The bottom line: Hidden prompts in Word documents are copied by Copilot into newly generated files and can trigger the same unwanted action there.
Hidden prompts embedded in Word documents can cause Microsoft 365 Copilot to falsify numbers in reports and incorporate the same instructions into the generated files. Security researcher Håkon Måløy disclosed the vulnerability publicly on July 28th.
Håkon Måløy demonstrated in a proof of concept that invisible instructions embedded in Word documents can cause Microsoft 365 Copilot to manipulate data in reports. The critical issue: these hidden prompts are copied into the newly generated files and lead to the same unwanted behavior when the document is used in a subsequent Copilot session.
The vulnerability was disclosed to Microsoft 365 Copilot on July 28th, 144 days after the initial report to Microsoft. The delayed disclosure process indicates complexity in remediation efforts.
For CISOs, this presents an additional risk in the context of “prompt injection”: documents prepared by attackers can be automatically manipulated through the AI assistant — with the additional effect that this manipulation persists in the output files and carries over to subsequent processing steps. This requires enhanced controls when using generative AI tools with internal company documents.
Source: thehackernews.com · Published July 30, 2026
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