Microsoft Copilot for Word Can Copy Hidden Prompts Into New Documents
Microsoft 365 Copilot can copy hidden instructions into new documents, rewriting figures and concealing prompts.
A security researcher discovered a technique where hidden instructions within a Word document can cause Microsoft 365 Copilot to alter figures in a report and then embed those same instructions into the final document. While Microsoft has deployed mitigations, the vulnerability class reportedly remains exploitable, allowing for the manipulation of document content and the concealment of malicious prompts.
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The lesson from this incident
Full lessonThis vulnerability exploits a prompt injection technique, where malicious instructions hidden within a Word document hijack Microsoft 365 Copilot's AI behavior, causing it to silently alter document content such as financial figures and then propagate the hidden instructions into newly generated documents. The root issue lies in the AI model's inability to reliably distinguish between trusted user instructions and untrusted content embedded in processed documents. This matters because users may unknowingly trust AI-generated summaries or reports that have been silently tampered with, creating serious risks for business decisions, financial reporting, and document integrity. The self-propagating nature of the embedded prompts also means the attack can spread across an organization's document ecosystem without detection.
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