Panos Anastassiadis didn't click on the fake subpoena that popped into his inbox on Monday morning, but he runs a computer security company. Others were not so lucky.

In fact, security researchers say that thousands have fallen victim to an email scam in which senior managers such as Anastassiadis are told that they have been sued in federal court and must click on a web link to download court documents. Victims of the crime are taken to a phony website where they are told they need to install browser plug-in software to view the documents. That software gives the criminals access to the victim's computer.

This type of targeted email attack, called "spear-phishing," is a variation on the more common "phishing" attack. Both attacks use fake email messages to try to lure victims to malicious websites, but with spear-phishing the attackers try to make their messages more believable by including information tailored to the victim.

The e-mail sent to Anastassiadis, CEO of Cyveillance, included his name, company's name and even the correct phone number, said James Brooks, director of product management with the security vendor. "Given the nature of our business, he suspected something right away and forwarded it to our operations centre."

However, Verisign's iDefense division has tracked more than 1,800 victims who clicked on the message. "This is probably one of the largest spear-phishing attacks we've seen to date in terms of number of victims," said Matt Richard, director of iDefense's Rapid Response Team.

Verisign believes that the criminals behind this scam are the same ones who launched an attack last month that used fake emails that appeared to be from the Better Business Bureau. And the US courts have been warning computer users for years now of an ongoing scam where victims are told that they have failed to show up for jury duty and then asked to enter sensitive information into a phishing site.

"The malware itself is not particularly interesting. It was clever that it went straight to CEOs and it didn't really blast the whole world," said John Bambenek, a security researcher at the University of Illinois at Urbana-Champaign and volunteer at the Internet Storm Center.

"For someone who doesn't know what a legal document looks like, it kind of passes the smell test," he added. "When they see they've been subpoenaed, people panic and they click on things they shouldn't."

The mail directs the victim to a Web site that ends in "...uscourts.com" and is very similar to a legitimate .gov domain used by California courts, Bambenek said. The Web server delivering the malware is based in China, while the computer that then controls the victim's computer is based in Singapore.

The malware used in this scam was not identified by the majority of antivirus companies, although most were updating their software to flag it, Bambenek added.

By Monday afternoon, the Administrative Office of the US Courts had posted a note to its website, warning of the fake emails. "This is not a valid subpoena," said Karen Redmond, a spokeswoman with the office. "Subpoenas are not issued like that to individuals unless they're a party in the case."

The US federal court system heavily relies on email messages to help lawyers communicate with each other and the court throughout cases, and IT staff in legal firms have traditionally had to work hard to make sure that these messages are not blocked by spam filters. Now they'll have one more thing to worry about: whether the court notices they're getting are legitimate notices or an online attack.