Cybersecurity researchers have discovered what they said is the first known malicious Microsoft Outlook add-in detected in the wild.

In this unusual supply chain attack detailed by Koi Security, an unknown attacker claimed the domain associated with a now-abandoned legitimate add-in to serve a fake Microsoft login page, stealing over 4,000 credentials in the process. The activity has been codenamed AgreeToSteal by the cybersecurity company.

The Outlook add-in in question is AgreeTo, which is advertised by its developer as a way for users to connect different calendars in a single place and share their availability through email. The add-in was last updated in December 2022.