Security researchers at Varonis uncovered a critical vulnerability in Microsoft 365 Copilot for enterprise that could be exploited to extract sensitive user data through a single click on a malicious link. The vulnerability centered on an undocumented prompt parameter that bypassed safety mechanisms requiring explicit user consent before executing powerful commands.
The research methodology proved unconventional. Rather than using traditional reverse engineering techniques, Varonis posed a series of increasingly specific questions to Copilot itself about its internal safety guardrails. Through this iterative dialogue, the AI assistant inadvertently disclosed technical details about its architecture and ultimately revealed the existence of the undocumented parameter string “?autorun=1.” When combined with the known parameter “?q=”, this hidden parameter allowed prompts to execute silently upon clicking a URL.
The researchers demonstrated multiple attack vectors using the vulnerability. One method involved crafting URLs that, when clicked by targets, would instruct Copilot to search inbox contents and extract sender email addresses or credentials, then transmit this information to attacker-controlled servers. Varonis also discovered a separate attack leveraging prompt injection embedded in webpages to poison Copilot’s permanent memory store, which retains user preferences and instructions across sessions. Such poisoning could persist even after password changes or session revocations.
Microsoft initially addressed the vulnerability in February by disabling the ability of the “?q=” parameter to inject text into the chatbot input, requiring users to manually type instead. The company implemented more comprehensive fixes on Tuesday, the publication date of the research. In a statement, Microsoft acknowledged the vulnerability and stated that customers are protected without requiring manual intervention, while noting that guardrails are continuously updated against similar techniques.
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