Security researchers at Varonis identified a critical vulnerability in Microsoft 365 Copilot for enterprise that enabled attackers to exfiltrate sensitive user data, including passwords, through a deceptively simple attack vector. The vulnerability centered on an undocumented prompt parameter that circumvented the AI assistant’s built-in safeguards requiring explicit user confirmation before executing commands.
The researchers discovered the flaw through an unconventional approach: they directly questioned Copilot about its security mechanisms and guardrails. Through iterative questioning designed to probe the system’s limitations, they eventually obtained the secret parameter string ?autorun=1. When combined with the existing parameter ?q=, this undocumented feature allowed malicious prompts to execute automatically the moment a user clicked a crafted link, bypassing all required user confirmation gestures.
Using the discovered parameter, Varonis created a proof-of-concept attack capable of searching a user’s inbox, extracting email addresses and credentials, and transmitting that data to attacker-controlled servers. The attack required only a single click on a specially formatted URL sent via email, text message, or other communication channels. Data theft was concealed through base64 encoding to prevent detection and transmission errors. The researchers also demonstrated a secondary attack that poisoned Copilot’s permanent memory store through prompt injection techniques embedded in webpage metadata.
Microsoft initially addressed the vulnerability in February by disabling the ?q= parameter’s ability to inject text into the chatbot input, requiring manual user interaction instead. However, this interim mitigation disrupted legitimate third-party browser integrations. The company deployed more comprehensive fixes on Tuesday. The incidents underscore broader security challenges in large language model systems, where protections rely largely on reactive restrictions rather than fundamental architectural safeguards. Security experts recommend users exercise caution with links from untrusted sources and monitor AI assistant outputs for unexpected behavior.
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