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The Call Is Coming From Inside the House: Microsoft Direct Send and Why You Need to Mitigate Now

Earlier this year, a new technique for sending spoofed emails was discovered. This new technique focused on sending an email from anywhere on the internet while spoofing the sender to land directly in the target’s inbox.

Microsoft “Direct Send,” a feature of all Microsoft 365 environments with an Exchange Online plan that cannot be removed, is intended to allow printers and scanners to send email inside a trusted network. However, this feature also allows for unauthenticated use and the ability to not only send from inside to inside, but also outside to inside. What could go wrong?

For a spoofed email to arrive in an end user’s inbox, an attacker must first find the “smart host” endpoint. Luckily, this endpoint is static across all deployments:

Basically, if an attacker knows your email format and associated domain for email, then they have what they need to perform this attack. We use this technique in almost every external penetration test we perform given its effectiveness and the ready availability of tools to take advantage of it.

With a basic PowerShell command, this process is very straightforward:

A brief rundown of the fields in use:

  • -SmtpServer: The Direct Send endpoint that we will send mail through.

  • -To: The user we are targeting at

  • -From: The internal user we are spoofing the mail coming from within

    • Note: This does not have to be a valid user. However, any user that does not exist in the domain will not appear to be as legitimate (no profile picture in Outlook, etc.) and the email is more likely to land in spam.

  • -Subject: The email subject line.

  • -Body: The email body, which can be plaintext or pulled from an HTML template the attacker creates to make it look more authentic.

Once this command is sent, the end user will likely get the email delivered directly to their inbox (assuming mitigations are not in place, more on that later).

Here are just a few examples of the ways we have leveraged this “feature”:

  • To entice a user to go to a malicious page we have setup that steals end user credentials. This can be an email from internal user A to internal user B that would naturally seem trustworthy.

  • To get an end user to complete a login via device code phishing

  • To convince an end user to take some internal action that opens up pathways for further network exploitation

So, about that mitigation…

Given that you cannot just turn Direct Send off, the best way to stop this attack is to ensure that all incoming mail to your organization is forced through your mail proxies like Mimecast, Proofpoint, etc. Email should not be allowed into the organization from an untrusted source.

This can be accomplished via Exchange Admin Center using connector rules, an example of which is included below (this is for Mimecast, but can be used in a similar fashion for other mail proxies):

Direct Send is a dangerous feature of Microsoft 365 if used by the wrong people. Given the success using it to help clients remediate risk, the relatively young age of the attack vector, and the ease with which the attack can be performed, I will not be surprised to see this attack become prevalent amongst threat actors.

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