Security researchers at Varonis uncovered a critical vulnerability in Microsoft 365 Copilot for enterprise that allowed attackers to extract passwords and other sensitive information from users without their knowledge or consent. The researchers employed an unconventional discovery method, systematically questioning Copilot about its safety mechanisms and guardrails. Through this interactive dialogue, Copilot inadvertently revealed technical details about its internal architecture, eventually disclosing an undocumented prompt parameter called ?autorun=1.
When combined with the known parameter ?q=, this secret parameter allowed prompts to execute automatically upon a user clicking a malicious URL, completely bypassing the standard requirement for explicit user confirmation. The researchers demonstrated the vulnerability by crafting URLs that, when clicked, would search a victim’s inbox for sensitive information and exfiltrate email addresses and stored credentials to attacker-controlled servers. The stolen data was converted to base64 format to conceal the theft.
The attack chain required minimal user interaction. A victim needed only to click a malicious link delivered via email, messaging, or QR code. Once clicked, Copilot would process injected prompts with full access to the user’s session context and connected applications, executing commands even if the Copilot tab was closed immediately after loading. The researchers also identified a separate attack vector that could poison Copilot’s permanent memory store, allowing attackers to modify stored user information and preferences that would persist across password changes and device re-enrollments.
Varonis reported the vulnerability to Microsoft in February, three months before publicly disclosing it. Microsoft implemented an initial mitigation by disabling the ?q= parameter’s ability to inject text into the chatbot input, requiring manual user interaction instead. The company released more comprehensive security fixes on Tuesday, the same day Varonis published its findings. Microsoft stated that customers are protected without requiring any manual action and indicated it continuously updates safeguards against similar attack techniques.
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