Spammers are using ostensibly clean Microsoft Office documents to trick anti-spam filters into letting their messages through, according to the security vendor Bitdefender.
Emails disguised as tax returns and other bills are said to be sent with attachments from Word or Excel which appear to be safe but contain dangerous macros, used to download malware from a remote server and execute it without the victim’s knowledge.
Catalin Cosoi, chief security strategist at Bitdefender, said: "The malware on the remote server is either a ransomware or an industrial espionage tool."
"Both are as dangerous as they look: the effect of the ransomware is immediate as it can encrypt a company’s important files and ask for a ransom. The espionage tool can be even more vicious, depending on what kind of files it can access."
Macros in Office are often used to automate repetitive tasks, but can be reconfigured to interact with the operating system, and even corrupt it if commands are malicious.
Whilst antivirus products are available that will block malicious macros, not all software does.
Cosoi added that the emails are targeted at management departments, and that recipients of "odd" bills or tax returns would do well not to open the attached file.
