First Malicious Outlook Add-In Found Stealing 4,000+ Microsoft Credentials

In early February 2026, a new kind of digital trap was discovered hidden right inside Microsoft Outlook. Known as "AgreeToSteal," this campaign involved a once-popular scheduling tool called "AgreeTo" that had been abandoned by its creator. Because the original developer stopped paying for the website that powered the app, a clever hacker stepped in and bought the web address. Since the app was still officially listed in the Microsoft Store and trusted by Outlook, the hacker was able to "inherit" that trust, instantly gaining a direct line into the sidebars of thousands of users without raising a single alarm.

The Perfect Disguise: A "Verified" Fake

This attack didn't rely on breaking through complex security walls; it exploited a loophole in how app stores work. When Microsoft first approved the app years ago, it was safe. However, because the app loads its content live from the internet every time you click it, the hacker was able to swap the helpful scheduling tool for a perfectly faked login screen. When users clicked the "AgreeTo" icon they had trusted for years, they were greeted with a "Microsoft Sign-in" box. As soon as they typed their password, a hidden script whisked those credentials—along with their location and device info—straight to the hacker via a private Telegram chat .

The Damage: More Than Just a Stolen Password

The impact was devastating and remarkably efficient. Security experts found that over 4,000 accounts were fully compromised. This wasn't just about reading emails; the hackers used the app's "permission to read and write" to hunt for bank security answers and credit card details. In many cases, they were able to intercept e-transfers and bank transfers, draining funds directly from victims' accounts. Because the attack happened inside an official Microsoft sidebar, it bypassed almost every standard email filter, making it virtually invisible to traditional security software.

How to Secure Your Organization

This incident proves that just because an app is in an official store doesn't mean it will stay safe forever. To protect your team, you should move from "automatic trust" to a "verify everything" approach.

• Immediate Cleanup: If your company used "AgreeTo," it must be removed across the entire organization immediately. Any user who had it installed should have their active sessions ended and their passwords changed right away.

• Lock Down the Store: IT teams should disable the setting that allows employees to install their own apps from the Microsoft Store. Instead, use "Centralized Deployment," where the security team reviews and approves an app before anyone can use it.

• Modern Protection: The best way to stop these fake login pages is to use Passkeys or physical security keys. These high-tech locks are "tied" to the real Microsoft website; if a hacker tries to use a fake "AgreeTo" page to steal your login, the security key will recognize the site is a fraud and refuse to work.

Reference

1. https://www.csoonline.com/article/4131632/dead-outlook-add-in-hijacked-to-phish-4000-microsoft-office-store-users-2.html

2. https://thehackernews.com/2026/02/first-malicious-outlook-add-in-found.html

3. https://www.bleepingcomputer.com/news/security/microsoft-store-outlook-add-in-hijacked-to-steal-4-000-microsoft-accounts/

4. https://radar.offseq.com/threat/first-malicious-outlook-add-in-found-stealing-4000-1d56e446A

5. https://windowsreport.com/hackers-hijack-outlook-add-in-to-steal-4000-microsoft-accounts/