C-level employees of publicly listed companies are being targeted by cybercriminals using malware-infected rich text file documents disguised as recruitment letters.
Security vendor MessageLabs reported that 1,100 emails containing malware-infected rich text file (RTF) attachments have been recorded over a 16-hour period this month. Four separate waves appeared between 13 and 14 September, the company said.
"All [the emails] were going after C-level management. The emails included the company name in the subject field, purporting to be a recruitment company. What it had in the attachment is an executable RTF file," a MessageLabs spokesperson said.
Similar emails were noticed in June this year, he said.
The email, which contains no body text, includes an .scr screen-saver dummy file within an executable RTF file, the spokesperson said. When recipients attempt to open the file, a message is displayed stating: "Microsoft has encountered an error and had to close." The recipient is then advised: "To view this, double click on the message."
Once activated, the RTF file starts a chain of downloads which establish a secure connection between the attacker's server and the infected computer.
The C-level nature of the targets clearly indicates that the attackers are after information, the MessageLabs spokesperson said, but the greater concern is the social-engineering technique used to spread the Trojan-harbouring email.
"The way that this works has the potential to be so effective. You are getting that top-down approach — if they forward that email on internally, that email is coming from a trusted source," he said.
The spokesperson added that all the emails were addressed to a single person, which helped diminish their conspicuousness.
F-Secure security expert Patrik Runald recently postulated that the perfect attack would be a zero-day attack using a rootkit-cloaked Trojan sent to an HR manager who, due to company policy, would be compelled to open the document.
Runald said: "These are scary cases because it's really hard to protect yourself against. We have to run Office and we have to allow Word, RTF, PowerPoint and Excel files through. It shows that signature-based antivirus is not enough; you need more technology than that."
Runald said there is little organisations can do to protect against these threat types besides educating users of the risks, because banning the receipt of common file types is impractical.
Heuristic or behavioural-based monitoring is proving to be more effective at blocking these attacks since the behaviour of the file remains the same despite different signatures being used, Runald said.