Zeus spyware Trojan variants have begun using PDF files to package exploits.

Thousands of spammed messages containing exploit-ridden attachments posing as delivery notices from the Royal Mail have been intercepted by net security firm Websense this week.

The PDF attachment contains an embedded executable containing the Zeus payload. Users get a warning before this executable is run, but long experience shows that many user will simply click through such warnings.

Once infected, compromised machines 'phone home' to hacker controlled servers in China.

Other Zeus variants using the same attack strategy also spotted this week come disguised as a billing invoice, warns M86 Security Labs.

Zeus (aka Zbot) is a family of information stealing, customised crimeware tools on sale on the digital underground and widely linked to bank fraud scams. The latest attack is a more sophisticated variant on previous attacks featuring malware-laced email attachments or (more inferquently) drive-by download attacks.

The techniques used by the latest Zeus attacks are similar to the launch embedded executable from PDF security shortcoming of multiple PDF reader packages first explained by security researcher Didier Stevens late last month: http://blog.didierstevens.com/2010/03/29/escape-from-pdf

Websense's advisory, which contains screenshots of the malware-laced spam emails and a detailed explanation of how the attack works, can be found here: http://securitylabs.websense.com/content/Alerts/3593.aspx?cmpid=slalert

(ElReg.)