A flaw in Office 365 could have been exploited by attackers to send out malicious emails and make them look as if they were coming from a legitimate microsoft.com address.
The issue was discovered by Utku Sen, a Turkey-based security enthusiast known for releasing an open source ransomware called Hidden Tear for educational purposes.
Sen found the issue while testing the spam filters of email services such as Outlook 365, Gmail and Yandex. During his tests, which he conducted using the Social Engineering Email Sender (SEES) tool, the expert noticed that Yandex identified some of his phishing emails as valid and marked them with a green icon after performing a DomainKeys Identified Mail (DKIM) verification.
It turned out that the emails detected as valid came from a spoofed microsoft.com email address and they were forwarded through Outlook 365 to Yandex. Further analysis showed that Gmail also accepted the fake microsoft.com emails forwarded from Outlook as legitimate.
The method only worked with emails coming from a spoofed microsoft.com address. When other domains were used, the fake emails went straight to the spam folder.
Sen was unable to figure out the cause, but Reddit user “ptmb” said the problem was likely that Outlook was signing redirected messages with its own DKIM key.
“That means that instead of having an email with a proof of identity from the original sender, you received an email with a proof of identity from the ‘redirector’,” ptmb explained. “And because Outlook was blindly signing these messages it was redirecting, if the message had a fake from field saying something(at)microsoft.com, then after Outlook blindly redirected it, it’d have a genuine DKIM signature from Microsoft by coincidence, even though the original email wasn’t from Microsoft at all.”
Sen informed both Microsoft and Yandex about his findings in September. Microsoft confirmed the issue and patched it in late October, and listed the researcher on its acknowledgements page. Yandex removed the green validation icon, but it’s unclear if it was due to the expert’s report.
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