Imagine having unlimited attempts to guess someone’s username and password without getting caught. That would make an ideal scenario for a stealthy threat actor—leaving server admins with little to no visibility into the attacker’s actions, let alone the possibility of blocking them.

A newly discovered bug in Microsoft Azure’s Active Directory (AD) implementation allows just that: single-factor brute-forcing of a user’s AD credentials. And, these attempts aren’t logged on to the server.

Invalid password, try again, and again…

In June this year, researchers at Secureworks Counter Threat Unit (CTU) discovered a flaw in the protocol used by Azure Active Directory Seamless Single Sign-On service.