Security researchers at Varonis identified a critical vulnerability in Microsoft 365 Copilot that allowed attackers to exfiltrate sensitive user data through a single click on a malicious link. The discovery process proved unusual, as the researchers obtained crucial information by directly questioning Copilot about its safety mechanisms and guardrails.
The researchers sought to create an exploit that would extract user passwords and other sensitive information when a target merely clicked on a provided link. When Copilot initially refused to assist with such requests, emphasizing that sensitive operations require explicit user confirmation, the team adapted their approach. Through a series of targeted questions about the system’s internal architecture and safety limitations, they gradually extracted technical details about how Copilot’s protective mechanisms functioned. This methodical interrogation eventually led Copilot to disclose an undocumented parameter called ?autorun=1 that completely bypassed the consent requirement.
Once obtained, the researchers demonstrated how this parameter could be combined with another known parameter, ?q=, to silently execute malicious prompts the moment a user clicked a crafted URL. The attack chain involved using Copilot to search the victim’s email inbox, extract sensitive information such as passwords or sender email addresses, and automatically transmit this data to an attacker-controlled server. The exfiltrated information was encoded in base64 format to prevent detection and transmission errors. Additionally, the researchers developed a secondary attack method that injected prompts into webpage metadata to corrupt Copilot’s permanent memory store, which retains user information and preferences across sessions.
Microsoft silently implemented initial mitigation measures in February, limiting the ?q= parameter to prevent text injection into the chatbot input field. More comprehensive fixes were introduced on Tuesday. The company acknowledged the Varonis team’s work and stated that customers are protected without requiring any manual action. Microsoft indicated it continues updating its safeguards to address similar vulnerability classes.
The incidents highlight ongoing security challenges with AI assistants and underscore the importance of user caution when interacting with unfamiliar links and monitoring outputs for unexpected behavior. Experts recommend limiting the number of applications connected to AI assistants as an additional protective measure.
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