Varonis reveals Microsoft Copilot auto-executes hidden prompts via URL parameter
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Security firm Varonis disclosed Tuesday that an undocumented `?autorun=1` parameter in Microsoft Copilot URLs auto-executes embedded prompts without user approval. A crafted link can make Copilot search the victim's inbox for the latest sender address or stored credentials and leak the data to an attacker-controlled server. Separately, Varonis demonstrated a webpage prompt injection that poisons Copilot's permanent memory when a user asks Copilot to summarize a page.
Key Facts
- Varonis disclosed Tuesday that an undocumented `?autorun=1` parameter in Microsoft Copilot URLs auto-executes embedded prompts without user approval.
- A crafted link can make Copilot search the victim's inbox for the latest sender email address and send it to an attacker-controlled webhook.site server.
- A separate prompt in the same URL format instructs Copilot to search the inbox for passwords or credentials, and any found secrets are leaked to the attacker-controlled server after base64 encoding.
- Varonis also showed a webpage-based prompt injection that updates Copilot's permanent memory when a user summarizes a page, enabling future output forwarding, information filtering, or biased responses.
Auto-Execution via URL Parameter
Varonis disclosed Tuesday that the URL format https://copilot.microsoft.com/?q=&autorun=1 can inject prompts directly into Microsoft Copilot. The undocumented `?autorun=1` parameter triggers automatic execution, while the `?q=` prompt fires without any user gesture. Copilot processes the injected prompt with full access to the victim's session context, connected apps, and memory. The prompt executes to completion, including network fetches and connector invocations, even if the user closes the Copilot tab immediately after loading the page.
Inbox and Credential Exfiltration
One injected prompt searches the victim's inbox for the latest email and extracts only the most recent sender's email address into a variable named SUPPORT. It then builds a webhook.site URL ending in that sender address and has Copilot summarize the URL, sending the address to an attacker-controlled server. A separate prompt in the same URL format instructs Copilot to search the inbox for passwords or other credentials sent to the address. Any secrets found are appended to a separate URL that Copilot automatically opens on the user's device, and the exfiltrated data is converted to base64 to prevent transmission errors.
Copilot Memory Poisoning
Varonis separately devised an attack that embeds prompt injection in a webpage to poison Microsoft Copilot's permanent memory store. The memory store saves user information, preferences, and instructions for use in future sessions without re-entry. When a user asks Copilot to summarize a malicious page, the assistant follows instructions hidden in the page metadata to update that memory. Varonis said the technique can forward outputs, filter information, bias responses toward attacker-chosen narratives, or execute attacker-defined actions on trigger conditions.
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Varonis reveals Microsoft Copilot auto-executes hidden prompts via URL parameter
