Hidden Threats in Rapidly Deploying AI Tools — Why Your Organization Should Take Notice

In our rush to adopt the latest productivity-boosting platforms, it’s easy to assume that enterprise AI tools are sealed, safe, and “just work”. The recent disclosure regarding the Microsoft 365 Copilot flaw shows how fast that assumption can break—if you haven’t explicitly guarded against indirect attack surfaces.

The Incident

Security researcher Adam Logue uncovered a clever exploit: an attacker crafts an Excel spreadsheet with hidden instructions (white text spread across multiple sheets) that, when summarized via Copilot, hijacks the AI’s workflow. The AI tool instead uses its enterprise email-search capability, grabs recent internal emails, hex-encodes them, fragments them into short lines, embeds them into a disguised diagram (via Mermaid syntax), and then lures a user into clicking what appears to be a benign “login” button — which silently uploads the data to the attacker’s server.

What makes it particularly worrying is how indirect it is: no obvious prompt injection in a chat UI, but rather hidden instructions embedded in document assets. The user simply asked the AI to summarize a file. Microsoft Corporation validated the vulnerability in early September 2025 and patched it by late September by removing interactive hyperlinks from Copilot’s Mermaid diagram outputs.

What This Means for You

• AI tools = new attack surface. Just because a tool is “internal” or “official” doesn’t mean it can’t be misused.

• Documents and visual assets matter. They’re not passive anymore—attackers embed logic in spreadsheets, diagrams, and hidden macros.

• User interaction matters. The attacker still needed the user to click a disguised link—but that link looked innocuous via a convincingly styled graphic.

• Access controls and monitoring are critical. The AI had access to internal emails and search APIs—without limiting and monitoring that, an attacker could exploit it.

• Patching isn’t enough. Even though Microsoft fixed this, not all AI integrations are as mature—so proactive defensive posture is required.

  
  

Final Word

Adopting AI tools in your organization is smart—but assuming they’re secure by default is a risk. The Copilot incident is a reminder that every interface, workflow and document is now part of your attack surface.If you want to ensure your AI-enabled workflows, document channels, and internal portals stay resilient, let’s talk!

Plexus IT is here to help you strengthen your defenses before the next headline.