Researchers at Avanan have spotted phishing emails that use a font size of one to fool email security scanners. The emails appear to be password expiration notifications from Microsoft 365. The attackers have inserted benign links that are invisible to the human eye, but trick security scanners into viewing the email as a legitimate marketing email.

“In this attack, hackers utilize a number of obfuscation techniques to get a credential harvesting page through to the inbox,” the researchers write. “First, all links are hidden within the CSS. This confuses natural language filters. Natural language filters see random text; human readers see what the attackers want them to see. In addition, hackers put links within the <font> tag, and brought the font size down to one. This breaks semantic analysis, which leads many solutions to treat it as a marketing email, as opposed to phishing. Beyond that, there are invalid parameters, as the ‘Padding Left’ is set to ‘;’ further confusing scanners.”

Avanan concludes that the phishing emails themselves appear suspicious, so a trained user would be able to spot them as malicious. The emails simply state, “Notification for Password 365. Access To Your Email will be Expired.”

“To the end-user, this email looks like a standard request from their IT department,” the researchers write. “The email is designed to fool both Natural Language Processing and human eyes. For a user to spot this attack, they should rely on their phishing training. They should notice the stilted grammar, such as ‘Notification Microsoft 365’ as a red flag. They should also ask their own IT department before resetting any passwords.”

Thus, insecurity by obscurity. Attackers are constantly coming up with new ways to bypass email security filters. New-school security awareness training can give your employees a healthy sense of suspicion so they can avoid falling for social engineering attacks.