Business Email Compromise Attacks Involving MFA Bypass Increase

Multifactor authentication (MFA) is widely regarded as a strong measure for protecting against account takeover attacks. But as with almost any security control, adversaries have devised several ways to bypass it.

Researchers from Abnormal Security this week reported observing a recent increase in attacks where threat actors used legacy apps with old email protocols, such as IMAP, SMTP, and POP, to access and take over business email accounts protected with MFA.

In these attacks, a threat actor who might have obtained the username and password to an MFA-protected email account — via a paste site, for instance — would access the account by signing in from a legacy app that does not enforce MFA. One example is an email client like MailBird, which allows Gmail to be set up via IMAP, says Erin Ludert, data scientist at Abnormal Security.

"Enterprises that allow access from legacy applications are vulnerable to business email compromise [BEC] due to lack of MFA controls" on older email clients, she says.

It is common for many organizations to allow email logins from both modern apps that support MFA and legacy apps that do not.

"Some, but not all, organizations apply conditional access policies to strengthen legacy app logins," Ludert says. It's when organizations don't apply or enforce such policies that they become vulnerable to BEC, she notes.

Some Microsoft Office 365 licenses give organizations the ability to implement conditional access policies for doing this. However, legacy access is still enabled by default, Abnormal Security said in a recent report. To block it, organizations need to disable legacy access on a per-tenant basis across the organization. Also, many enterprises allow legacy access on a widespread basis, so completely blocking users from using legacy apps could be disruptive. 

Even so, "the best protection an organization can implement is MFA and conditional access policies for legacy apps," Ludert says.

In recent years, numerous organizations have suffered substantial losses from BEC attacks. In many of these attacks, threat actors obtained access to high-value business email accounts — such as those belonging to a CFO or other executive with signing authority — and used the access to initiate fraudulent wire transfers.

In other instances, adversaries have used the email accounts to divert payroll payments, for gift card scams, and to steal W-2 data. The FBI has estimated that US businesses lost some $1.7 billion to BEC-related fraud in 2019.

Making BEC Attacks Harder to Pull Off
The FBI and numerous security researchers have urged the use of MFA as one way to make email account takeovers harder. While the measure can help, organizations need to be aware that strong authentication and conditional access policies are not silver bullets, Abnormal said in its report.

"SMS-based MFA, in particular, can be exploited by attackers in several ways," says Charles Ragland, security engineer at Digital Shadows.

One of the most common is SIM-jacking involving the use of social engineering to get network providers to switch a victim's mobile service to an attacker-controlled SIM card. Another common tactic is SS7 hijacking, where attackers intercept and eavesdrop on mobile devices by exploiting a weakness in the SS7 protocol, Ragland says.

Other methods to bypass non-SMS-based MFA are also commonly discussed on cybercriminal forums. One Russian-language cybercriminal forum Exploit, for instance, recently advertised the sale of a method to bypass MFA systems for $5,000, Ragland says.

MFA, or MFA with Single Sign On (SSO), is a great way to provide a secure access policy to a network, he says. But organizations need to be aware that legacy protocols do not all support modern authentication methods.

"In these instances, organizations should perform a risk review and determine if an upgrade is worthwhile, or if they'll accept the risk," Ragland says.

Brandon Hoffman, chief information security officer at Netenrich, says MFA is a great second step to helping secure critical services even though they can be bypassed in several different ways. Where organizations don't use MFA, strong conditional access policies can help. However, care needs to be taken to how they are configured.

"Many of these policies are dependent on static browser elements that are easily manipulated," Hoffman warns.

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