It was a busy weekend for the international hacktivist movement Anonymous, as hackers claiming affiliation with the group knocked websites belonging to NATO and the city of Chicago offline.
The distributed denial-of-service (DDoS) attacks that blocked access to the sites Sunday (May 20) were timed to coincide with street demonstrations in Chicago against the summit of NATO leaders taking place in the city Saturday and Sunday.
They were unreachable again as of noon ET today (May 21).
"We are aware of the potential issue with the city's website and are working with the appropriate federal authorities to address the situation," a city spokesman told Reuters.
Forty-five people were arrested by Chicago police in connection with street protests Sunday, according to the Chicago Tribune and the Associated Press.
The Chicago street protests were led by the Occupy Wall Street movement, which has been praised by Anonymous followers since it began last fall in New York.
President Barack Obama, who is hosting the summit, is from Chicago, and former White House chief of staff Rahm Emanuel is now the city's mayor.
"This is a time we all need to come together and rally around our brothers and sisters bravely in the streets of Chicago risking their lives to make the one voice they have heard," said an Anonymous-related YouTube video, which was later taken down.
"We are actively engaged in actions against the Chicago police department, and encourage anyone to take up the cause and use the AntiS3curityOPS Anonymous banner. For those able, chicagopolice.org should be fired upon as much as possible. We are in your harbor Chicago, and you will not forget us."
Anonymous-related Twitter feeds also claimed to have knocked the NATO website offline Sunday, but a NATO official brushed off the claim.
"We shouldn't imagine that a sixteen-[year]-old from the darkness of his room can bring very, very large networks down," NATO Assistant Secretary General for Emerging Security Challenges Gabor Iklody told WBEZ-TV.
The NATO site was reachable at noon today.
DDoS attacks occur when servers hosting websites are flooded with malformed requests for information, overwhelming smaller or poorly defended sites to the point where they cannot respond to legitimate requests.
DDoS attacks are not hacks or intrusions in that no security is breached, nor do they cause lasting damage, but they are inconvenient and can result in lost revenue for site operators.