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Security researchers at Proofpoint are warning of a new threat that’s only likely to become more serious as time goes on: Hackers who publish phishing kits are beginning to add multi-factor authentication bypassing capabilities to their software.

Proofpoint said that a recent study from MFA company Duo found that, as of 2021, 78% of people have or do use MFA, compared to just 28% in 2017. That rapid increase surely ruffled some cybercriminal feathers in the past few years, but that hardly means they’re down for the count. If anything, enterprising hackers are motivated by a challenge like the one posed by MFA, and Proofpoint seems to have evidence that they’ve succeeded.

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According to Aimei Wei, founder and CTO of Stellar Cyber, the man-in-the-middle phishing technique that has evolved to combat MFA “is already out there and happening. Consumers as well as enterprise users are already being targeted.”

The evolution of phishing by proxy

Traditionally, Proofpoint said in its report, phishing kits available for sale online range from “simple open-source kits with human readable code and no-frills functionality to sophisticated kits utilizing numerous layers of obfuscation and built-in modules that allow for stealing usernames, passwords, MFA tokens, social security numbers and credit card numbers.” The way they typically do that is to recreate a target website, like a login page, in the hopes of tricking unaware users.

With MFA in the mix, fake pages are rendered useless: While an attacker may have a username and password, the second factor remains out of reach. Enter what Proofpoint calls “a new kind of kit” that, instead of recreating a page, uses a transparent reverse proxy to act as a man-in-the-middle. By intercepting all the traffic between a victim and their destination server, these transparent proxy MitM attacks allow the user to carry on without ever knowing that their credentials, and their session cookie, have been stolen.

In addition to allowing an attacker to hijack credentials and MFA codes, Proofpoint said that this new technique also gives attackers more staying power. “Modern web pages are dynamic and change frequently. Therefore, presenting the actual site instead of a facsimile greatly enhances the illusion an individual is logging in safely,” the report said.

Proofpoint noted that there are three phish kits that have emerged as the big players in the transparent reverse proxy MitM sphere: Modlishka, Muraena/Necrobrowser and Evilginx2. All have different capabilities making them better suited to certain purposes, but they also have a big feature in common: They were created for legitimate purposes, like penetration testing.

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“Although online services may utilize [any of those three tools] to stop phishing attempts as they occur, with the ever increasing online services that enterprises are using today, it is hard to make sure that [every vendor] has this protection in place,” Wei said.

Both Wei and Proofpoint warn that transparent proxy MitM phishing attacks are only going to grow as more businesses adopt MFA. Basically, it’s a bad idea to rely on multiple authentication factors as the only insurance against stolen accounts.

Noting that Google began requiring MFA for all of its users, Proofpoint said that as more organizations, both enterprises and consumer-facing ones, adopt similar technology, hackers will be more motivated to turn to cheap, ready-to-use, hosted malware solutions.

“They are easy to deploy, free to use and have proven effective at evading detection. The industry needs to prepare to deal with blind spots like these before they can evolve in new unexpected directions,” Proofpoint said.