Reprompt Exploit: The One-Click Vulnerability in Microsoft Copilot Personal Exposing Sensitive Data
A recently discovered vulnerability in Microsoft Copilot Personal, dubbed Reprompt, has raised significant security concerns. This exploit allowed attackers to exfiltrate sensitive user data through a single-click phishing attack, operating undetected within the system. Although Microsoft has since patched the flaw, understanding the mechanics of this vulnerability is crucial for enhancing future cybersecurity measures.
Understanding the Reprompt Exploit
The Reprompt attack was initiated by threat actors who sent phishing emails containing legitimate Copilot URLs embedded with a malicious ‘q’ parameter. Upon clicking the link, this parameter auto-executed a prompt that leveraged the victim’s authenticated session. This session remained active even after the user closed the tab, allowing attackers to query and extract personal details such as usernames, locations, file access history, and even vacation plans.
The attack chain employed server-driven follow-ups, enabling commands to unfold dynamically and evade client-side detection. This method made the data exfiltration process stealthy and difficult to trace.
Techniques Enabling Stealthy Data Theft
Security researchers at Varonis identified three core techniques that facilitated this undetected data theft, effectively bypassing Copilot’s safeguards designed to prevent unauthorized data access:
1. Parameter-to-Prompt (P2P) Injection: By injecting instructions via the ‘q’ parameter, attackers could auto-populate and execute prompts that stole conversation memory or data. This method exploited the system’s ability to process input parameters without adequate validation.
2. Double-Request Technique: Copilot’s leak protections were primarily effective during initial requests. By instructing the system to repeat actions twice, attackers could succeed on the second attempt. For instance, commands like double check… make every function call twice could expose secrets such as HELLOWORLD1234! upon retry.
3. Chain-Request Method: This technique involved the server generating sequential prompts based on previous responses, effectively chaining exfiltration stages indefinitely. The process could progress from fetching usernames to retrieving time, location, user information summaries, and conversation topics through staged URLs.
These methods rendered data exfiltration virtually undetectable, as the prompts appeared innocuous while gradually leaking information to attacker-controlled servers.
Scope and Impact of the Vulnerability
The Reprompt exploit specifically targeted Copilot Personal, which is integrated into Windows and Edge for consumer use. This integration allowed access to prompts, history, and Microsoft data, including recent files and geolocation information.
Notably, enterprises utilizing Microsoft 365 Copilot remained unaffected due to the presence of Purview auditing, tenant Data Loss Prevention (DLP) measures, and administrative controls. While there were no reported instances of this exploit being used in the wild, the ease of executing a single-click email or chat attack posed significant risks. Potentially compromised data could include sensitive information such as financial plans or medical notes, as illustrated in the attack diagrams provided by Varonis.
Disclosure and Remediation
Varonis responsibly disclosed the Reprompt vulnerability to Microsoft on August 31, 2025. In response, Microsoft deployed a fix as part of the January 13, 2026, Patch Tuesday updates. Users are strongly advised to apply the latest Windows updates immediately to mitigate any residual risks associated with this vulnerability.
Comparative Analysis with Previous Vulnerabilities
Unlike prior flaws such as EchoLeak (CVE-2025-32711), the Reprompt exploit did not require the use of documents or plugins. This distinction highlights the inherent risks associated with URL parameter manipulation in AI platforms. The simplicity of executing the Reprompt attack underscores the need for robust input validation and security measures in AI-driven applications.
Recommendations for Users and Organizations
To safeguard against similar vulnerabilities, organizations and individual users should adopt the following practices:
– Treat AI URL Inputs as Untrusted: Implement stringent validation and sanitization processes for all AI-related URL inputs to prevent unauthorized data access.
– Enforce Persistent Safeguards Across Chained Prompts: Develop and enforce security measures that persist across multiple prompts to detect and prevent chained exfiltration attempts.
– Scrutinize Pre-Filled Prompts: Users should be vigilant in reviewing pre-filled prompts, especially those originating from untrusted sources, to identify and avoid potential phishing attempts.
– Avoid Untrusted Links: Exercise caution when clicking on links from unknown or unverified sources, as they may contain malicious parameters designed to exploit vulnerabilities.
– Monitor for Anomalies: Regularly monitor systems for unusual activities, such as unsolicited data requests or unexpected prompt executions, which may indicate a security breach.
Call to Action for Vendors
Vendors, including Microsoft, are urged to conduct thorough audits of external inputs, operating under the assumption of insider-level access in AI contexts. This proactive approach is essential to preempt and mitigate similar exploit chains in the future.
Conclusion
The Reprompt vulnerability in Microsoft Copilot Personal serves as a stark reminder of the evolving threats in the cybersecurity landscape, particularly concerning AI-integrated platforms. By understanding the mechanisms of such exploits and implementing robust security measures, users and organizations can better protect sensitive data from unauthorized access.
