Cybersecurity Alert: First Known Malicious Microsoft Outlook Add-in Discovered in the Wild

In a groundbreaking revelation that’s sending shockwaves through the cybersecurity community, researchers have uncovered what appears to be the first documented malicious Microsoft Outlook add-in operating in real-world conditions. The discovery exposes a critical vulnerability in how Office add-ins are monitored and raises serious questions about the security of third-party applications in enterprise environments.

The Anatomy of a Supply Chain Attack

The malicious campaign, dubbed AgreeToSteal by Koi Security researchers, represents a sophisticated evolution of supply chain attacks that have previously targeted browser extensions, npm packages, and IDE plugins. What makes this incident particularly alarming is the exploitation of Microsoft’s own official marketplace—a platform that users inherently trust.

The attack began when an unknown threat actor claimed control of a now-abandoned domain associated with a legitimate Outlook add-in called AgreeTo. This add-in, which had been available on Microsoft’s official marketplace since December 2022, was designed to help users connect different calendars in a single interface and share their availability through email. However, when its original developer ceased maintenance and the associated Vercel deployment was deleted, the infrastructure became vulnerable to takeover.

How the Attack Unfolded

The mechanics of the attack exploit a fundamental characteristic of how Office add-ins function. Rather than shipping static code, these add-ins use a manifest file that declares a URL, with content being fetched and served in real-time from the developer’s server every time the add-in is accessed within Outlook’s iframe environment.

Once the threat actor gained control of the expired domain, they deployed a sophisticated phishing kit at “outlook-one.vercel[.]app” that displayed a convincing fake Microsoft sign-in page. When unsuspecting users entered their credentials, the information was immediately exfiltrated via the Telegram Bot API before victims were redirected to the legitimate Microsoft login page—leaving many unaware they had been compromised.

The Scale of the Breach

The impact has been substantial, with Koi Security confirming that over 4,000 credentials were stolen during the campaign. What makes this particularly concerning is that the malicious add-in remains listed in Microsoft’s official store, complete with its original “ReadWriteItem” permissions that allow it to read and modify users’ emails.

Why This Matters: The Broader Implications

According to Idan Dardikman, co-founder and CTO of Koi Security, this incident represents a dangerous expansion of supply chain attack vectors. “This is the same class of attack we’ve seen in browser extensions, npm packages, and IDE plugins,” Dardikman explained to The Hacker News. “What makes Office add-ins particularly concerning is the combination of factors: they run inside Outlook, where users handle their most sensitive communications, they can request permissions to read and modify emails, and they’re distributed through Microsoft’s own store, which carries implicit trust.”

The structural problem extends far beyond Microsoft’s ecosystem. Similar vulnerabilities exist across all marketplaces that host remote dynamic dependencies—platforms that review a manifest at submission without monitoring what referenced URLs actually serve afterward. This “approve once, trust forever” model creates a dangerous blind spot in cybersecurity defenses.

The Security Gap: What Went Wrong

Microsoft’s current review process examines the manifest during initial submission but lacks mechanisms to monitor the actual content served by the developer’s URL after approval. This creates a critical vulnerability window between when a developer abandons a project and when the platform notices the change.

The situation is compounded by the fact that the original AgreeTo developer did nothing wrong—they built a legitimate product and moved on, unaware that their abandoned infrastructure would become a weapon in a cybercrime campaign.

Potential for Greater Damage

While the current attack focused on credential theft, Koi Security warns that the incident could have been far more devastating. Given the add-in’s “ReadWriteItem” permissions, threat actors could have deployed JavaScript to covertly siphon entire mailbox contents, potentially exposing years of sensitive business communications, financial data, and personal information.

Microsoft’s Response and Industry-Wide Challenge

The Hacker News has reached out to Microsoft for comment regarding this critical security issue. The discovery comes at a time when other major platforms are recognizing similar vulnerabilities. Just last month, Open VSX announced plans to enforce security checks before Microsoft Visual Studio Code extensions are published to their open-source repository, while Microsoft’s own VS Code Marketplace conducts periodic bulk rescanning of all packages.

Recommended Solutions

Koi Security has proposed several critical steps Microsoft should implement immediately:

• Trigger re-reviews when an add-in’s URL starts serving different content than what was reviewed initially
• Verify domain ownership to ensure infrastructure hasn’t changed hands
• Implement mechanisms for delisting or flagging add-ins that haven’t been updated within specified timeframes
• Display installation counts to help assess potential impact

The Future of Add-in Security

This incident serves as a wake-up call for the entire technology industry. As organizations increasingly rely on third-party integrations and marketplace applications, the need for continuous monitoring and dynamic security assessment becomes paramount. The traditional model of one-time approval is no longer sufficient in an environment where threat actors are becoming increasingly sophisticated in their exploitation of abandoned digital assets.

The AgreeToSteal campaign demonstrates that even the most trusted platforms can harbor hidden dangers, and it underscores the critical importance of implementing robust, ongoing security monitoring for all third-party applications and integrations.

Tags

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