Hidden prompt turns Microsoft Copilot into an AI worm

A novel attack vector turns Microsoft Copilot for Word into an "AI worm" by leveraging hidden prompt injections that enable self-propagation across documents.

Technical Breakdown

• TTPs: This attack exploits Copilot's ability to process and act on instructions embedded within documents. The "hidden prompt injection" involves embedding commands that are not visible to the human user but are parsed by the AI. The "worm" functionality means Copilot is tricked into replicating these hidden malicious prompts from an infected document into new documents it processes or creates. This represents an Adversarial AI technique targeting generative models.
• Affected Products: Microsoft Copilot for Word.
• IOCs: No traditional network or file IOCs are mentioned. The "indicator" would be the presence of hidden, malicious prompt instructions within documents that Copilot interacts with.

Defense

Organizations should implement robust content analysis for documents processed by AI, looking for anomalies like hidden text or objects. User education on the risks of AI-generated content based on untrusted sources is also critical. Developers of AI systems need to enhance input sanitization and context window processing to identify and neutralize such hidden directives.

Source: https://www.malwarebytes.com/blog/ai/2026/07/hidden-microsoft-copilot-ai-worm