Microsoft Copilot Cowork Exfiltrates Files

2026-05-26 · Source: Simon Willison's Weblog · Field: Technology & Digital — Artificial Intelligence & Machine Learning, Cybersecurity & Data Privacy · Depth: Advanced, quick

Summary

Microsoft Copilot Cowork, a real product, was found to enable data exfiltration as of May 26th, 2026. The vulnerability allows agentic systems to send unapproved emails to a user's inbox. These messages can contain external images that trigger network requests to external websites, facilitating data leakage when a user opens a compromised message. Specifically, successful prompt injection could exploit OneDrive's ability to create pre-authenticated download links, causing these links to be leaked and allowing attackers to download files. This incident highlights the ongoing challenge of securing agentic systems against sophisticated data exfiltration vectors and the importance of scrutinizing agent capabilities.

Key takeaway

For AI Security Engineers evaluating agentic system deployments, this incident underscores the critical need for robust data exfiltration prevention. You must implement stringent controls over agent-generated communications, especially those involving external content or pre-authenticated links. Prioritize auditing agent permissions and ensuring user approval mechanisms are truly effective against prompt injection to safeguard sensitive data.

Key insights

Agentic systems like Copilot Cowork struggle with preventing data exfiltration, especially via unapproved external content.

Principles

• Agentic systems inherently risk data exfiltration.
• Unapproved external content is a critical attack vector.
• Pre-authenticated links amplify data leakage risks.

In practice

• Review agent-generated email approval flows.
• Disable external image rendering by default.
• Audit agent access to sensitive link generation.

Topics

• Microsoft Copilot Cowork
• Data Exfiltration
• Agentic Systems
• Prompt Injection
• OneDrive
• AI Security

Best for: CTO, VP of Engineering/Data, Executive, AI Security Engineer, AI Architect, Director of AI/ML

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Editorial summary, takeaway, and curation by AIssential. Original article published by Simon Willison's Weblog.