Malicious Outlook Add-In Hijack Steals 4,000 Microsoft Credentials

A breach transformed the AgreeTo plug-in for Microsoft Outlook – once meant for organizing meetings – into a weapon that harvested over four thousand login details. Though built by a third-party developer and offered through the official Office Add-in Store starting in late 2022, it turned against its intended purpose. Instead of simplifying calendars, it funneled user data to attackers. What began as a practical tool ended up exploited, quietly capturing credentials under false trust. 

Not every tool inside Office apps runs locally – some pull data straight from web addresses. For AgreeTo, its feature lived online through a link managed via Vercel. That address stopped receiving updates when the creator walked away, even though people kept using it. With no one fixing issues, the software faded into silence. Yet Microsoft still displayed it as available for download. Later, someone with harmful intent took control of the unused webpage. From there, they served malicious material under the app’s trusted name.

A login screen mimicking Microsoft’s design appeared where the real one should have been, according to analysts at Koi Security. 

Instead of authentic access points, users faced a counterfeit form built to harvest credentials. Hidden scripts ran alongside, silently sending captured data elsewhere. After approval in Microsoft’s marketplace, the add-in escaped further checks. The company examines just the manifest when apps are submitted – nothing beyond that gets verified later. Interface components and features load externally, pulled from servers run by developers themselves. 

Since AgreeTo passed initial review, its updated files came straight from machines now under malicious control. Oversight ended once publication was complete.

From inside the attacker’s data pipeline, Koi Security found over 4,000 Microsoft login details already taken. Alongside these, information such as credit card records and responses to bank verification questions had also been collected. While analyzing activity, experts noticed live attempts using the breached logins unfolding in real time. 

Opening the harmful AgreeTo add-on in Outlook displayed a counterfeit Microsoft login screen within the sidebar rather than the expected calendar tool. Resembling an authentic authentication portal, this imitation proved hard to recognize as fraudulent. Once victims submitted their details, those credentials got sent through a Telegram bot interface. Following that transfer, individuals saw the genuine Microsoft sign-in page appear – helping mask what had just occurred.

Despite keeping ReadWriteItem access, which enables viewing and editing messages, there’s no proof the tool tampered with any emails. 

Behind the campaign, investigators spotted a single actor running several phishing setups aimed at financial services, online connectivity firms, and email systems.

Notable because it lives inside Microsoft’s official store, AgreeTo stands apart from past threats that spread via spam, phishing, or malvertising. This marks the first time a verified piece of malware has appeared on the Microsoft Marketplace, according to Oren Yomtov at Koi. He also notes it is the initial harmful Outlook extension spotted actively used outside test environments. 

A removal of AgreeTo from the store was carried out by Microsoft. Anyone keeping the add-in should uninstall it without delay, followed by a password change. Attempts to reach Microsoft for input have been made; no reply came so far.

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Malicious Outlook Add-In Hijack Steals 4,000 Microsoft Credentials