How Attackers Bypass MFA and Conditional Access to Compromise Email Accounts

Abnormal Security has detected an increase in BEC attacks that successfully compromise email accounts despite multi-factor authentication (MFA) and Conditional Access. While MFA and modern authentication protocols are an important advancement in account security and should be used whenever possible, many common applications — such as those used by mobile email clients (for example, iOS Mail for iOS 10 and older) — do not support modern authentication. This means that it is not possible to enforce MFA when a user signs into their account using one of these applications.

How do attackers do this? 

Legacy email protocols, including IMAP, SMTP, MAPI and POP, do not support MFA, making it possible for attackers to easily bypass MFA using these legacy applications.

To further protect against compromise, many Office 365 licenses provide the ability to configure Conditional Access policies, which are an excellent way to increase security by blocking access from legacy applications that are often targeted for password-spraying campaigns.

However, they have a few weaknesses and drawbacks. First and foremost, Conditional Access is not included with all licenses, meaning that many enterprises simply have no way to protect themselves from this type of attack. Additionally, legacy applications are still in widespread use in most enterprises. Completely blocking all users from legitimate access using these applications will be quite disruptive to the workforce. Also, legacy access is enabled by default on O365. In order to effectively block legacy access, it must be disabled on a per-tenant basis – for all users and platforms.

Even with Conditional Access blocking of legacy apps, Abnormal has observed that attackers are still able to access accounts by obscuring the app that they are using.

To avoid this, Microsoft recommends first examining the sign-in logs to determine which users are not using legacy applications, and applying the Conditional Access policy to them only.  Of course, this leaves those who do use these applications vulnerable. Attempting to apply legacy blocking based on the platform (Windows, mobile, etc.) creates another known vulnerability: the platform is determined by Microsoft based on the user agent, which is very easy to falsify. Attackers simply have to cycle through user agents until they find one with more lax access policies that are easier to bypass.

What does this type of attack look like for companies?

Many enterprises are under the mistaken impression that they are fully protected by MFA and do not need to worry about account takeovers. This is a dangerous assumption, because MFA and Conditional Access policies are not silver bullets. Any security team that believes they are will be vulnerable to these rare but costly events.

A common pattern we have observed in account takeovers is that after being blocked by MFA, an attacker will immediately switch to using a legacy application. In fact, most credential stuffing campaigns utilize legacy applications such as IMAP4 to ensure they do not encounter difficulties from MFA at any point. 

Additionally, even with a Conditional Access policy enabled (which blocks all legacy app access), Abnormal has observed successful account takeovers wherein the attacker bypasses the policy by obscuring the name of the app they were using.  In one case, the attacker initially attempted to sign in using a legacy application but was blocked by Conditional Access. The attacker then waited several days before trying again, this time with the app information obscured, and successfully gained access to the account. 

This example demonstrates that while most account takeover attempts utilize brute force attacks and password spraying techniques, some attackers are methodical and deliberate. These attackers are able to gain access to accounts even with the most secure protocols in place. Fully protecting users from account takeover requires understanding and modeling normal user behavior in order to identify these types of attacks.

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