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Quote:A recently addressed vulnerability in Office 365 allowed attackers to bypass existing phishing protections and deliver malicious messages to victims’ inboxes.

The issue, cloud security firm Avanan says, resided in the use of zero-width spaces (ZWSPs) in the middle of malicious URLs within the RAW HTML of the emails. This method breaks the URLs, thus preventing Microsoft’s systems from recognizing them and also preventing Safe Links from successfully protecting users.

What’s more, these zero-width spaces don’t render, meaning that the recipient would not notice the random special characters in the URL. The first wave of emails abusing this vulnerability was observed on November 10, and Microsoft addressed the issue on January 9, Avanan’s security researchers say.

The vulnerability apparently rendered all Office 365 users vulnerable to phishing attacks, even those who were using Microsoft’s Office 365 Advanced Threat Protection. Both URL reputation check and Safe Links protections are bypassed in the attack.

“The vulnerability was discovered when we noticed a large number of hackers using zero-width spaces (ZWSPs) to obfuscate links in phishing emails to Office 365, hiding the phishing URL from Office 365 Security and Office 365 ATP,” the security researchers say.

ZWSPs, Avanan explains, are characters that render to spaces of zero-width, and can be looked at as "empty space" characters. There are 5 ZWSP entities, namely ​ (Zero-Width Space), ‌ (Zero-Width Non-Joiner), ‍ (Zero-Width Joiner),  (Zero-Width No-Break Space), and ０ (Full-Width Digit Zero).