All You Need is Guest: Beyond Enumeration

Azure Active Directory guest accounts are commonly invited into corporate tenants so outside partners and contractors can collaborate, and they are often treated as low risk. Michael Bargury argues that this assumption breaks down in practice. He shows how a guest can join a host tenant through ordinary business-user invitation flows and then reach low-code applications built on the Power Platform. Those applications store "connections" that wrap a user's credentials, and the sharing controls let business users share them widely, so a guest can view data behind SQL servers and file storage they were never explicitly granted.

The talk walks through the obstacles along the way: a missing license, a data loss prevention policy that the speaker describes as an allow and deny list for connectors rather than real DLP, and an internal token exchange. He explains how an API gateway injects stored tokens into requests, and how a public Microsoft client identity can be used to obtain the token needed to query it. He presents PowerGuest, a tool that enumerates the connections a guest can reach, dumps the data behind them, and offers an interactive interface for running queries. One step was withheld pending a fix from Microsoft.

For defenders, the takeaway is that guest access should not be assumed harmless, and that business-built applications are part of the shared responsibility model rather than the vendor's problem alone. The speaker recommends hardening Azure AD and Office 365 guest settings, reviewing who shares connections and what data they expose, and adding detection logic where configuration changes are not possible.