Networking site LinkedIn is being used as the lure for a huge spam campaign designed to infect UK and US businesses with the data-stealing Zeus/Zbot Trojan, Cisco has reported.
After appearing on 27 September, by 11am BST (6am eastern), spam featuring a bogus LinkedIn reminder accounted for up to a quarter of all spam detected by the company in a 15-minute period.
Users are asked to review the contact request for a fictitious user by clicking on an embedded link in the usual LinkedIn style. This takes victims to a page that asks them to wait before sending them to Google, unaware that anything untoward has happened. By that point, Zeus will have attempted to jump on to the target PC.
Assuming it is not detected by the PC's antivirus software - and there is plenty of evidence that Zeus variants can get past many such defences - this particular Zeus variant monitors browser entries for online bank account credentials.
"This strongly suggests that the criminals behind this attack are most interested in employees with access to financial systems and online commercial bank accounts," said a Cisco statement.
The problem with the attack is that LinkedIn thrives on members being contacted by new members, so the fact that the apparent message sender is unknown would not necessarily alert users not to click on the link.
The defence against this attack is most businesses would be anti-spam filters at gateway level and antivirus, plus any defences such as Trusteer's Rapport browser plug-in.
Another barrier is that companies accessing online bank account use a dedicated PC that does not run any other application.
LinkedIn has been used for spam campaigns in the past but relatively infrequently compared to consumer rivals such as Facebook and Twitter, which can also be used to attack business users.
"Targeting social network users for distributing financial malware is a smart move for the criminals," commented Trusteer CEO, Mickey Boodaei on the latest attack.
"These attacks are much more likely to succeed than phishing attacks on banks. Once Zeus is installed on the user's computer then the criminals get access not only to login information but also to real-time transactions and other sensitive information on the victim's computer," he said.