Microsoft Outlook Add-in Stolen 4,000 Microsoft account Credentials and Credit Card Numbers
Security researchers have identified a malicious Microsoft Outlook add-in used in real-world attacks. The compromised meeting scheduler, known as AgreeTo, was utilized to steal over 4,000 Microsoft account credentials, credit card numbers, and answers to…
Security researchers have identified a malicious Microsoft Outlook add-in used in real-world attacks. The compromised meeting scheduler, known as AgreeTo, was utilized to steal over 4,000 Microsoft account credentials, credit card numbers, and answers to banking security questions.
Originally launched as a legitimate open-source project on the Microsoft Office Add-in Store in December 2022, AgreeTo was a functional meeting tool that received positive reviews. However, the project was abandoned by its developer, leaving the hosting URL (outlook-one.vercel.app) available for registration. An attacker subsequently claimed the URL and deployed a phishing kit.
Since the add-in was never removed from Microsoft's store, the attacker's phishing page was accessible directly in the trusted Outlook sidebar to users who had the add-in installed or downloaded it.
The Technical Flaw: Remote Dynamic Dependencies
The attack exploited the architecture of Office add-ins, which function as "remote dynamic dependencies." These are XML manifests that load a URL in an iframe.
Stage Description
Security researchers have identified a malicious Microsoft Outlook add-in used in real-world attacks.
The Gap Microsoft reviews the manifest only during the initial submission (2022 in this case).
The Exploit Microsoft does not continuously verify the live content hosted at the approved URL.
The Result The attacker replaced the scheduling tool with a fake Microsoft login page without initiating a new security review.
When users opened AgreeTo, they were prompted to log in, and a script collected their credentials and IP addresses, sending the data to the attacker via a Telegram bot.
Koi Security discovered the campaign after identifying the attacker's poorly secured exfiltration channel. They recovered the full dataset of 4,000 victims, which included banking details and security answers targeting Canadian institutions.
The add-in had "ReadWriteItem" permissions, allowing potential access to read and modify users' emails. However, the primary focus of the attack was credential harvesting.
Microsoft has removed the add-in following the report. This incident highlights a critical supply chain risk: trusted software can become malicious if its infrastructure is abandoned.
Based on reporting by Cyber Security News.