Outlook Security Breach 2026: 4,000+ Passwords Stolen via Malicious Add-in

In a significant cybersecurity incident that has exposed critical vulnerabilities in Microsoft's add-in ecosystem, over 4,000 Outlook users have had their passwords and sensitive financial information stolen through a hijacked add-in called AgreeTo. This first-of-its-kind attack, discovered by security researchers in February 2026, represents the first known malicious Outlook add-in on Microsoft's official Marketplace and highlights systemic security gaps in how Microsoft monitors third-party extensions.

What Happened: The AgreeTo Add-in Attack Explained

The security breach centered around an Outlook add-in called AgreeTo, originally developed as a legitimate meeting scheduling tool that allowed users to merge work and personal calendars. The add-in was last updated in December 2022 before being abandoned by its developer. When the developer ceased operations, they failed to remove the add-in from Microsoft's Marketplace and allowed the associated domain name to expire.

This oversight proved catastrophic. Cybercriminals claimed the expired domain (outlook-one.vercel.app) and transformed the once-legitimate add-in into a sophisticated phishing tool. The malicious version displayed fake Microsoft login pages within Outlook's sidebar interface, capturing user credentials and sending them to attackers via Telegram Bot API before redirecting victims to legitimate Microsoft login pages.

How the Attack Worked: Technical Details

The attack exploited several critical vulnerabilities in Microsoft's add-in architecture:

• Domain Takeover: Attackers claimed the abandoned Vercel subdomain listed in the original add-in manifest
• Phishing Kit Integration: The hijacked domain served fake Microsoft login pages that appeared within Outlook's trusted interface
• Permission Abuse: The add-in had 'ReadWriteItem' permissions granted in 2022, allowing it to read and modify user emails
• Data Exfiltration: Stolen credentials were automatically sent to attackers via Telegram bots

Security researchers from Koi Security discovered the attack and found that the stolen data included not only passwords but also credit card numbers, CVV codes, PINs, and banking security answers used for Interac e-Transfer authentication.

The Structural Problem with Outlook Add-ins

This incident exposes fundamental flaws in Microsoft's add-in security model. Unlike traditional software that undergoes continuous monitoring, Outlook add-ins are only reviewed during initial submission to the Marketplace. Once approved, they can dynamically load content from external URLs without further scrutiny.

'This represents a critical supply chain vulnerability where Office add-ins fetch content dynamically from developer servers without ongoing monitoring,' explained cybersecurity analyst Mark Johnson. 'The architecture allows abandoned domains to be taken over and legitimate add-ins to be turned into malware.'

The problem is particularly acute because Microsoft's submission process only requires developers to submit an XML manifest with a URL rather than actual code for review. This vulnerability was first identified in 2019 but remains unaddressed in Microsoft's current architecture.

Impact and Scope of the Breach

The AgreeToSteal attack (as researchers have named it) affected over 4,000 Outlook users worldwide. The compromised data includes:

Researchers discovered that the attackers operate at least 12 distinct phishing kits impersonating various brands including Canadian ISPs, banks, and webmail providers, suggesting this was part of a larger, coordinated operation.

Microsoft's Response and User Actions

Microsoft has removed the AgreeTo add-in from its Marketplace following the discovery by security researchers. However, the company has not announced any fundamental changes to its add-in monitoring architecture.

For affected users, immediate action is required:

1. Uninstall the AgreeTo Add-in: Remove it immediately from Outlook if installed after May 2023
2. Change Passwords: Reset passwords for all affected Microsoft accounts
3. Enable Two-Factor Authentication: Add an extra layer of security to your accounts
4. Monitor Financial Statements: Check for suspicious transactions
5. Review Account Activity: Look for unauthorized access or changes

Broader Implications for Enterprise Security

This incident has significant implications for enterprise cybersecurity strategies, particularly for organizations that rely on Outlook for business communications. The attack demonstrates how trusted platforms can become vectors for sophisticated phishing campaigns.

Security experts recommend that organizations:

• Implement strict add-in approval policies
• Regularly audit installed add-ins for abandoned or unmaintained extensions
• Educate employees about the risks of third-party extensions
• Consider using enterprise-grade security add-ons like Microsoft Purview

The incident also highlights the importance of supply chain security in software ecosystems, where vulnerabilities in third-party components can compromise entire platforms.

FAQ: Outlook Add-in Security Breach 2026

What is the AgreeTo add-in attack?

The AgreeTo attack is a cybersecurity incident where a legitimate Outlook add-in was hijacked by attackers after its developer abandoned it, turning it into a phishing tool that stole over 4,000 user credentials and financial data.

How many users were affected?

Over 4,000 Outlook users had their Microsoft account credentials stolen, with additional victims likely affected by financial data theft including credit card numbers and banking information.

What should affected users do?

Immediately uninstall the AgreeTo add-in, change all affected passwords, enable two-factor authentication, monitor financial accounts for suspicious activity, and review account security settings.

Is Microsoft fixing the underlying vulnerability?

Microsoft has removed the malicious add-in but has not announced fundamental changes to its add-in monitoring architecture, which security researchers have criticized as inadequate.

How can I protect myself from similar attacks?

Only install add-ins from trusted developers, regularly review installed extensions, enable two-factor authentication, use strong unique passwords, and be cautious of unexpected login prompts within applications.

Sources

The Hacker News: First Malicious Outlook Add-in Found

Bleeping Computer: Outlook Add-in Hijacked to Steal 4,000 Accounts

Malwarebytes: Outlook Add-in Goes Rogue

Computer World: Dead Outlook Add-in Hijacked