Microsoft Outlook shows real contact details in some phishing emails

Microsoft Outlook is susceptible to phishing attacks using internationalized domain names (IDNs), according to reports from two separate security researchers.

Phishing attacks sent from IDNs are also known as homograph attacks. They use Unicode characters from non-Latin character sets, such as Cyrillic or Greek, that look like regular Latin characters. An attacker might register the domain tωitter.com, which uses an international alternative to a regular 'w'.

Browsers have long recognized and flagged IDNs, displaying them in their original Unicode format (known as Punycode), making them easier to spot. The tωitter.com IDN would show up as xn–titter-i2e.com, for example.

However, researcher dobby1kenobi revealed that Microsoft Outlook does not highlight them. Moreover, if a spoofed email using an IDN resembles a legitimate email address in the recipient's Outlook contact book - for example, real.person@tωitter.com instead of real.person@twitter.com - the software will display the legitimate person's contact details next to the phishing email.

For the attack to work, the sender must include the real email address in the 'Sender' field, which is trivial.

"This means if a company’s domain is “somecompany[.]com”, an attacker that registers an IDN such as “ѕomecompany[.]com” (xn–omecompany-l2i[.]com) could take advantage of this bug and send convincing phishing emails to employees within “somecompany.com” that used Microsoft Outlook for Windows," he reported.

Because a spoofed email address would cause the real employee's contact details to appear, many employees might be fooled into thinking the email was legitimate.

Mike Manzotti, senior consultant at security company Dionach, also noted the issue. He reported the same response from Microsoft as dobby1kenobi:

"We’ve finished going over your case, but in this instance it was decided that we will not be fixing this vulnerability in the current version and are closing this case," the company said. "In this case, while spoofing could occur, the senders identity cannot be trusted without a digital signature. The changes needed are likely to cause false positives and issues in other ways."

However, Manzotti noticed that the latest version of Microsoft Outlook (16.0.14228.20216) is no longer vulnerable. Microsoft was unable to confirm if it had issued a fix, he said.

Companies with versions of Outlook still susceptible to this flaw can work around the issue by digitally signing their emails and visually classifying all mails from external sources, dobby1kenobi said.