As email security providers strengthen their defenses, attackers develop new ways to work around them. A new report from Inky sheds light on how these techniques are evolving to become more complex and difficult to detect by unsuspecting targets.
"They're clearly testing those systems, and then they're exploring ways to get through," says Inky co-founder Dave Baggett of email security defenses. Today's adversaries are looking for ways to hide from legacy protection systems while avoiding suspicion from users, he adds.
One of the strategies Inky highlights is the use of hidden text and zero-font attacks to disguise malicious emails from security software. Someone may use an Office 365 logo to make a phish appear legitimate; the problem is, this would likely trigger defenses to see whether the email came from a Microsoft domain. Adding hidden text in the logo can confuse defensive tools.
"Attackers will put invisible [characters] in between the letters so the end user doesn't see it," Baggett says. Keyword stuffing is a similar tactic. With this technique, the attacker adds hidden text – white text on a white background – that contains keywords to make the email appear to be a conversation between two people rather than a transaction.
Malicious fake attachments are not a new technique, but with these attackers are also shifting their techniques to slip past defenses. Now they're embedding local images into emails. If a victim sees an attachment that appears to be a PDF and clicks to open it, they may instead be redirected to a malicious website that asks for their credentials, Inky explains in the report.
A third tactic is the use of homograph attacks. Attackers may use Unicode lettering to trick people into thinking a fake domain name is real. Instead of spelling Microsoft with a normal "M" character, for example, they may use a visually similar letter from a different alphabet. The result is a domain name that appears real but bypasses detection from security software.
"Some of these things end up being a tell," Baggett says. "Microsoft themselves would never spell their name with a funny Unicode M." Still, he points out, chances are unsuspecting users wouldn't notice.
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