Beware of Web postcards bearing greetings. That's the advice from The SANS Institute's Internet Storm Center, which is warning about e-mail messages that pose as Web postcards, then direct recipients to a Web site that installs a Trojan horse program.

The new attacks use sophisticated social engineering techniques to trick users into installing Trojan remote access programs that can fool anti-virus and firewall software by appearing to be authorized applications like Internet Relay Chat software, the Internet Storm Center (ISC) said.

ISC has received an increasing number of reports of the postcard scams in recent days. Victims receive e-mail messages with subject lines such as "You have received a virtual postcard from a family member," with a link to a pickup site that installs the Trojan, according to a post on the ISC Web site Sunday.

Another recent scam posed as a message from Blue Mountain Greeting Cards, a service operated by American Greetings of Cleveland, Ohio. The messages use a spoofed sender address and appear to come from bluemountain.com. A link in each e-mail claims to go to Blue Mountain's card pickup Web page, where recipients are asked to enter a unique card ID number provided in the e-mail. However, victims who click on the link pass first through one of a number of sites that may have installed malicious software, ISC said.

Those Web sites were not available Tuesday. A customer service representative from Blue Mountain said in an e-mail message that the company has recently received reports of false e-mail purporting to come from the company and that the company's abuse team is investigating.

Web greeting cards have been used to spread malicious code before. Variants of the Zafi worm in December arrived in e-mail attachments claiming to be postcards offering Christmas greetings. Some variants of the Cult worm in 2003 also spread in e-mail attachments that were said to be Blue Mountain greeting cards.

While the method of attack is well-established, the latest threats are becoming harder to detect because they install programs that piggyback on existing, authorized applications to carry out malicious acts. For example, the Trojan horse installed through one recent Web postcard attack took over, or "hooked," IRC applications on compromised systems. Anti-virus and firewall software is often instructed to trust such programs and allow traffic from them to pass to and from a protected computer unmolested, which gives remote hackers access to infected systems, ISC said.

The IDG News Service is a Network World affiliate.