Photo

After Israel killed a top military commander of Hamas on Wednesday, Anonymous, the loose affiliation of hackers, retaliated with a series of attacks on Israeli Web sites.

In a coordinated action that began at 3 a.m. New York time Thursday, hackers attacked Web sites belonging to the Israel Defense Forces, the prime minister’s office, Israeli banks, airlines and security companies by flooding them with Web traffic, in a campaign they called #OpIsrael.

Though hackers boasted on Twitter that they had taken some 40 Israeli sites offline, Radware, a computer security company, said that in all but a few cases they were unsuccessful. But they did take down a blog page belonging to the I.D.F. and replaced the home page of what they said was a private Israeli surveillance and security company with an image of Gaza in flames and the following message: “Stop bombing Gaza! Millions of Israelis & Palestinians are lying awake, exposed and terrified.”

According to Radware, the hackers utilized very basic attack tools like a Low Orbit Ion Cannon (LOIC), a simple open-source application that requires very little technical know-how. Once the application is downloaded — either voluntarily or via a malicious link — the application recruits computers into a botnet, or a network of computers, that floods a designated Web site with traffic until it slows or collapses under the load.

The hackers have recruited volunteers on Twitter, in Internet Relay Chat and on Pastebin, a site frequently used by Anonymous to publicize such attacks.

The attacks, while not technically sophisticated, can be difficult to thwart. The best the owner of a site under attack can hope for is to get between the targeted site and the attacking site. But blocking the attack is nearly impossible because of the innumerable sites on the Internet on which Anonymous can post their attack tools.

Instead, some sites will invest in defenses that can absorb the flood of Anonymous traffic.

But even then, so-called hacktivists — hackers who attack for political reasons rather than profits — have found ways around such security before. In September, a group of hackers caused intermittent outages at several American banks, by infecting powerful Web servers around the globe with malware, and then using them to simultaneously fire unprecedented amounts of traffic at each bank until it slowed or collapsed.

In a message posted on Pastebin, the hackers said: