[Heads-up] New Attack Blindsides Microsoft Office 365 Anti-Phishing Filter And Blacklists



Phishers have found a way of moving the malicious URLs in their emails past Office 365's protections. The security company Avanan says they've observed criminals using a <base> tag in the HTML header used with the URL.

The hack works because newer email clients are equipped to handle the <base> tag, and they render the split URL as a link. Office 365's Advanced Threat Protection includes Safe Links, which checks links in emails against a blacklist. Safe Links would catch a malicious link like this one—Avanan's example, which we hasten to state is a fake:

baseStriker_HTML_Example

Avanan calls the technique "baseStriker." It works against Microsoft Outlook clients that support the <base> tag. Gmail is said to be immune. Avanan has informed Microsoft, which is investigating. In the meantime Microsoft has told SecurityWeek, "We encourage customers to practice safe computing habits by avoiding opening links in emails from senders they don’t recognize."

That's good advice, and something any organization might wish to reinforce with some realistic, interactive security awareness training. And remember that technical means of defense do have their limitations, which criminals are always actively seeking out. SecurityWeek has the story: https://www.securityweek.com/phishers-use-new-method-bypass-office-365-safe-links

Note that other email clients may be vulnerable to this exploit as well. 

Companies like MimeCast and Proofpoint have built-in defenses against this, but there is a cheaper alternative. As a matter of fact, this one costs nothing. 

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Topics: Phishing



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