiment, calling it the "biggest cyber-attack in history."

As a result, some sites have seen some slowdowns, according to reports. Spamhaus is reportedly now working with various law enforcement agencies around the globe.

In an email, Cyberbunker wrote: "The thing we would like to say is that we (including our clients) did not and never have been sent any spam." Spamhaus did not immediately respond to a request for comment.

Update: CloudFlare posted an updated blog post this afternoon that said "while we don't have direct visibility into the traffic loads they saw, we have been told by one major Tier 1 provider that they saw more than 300Gbps of attack traffic related to this attack. That would make this attack one of the largest ever reported."

Update 2: There's some skepticism as to how big a deal this DDoS attack actually was. For more on that, check out Internet Apocalypse Now? Experts Say No.