Based on research published by Varonis Threat Labs, the Reprompt vulnerability exposed a design flaw in how Microsoft Copilot Personal handled URL based prompts.

With a single click on a legitimate Copilot link containing a malicious ?q= parameter, attackers could auto execute hidden instructions, silently hijack an active Copilot session, and begin exfiltrating sensitive data (e.g., conversation history, recent files, location, and personal details) without any further user action.

Varonis showed that Copilot's guardrails could be bypassed by instructing it to repeat the same action twice - the first attempt would sanitize sensitive data, but the second would leak it - while a server driven "chain request" kept the attack going even after the chat window was closed, making client side monitoring largely blind to the ongoing data theft.

Microsoft patched the issue in the Patch updates, and both Varonis and Microsoft confirmed that Microsoft 365 Copilot (Enterprise) was not affected due to tenant level protections like Purview auditing and DLP.

Summary of the Reprompt Vulnerability

1. Affected vs. Unaffected Systems

• - Affected: Copilot Personal (Consumer)
  The vulnerability abused how the consumer version of Copilot processed URL parameters - particularly the ?q= query string. A malicious link could auto populate and auto execute a hidden prompt, enabling attackers to extract conversation history, file access information, and personal data without the user realizing it.

• - Not Affected: M365 Copilot (Enterprise)
  Both Microsoft and Varonis confirmed that Microsoft 365 Copilot for Enterprise customers were not impacted by Reprompt.

2. Why M365 Copilot (Enterprise) Was Protected

Enterprise environments include several security layers not present in the consumer version:

• - Purview Auditing and Data Loss Prevention (DLP)
  Enterprise DLP can automatically block, mask, or log sensitive data movement before it leaves the tenant.

• - Tenant Level Administrative Controls
  Administrators can restrict Copilot's data access, manage connectors, and enforce tighter organizational policies.

• - Enhanced Identity and Session Management
  Stronger authentication flows and stricter session handling reduce the likelihood of unauthorized or persistent access.

3. How the "Reprompt" Attack Worked

Varonis identified a three step attack chain:

1. - Parameter to Prompt (P2P)
   A crafted URL using the ?q= parameter auto filled and triggered a malicious prompt immediately upon clicking.

2. - Double Request Bypass
   By instructing Copilot to repeat a request twice, the attacker bypassed guardrails that applied only to the initial attempt - leading to sensitive data being returned on the second try (e.g., the "HELLOWORLD1234!" demo secret).

3. - Chain Request Exfiltration
   After the initial execution, Copilot continued a hidden, server driven back and forth with the attacker's endpoint, enabling ongoing, incremental exfiltration - even after the user closed the chat window. Because follow up commands originated from the attacker's server, client side tools struggled to detect what data was being leaked.

4. Mitigation and Recommendations

• - Vulnerability Patched
  Microsoft fixed the issue in its updates release - (keep your windows updates - up to date)

• - Keep Software Updated
  Ensure Windows, browsers, and Copilot components are fully updated to current versions.

• - Stay Cautious with Links
  Avoid clicking unsolicited or suspicious URLs - especially those that launch AI tools or pre populate chat prompts via ?q=.

At a Glance Takeaways for Readers

• - One click compromise on a legitimate Microsoft Copilot URL - no plugins or further interaction required.
• - Guardrail bypass via a double request trick that leaked data on the second attempt.
• - Stealthy persistence with server driven chains that continued even after closing the chat.
• - Enterprise M365 Copilot was not impacted, thanks to tenant controls and DLP.
• - Patched by Microsoft in January 2026; keep systems up to date.

Should admin offices uninstall Microsoft Copilot "because of the risk"?

Short answer: Usually no - prefer disablement and scoping over wholesale uninstall, especially in enterprise contexts.

A recently disclosed single click attack targeting Microsoft Copilot Personal demonstrated how attackers could silently exfiltrate sensitive user data by embedding malicious instructions inside a phishing link. Once clicked, the attacker could hijack the active session without any further interaction from the user. This vulnerability has since been patched, but it underscores how easily convenience features - like URL based prompt loading - can be abused when combined with social engineering.

To reduce the risk of sensitive personal data disclosure through phishing, organizations should enforce strong email filtering, adopt phishing resistant authentication, educate users to avoid opening unsolicited Copilot related links, and apply security updates promptly. Strengthening link handling policies, verifying sender legitimacy, and avoiding interaction with unexpected AI generated URLs are essential first line defenses.

With that context, a blanket uninstall is rarely the best path for admin offices. A more sustainable approach is to minimize exposure through policy and configuration, especially if you're using Microsoft 365 Copilot (enterprise) rather than the personal/consumer variant:

• - License scoping: Only assign Copilot licenses to roles that genuinely need them. No license means no access, which cleanly limits your surface area.
• - UI and feature controls: Where AI is not required, disable the Windows/Edge Copilot surfaces via Intune or Group Policy.
• - Browser hardening: Enforce extension allow lists and block unapproved AI sidebars; the browser remains a high risk exfil channel.
• - Data controls: Apply Purview DLP, auditing, and conditional access so Copilot interactions inherit your existing guardrails.
• - User training: Reinforce anti phishing practices, especially around pre filled Copilot links and unexpected prompts.

When to fully uninstall?

If a specific admin office has no AI use case or must meet strict compliance obligations, it can be operationally simpler to remove Copilot UI and withhold licenses for those users.

Treat this as a scoping decision - not a panic response - because enterprise controls and current patches allow for a measured, risk based deployment.