The US Postal Service website received an unwelcome delivery this week of a new attack rapidly spreading among legitimate websites. USPS became the latest victim of the so-called "Blackhole" toolkit, a wildly popular website attack kit that's easy to use and provides obfuscation features that help it evade antivirus detection.

USPS officials have since taken down the Rapid Information Bulletin System pages for intelligent mail services, such as tracking and logistics.

Michael Sutton, vice president of research at Zscaler, which discovered the infected site and alerted the USPS, says the attack contained multiple payloads. "It's quite robust: It doesn't just have one or two payloads. We tracked down seven or eight, and it's intelligently delivering them," Sutton says. "Blackhole has done a good job at encrypting the payloads, and that's why [many antivirus packages] are not detecting it."

Researchers at Symantec say Blackhole is the most popular toolkit among website attackers and is the most prevalent one they have seen in the wild.

The kit was developed by Russian hackers and sells for $1,000 to $1,500. "It has a nice, neat, point-and-click interface," Zscaler's Sutton says. "It's popping up on a more frequent basis."

Last week, Zscaler revealed that it had been used to infect a Texas musical festival website, Worldfest.

The attack begins by exploiting an injection flaw on the website, usually SQL injection, and sticking iFrames on the page. It then places encoded JavaScript somewhere at the bottom of the page, for instance. When a user visits the site, the iFrame redirects him to the Blackhole exploit kit server without the user knowing.

Sutton says he's not sure what the attackers who hit the USPS site were after exactly, but the goal is to infect as many machines as possible, and attackers can use the Blackhole kit for various purposes.

"To me, this is the way attackers are doing things. They figured out end users are the weak point in the security chain. They're not trying to break into your servers -- they're trying to infect your end user machines," he says.

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