A large Zeus spam campaign is being delivered via fake emails from the US Federal Reserve and the Internal Revenue Service.
Researchers at Barracuda Labs first spotted the huge uptick in the malicious messages on Monday when the emails were blocked before reaching some 120,000 users within 10 minutes.
The bogus Federal Reserve emails appeared to target staff in charge of an organisation's finances. The body of the messages encourage recipients to click on a malicious link for more information about a wire fund transfer that was not processed.
Users who click on the link are then asked to install an executable, which actually is the data-stealing Zeus trojan, notorious for keylogging the corporate banking credentials belonging to small and midsize businesses, school districts and charities.
"The goal is to steal money from you," Barracuda researcher Luis Chapetti said.
On Wednesday, the fraudsters switched their tactics to leverage the IRS name in their emails. The messages contained the same payload, but victims were told that their federal tax payment was cancelled by their bank and were encouraged to click on the malicious link for further details.
"The potential is there for people to be affected, especially at these volumes," said Barracuda researcher David Michmerhuizen.