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Why Did a Trusted Microsoft Store App Just Steal 4,000 Passwords?
The recent Outlook add-in security incident offers a prime example of why third-party software vetting is crucial for digital safety. The following guide provides a comprehensive breakdown of the “AgreeTo” add-in breach, explaining how it happened and what steps you can take to protect your data.
Anatomy of the “AgreeTo” Add-in Breach
The “AgreeTo” incident reveals a critical vulnerability in how modern software supply chains operate. Originally, this Outlook add-in was a legitimate scheduling tool launched in 2022 by a well-intentioned developer. It gained popularity for its utility in managing calendars and appointments. However, after the developer discontinued the project, the underlying infrastructure was left vulnerable.
The breach occurred when the domain hosting the add-in’s content, specifically a subdomain on Vercel, expired. A cybercriminal recognized this opportunity and registered the abandoned subdomain. Because Microsoft Office add-ins function as XML manifests that load content from an external URL, the attacker was able to seamlessly inject malicious code into the existing, trusted add-in without altering the file users had already installed.
This “zombie” attack method bypassed standard security checks because Microsoft’s vetting process typically occurs only upon initial submission. Once an add-in is approved, the external content it loads is not continuously monitored for changes in ownership or intent.
The Phishing Mechanism and Data Theft
Once the attacker controlled the backend, they transformed “AgreeTo” into a potent phishing tool. When users opened the add-in, they were presented with a counterfeit Microsoft sign-in prompt within the trusted Outlook sidebar. This context made the request appear legitimate, significantly increasing the likelihood of user compliance.
The scope of the theft was extensive. Security researchers at Koi.ai discovered that the attacker successfully harvested over 4,000 sets of credentials. The stolen data included:
- Microsoft Account Credentials: Usernames and passwords for email and cloud services.
- Financial Information: Credit card numbers, CVVs, and PINs.
- Security Verification: Answers to banking security questions used for identity verification.
All harvested data was exfiltrated to the attacker via a Telegram bot. Researchers were able to access this channel and confirm that the attacker was actively testing the stolen credentials at the time of discovery.
Why This Matters: Supply Chain Vulnerabilities
This incident highlights a “blind spot” in digital security known as a software supply chain attack. Users and organizations often trust software based on its source—in this case, the official Microsoft Office Add-in Store. However, this trust can be exploited if the components that power that software (like domains or servers) are abandoned by their creators.
The “AgreeTo” add-in had extensive permissions, including ReadWriteItem, which theoretically allowed it to read and modify emails. While the attacker primarily focused on credential harvesting, these permissions could have been used for even more invasive surveillance or data manipulation. This serves as a stark reminder that “abandonware”—software that is no longer supported or updated—poses a significant security risk, even if it remains available on official platforms.
Actionable Steps to Protect Yourself
To safeguard your organization and personal data from similar threats, consider the following advisory steps:
- Audit Your Add-ins: regularly review all installed extensions and add-ins in Outlook and other Office apps. Remove any that are no longer essential or appear to be from developers who have ceased support.
- Verify Developer Activity: Before installing new tools, check if the developer is still active. A lack of recent updates or a defunct website can be a red flag.
- Enable Multi-Factor Authentication (MFA): MFA provides a critical layer of defense. even if an attacker steals your password, they cannot access your account without the second factor.
- Be Wary of In-App Logins: If an add-in asks you to sign in again, especially if you are already logged into Outlook, proceed with caution. Verify the legitimacy of the prompt before entering credentials.