Security researchers at Varonis identified a significant vulnerability in Microsoft 365 Copilot that allowed attackers to exfiltrate sensitive user data through a single click. The discovery process was unconventional—rather than using traditional vulnerability research methods, the team systematically questioned the AI assistant about its safety mechanisms and guardrails. Through a series of inquiries, Copilot gradually disclosed technical details about its internal architecture that ultimately revealed an undocumented prompt parameter designed to bypass user consent requirements.
The parameter in question was the string ?autorun=1, which when combined with the well-known parameter ?q=, allowed prompts to execute automatically upon a user clicking a malicious URL. This capability enabled attackers to craft links that, when clicked, would instruct Copilot to search a victim’s inbox, extract sensitive information such as email addresses or passwords, and transmit that data to attacker-controlled servers. The exfiltrated information was converted to base64 format to conceal the theft and prevent transmission errors.
The attack chain required minimal interaction from victims. Upon clicking a crafted URL delivered through email, messaging, or phishing pages, the browser would load Copilot in the user’s authenticated session. The ?autorun=1 parameter would trigger automatic execution of the injected prompt without any user gesture, allowing the assistant to process commands with full access to the victim’s session context and connected applications. Even if the user closed the Copilot tab immediately, the prompt would execute to completion.
Variants of the exploit also targeted Copilot’s permanent memory storage system, which retains user information and preferences across sessions. Attackers could inject prompts through webpage metadata to poison this memory store, potentially redirecting outputs, filtering information, or biasing responses toward attacker-chosen narratives. Such poisoned memories would persist across password changes and device re-enrollments.
Microsoft implemented an initial mitigation in February by disabling the ?q= parameter’s ability to inject text directly into the chatbot input, requiring users to click and type manually instead. The company released more comprehensive fixes on Tuesday and stated that customers are protected without requiring any manual action. Microsoft also committed to continuously updating its guardrails against similar techniques.
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