[BreachExchange] Business Email Compromise Attacks Involving MFA Bypass Increase

[Destry Winant]

https://www.darkreading.com/attacks-breaches/business-email-compromise-attacks-involving-mfa-bypass-increase/d/d-id/1338667

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 Lundert,
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," Lundert 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," Lundert 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.