Office 365 MFA Vulnerability Bypass

Microsoft 365 (M365) has emerged as one of the most widely used email platforms, equipped with a suite of productivity and communication tools deeply integrated into enterprise workflows. As a result, it has become a prime target for cybercriminals. Microsoft acknowledges this threat and has implemented robust security measures, including multifactor authentication (MFA). However, even MFA Office 365 security control has some vulnerabilities you must be aware of. In this article we’ll go over Microsoft MFA issues, three tactics employed by threat actors to bypass MFA controls and gain access to your network, and explore some remediation strategies to strength your security posture. Keep reading!

Legacy Authentication

The usage of legacy authentication is a common method used by hackers to get around MFA. Legacy authentication can be used for mail protocols such as IMAP4, POP3, or SMTP, as well as older Outlook and mobile clients that do not support MFA. After getting credentials via phishing, dark web sources, or other means, attackers can use legacy authentication to get access to an M365 email account, even if the user has MFA enabled. As long as legacy authentication is allowed, threat actors will be able to bypass authentication in Office 365. This gives them complete access to the user’s inbox content, which they can then move to their own systems.

Wireless Guest Network

Administrators in M365 can designate specific IP addresses as “named locations” where MFA is not required for authentication, generally within trusted company offices. An attacker with legitimate credentials can, however, enter an M365 account without Multi factor Authentication checks by Microsoft if they acquire access to the wireless guest network, which often uses the same IP address range as the corporate network. Threat actors have also showed the ability to circumvent geolocation bans by discovering hop points within the victim’s city and state of residence and using the victim’s primary IP range associated with that location to appear authentic.

Third-Party MFA Application Providers via Azure

In cases where threat actors compromise M365 administrator credentials, they can exploit third-party MFA applications set up through Azure Conditional Access policies, such as Duo. By gaining unauthorized access to an administrator account and dismissing risky logins, they effectively override Office 365 2 factor authentication requirements for multiple accounts. They can also modify Conditional Access policies to exempt additional accounts from MFA requirements. Another tactic observed involves adding an extra mobile device to the compromised user’s account to intercept MFA prompts. When hacker logs in, MFA prompts are redirected to their mobile device, and the unauthorized login is approved.

Remediation Strategies

To mitigate these vulnerabilities and enhance security, M365 administrators should consider the following steps:

• Disable basic authentication and legacy protocols, enforcing modern authentication.
• Avoid configuring trusted IP addresses as “named locations.”
• Enable the “Impossible Travel” report in the Microsoft Azure portal.
• When using third-party MFA application Conditional Access settings, make sure they apply to “All Cloud Apps” rather than specific apps.
• Monitor the Azure portal for suspicious sign-ins, review unified audit logs, and scrutinize login locations and user activities within M365 Compliance Center.

Many organizations trust Office 365 to enhance their productivity and streamline collaboration. While Microsoft 365 offers substantial productivity benefits, administrators must prioritize security configurations and policies to safeguard against vulnerabilities that threat actors could exploit to bypass MFA settings. A proactive approach to security is essential to protect M365 environments from evolving threats.

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