With more than 100 million monthly active subscribers, Office 365 has attracted the attention of hackers who’ve revamped an age-old trick. This time, they come up with a highly targeted, well-crafted spear-phishing scam that’s even more difficult to identify.

Fancy Bear, also known as APT28, is the Russian cybercriminal group responsible for hacking the Democratic National Party last year. Since then, the group has developed a host of tools to exploit Linux, Android, and Windows vulnerabilities. Last month, they spread a new strain of malware that specifically targets Apple Macbooks.