Security researchers at Varonis identified a significant vulnerability in Microsoft 365 Copilot for enterprise that could allow attackers to steal sensitive user data through a single malicious link click. The discovery process was unconventional—rather than employing traditional reverse engineering techniques, the researchers simply questioned Copilot about its built-in safety mechanisms. Through iterative questioning about how the assistant’s guardrails functioned, what URL structures were involved, and how prompts were processed, Copilot progressively disclosed technical details about its internal architecture.
Eventually, the AI revealed an undocumented prompt parameter called ?autorun=1 that could completely circumvent the requirement for explicit user confirmation before executing commands. When combined with the parameter ?q=, this parameter allowed prompts to execute silently the moment a user clicked on a malicious URL. The researchers demonstrated the vulnerability by crafting URLs that, when clicked by a target, could extract sensitive information such as email addresses and passwords from the victim’s inbox and exfiltrate the data to an attacker-controlled server. The stolen information was converted to base64 format to prevent detection and transmission errors.
The attack chain functioned in stages: the victim would click on the crafted URL delivered via email or messaging, the browser would load Copilot in the authenticated session, the ?autorun=1 parameter would trigger automatic execution, and Copilot would process the injected prompt with full access to the victim’s session context and connected applications. The prompt would complete execution even if the Copilot window was closed immediately after loading.
Varionis also identified a separate attack method involving prompt injection embedded in webpage metadata to poison Copilot’s permanent memory store. This attack could be used to forward outputs, filter information, introduce attacker-chosen biases, or execute specific actions based on trigger conditions. The malicious memories would persist across password changes and device re-enrollments, detectable only through manual inspection.
Microsoft addressed the initial vulnerability in February by disabling the ?q= parameter’s ability to inject text directly into the chatbot. More comprehensive security fixes were introduced later. The company stated that customers are protected without requiring action and noted that its guardrails are continuously updated to defend against similar techniques.
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