The USPS has taken down the infected site and it appears the page the attackers redirect visitors to is offline as well. As of early April 8, Google still reported the site as "may harm your computer" on its search results page and Firefox flagged it as a "Reported Attack Page."
The users never saw the intermediary page, which may have been another legitimate site that had been compromised, Zscaler researchers speculated.
There were nine Trojans to choose from, including executable files, malicious PDF files and PHP scripts. The PHP files exploited known Java vulnerabilities. According to Virus Total, none of them would have been detected by most major antivirus programs. Three of the potential downloads - an executable and two malicious PDF files - were the best recognized, with five out of 42 antivirus programs tracked by Virus Total able to detect them.
The Blackhole Exploit kit, a commercial kit developed by Russian developers, has been used in a number of attacks, according to Zscaler. The attack kit used allows individuals with little or no coding knowledge to deliver a hostile payload, whether that is scareware software or something else. A recent Symantec report found that automated attack kits targeting Websites accounted for two-thirds of all Web-based attacks in 2010.
The same Blackhole Exploit was behind the malicious advertisements that appeared on the free version of Spotify last month.
The name of the affected site, http://ribbs.usps.gov, stands for Rapid Information Bulletin Board System. The site handles the postal service's Intelligent Mail services, such as barcode-based tracking for business mail.