Cybersecurity researchers have identified a “critical” security vulnerability in Microsoft’s implementation of multi-factor authentication (MFA) that allows an attacker to trivially bypass protections and gain unauthorized access to a victim’s account.
“The bypass was simple, taking about an hour to execute, requiring no user interaction, generating no notifications, and giving the account owner no indication of a problem,” Oasis Security researchers Elad Luz and Tal Hassan said in a report shared with The Hacker News.
After responsible disclosure, the question is a code name AuthQuake – Microsoft appealed in October 2024.
As long as the Windows manufacturer supports different ways to authenticate users via MFA, one method involves entering a six-digit code from an authenticator after providing credentials. Up to 10 consecutive failed attempts are allowed per session.
The vulnerability discovered by Oasis essentially concerns the lack of rate limiting and an extended time interval when providing and validating these one-time codes, allowing an attacker to quickly create new sessions and enumerate all possible code permutations (ie one million) without even alerting the victim about failed login attempts.
It should be noted here that such codes are time-based, also called one-time passwords (TOTP), where they are created using the current time as a source of randomness. Moreover, the codes only remain active for about 30 seconds, after which they change.
“However, due to the potential time difference and delays between the validator and the user, the validator is encouraged to accept a larger time window for the code,” Oasis noted. “In short, this means that a single TOTP code can be valid for more than 30 seconds.”
In the case of Microsoft, the New York-based company found that the code was valid for 3 minutes, thus opening up the possibility of a scenario where an attacker could take advantage of the extended time window to initiate new brute-force attempts to simultaneously crack the six-digit code.
“It is important to introduce rate limits and make sure they are properly enforced,” the researchers said. “Furthermore, speed limits may not be enough—consecutive failed attempts should cause the account to be locked.”
Since then, Microsoft has implemented a stricter speed limit that kicks in after a number of failed attempts. Oasis also said the new limit lasts about half a day.
“The recent discovery of the AuthQuake vulnerability in Microsoft’s multi-factor authentication (MFA) serves as a reminder that security is not just about deploying MFA – it must also be properly configured,” said James Scobie, CIO at Keeper Security. the statement said.
“While MFA is undoubtedly a powerful defense, its effectiveness depends on key settings such as rate limiting to prevent brute force attempts and user notifications for failed login attempts. These features are optional; they are important for improving visibility, allowing users to detect suspicious activity early and respond quickly.”
