In early 2026 researchers found a new exploit method called Reprompt that could hijack Microsoft Copilot sessions through a crafted link, allowing attackers to steal data without the user knowing if the latest updates were not applied. Microsoft has patched the issue in January 2026, but this incident highlights why Microsoft Copilot security risk protection remains crucial for users and enterprises.

Microsoft Copilot security risk protection

When Varonis Threat Labs looked into Copilot’s security, they found that the AI assistant accepted natural language instructions directly from a URL parameter labeled “q” in the Copilot web session. An attacker could send a user a link that looked normal but had hidden malicious instructions embedded. When someone clicked that link, Copilot would automatically execute those instructions using the user’s authenticated session.

Researchers call this the Reprompt attack flow because it involves reprompting Copilot with follow‑up instructions pulled from the attacker’s server. The first step used phishing tactics to get someone to click a link. After that, Copilot continued to receive new prompt instructions without additional user actions. That made it hard for client‑side security tools to notice what was happening.

The attack combined several techniques to bypass protections that only applied to the first request. In simple terms, Copilot would accept a hidden instruction embedded in the URL and then carry out a chain of commands from the attacker. This could include reading stored conversations, accessing data Copilot already knew about, or even making Copilot send data out to a server controlled by the attacker.

Microsoft addressed this specific vulnerability in a security update released on Patch Tuesday in January 2026. The vulnerability was fixed and is no longer exploitable in updated systems.

Why Reprompt Matter for Copilot Security

Even though the Reprompt attack was patched, the way it worked shows deeper security challenges for AI assistants like Copilot, especially around session handling and prompt processing. Here are key lessons that relate to Copilot session hijack protection and AI assistant data theft prevention:

Session Handling Complexity

Copilot’s web interface kept a session alive even after someone closed the tab or moved away from the app. Attackers used this long‑lived session to inject commands and keep Copilot executing tasks behind the scenes. That is a classic session hijacking risk, but applied in a new context where AI continues processing after the user stops paying attention.

Prompt Manipulation

Attackers did not break into Microsoft’s systems or obtain credentials. They tricked Copilot into executing instructions by manipulating how prompts were passed to it. This is why Copilot prompt manipulation mitigation and prompt injection protection tools matter now more than ever.

Data Exfiltration

Once Copilot is hijacked, the real danger is sensitive data leaving the system. In this case researchers showed that conversation history, personal details, or other stored context could be exfiltrated without the user realizing it. That raises serious questions about Copilot privacy protection and preventing Copilot data leakage for enterprises.

How Prompt Injection Risks Affect AI Assistants

Prompt injection is a broader class of attack that Reprompt falls under. Prompt injection risks involve attackers embedding malicious instructions inside user input or content that AI systems trust and interpret. These risks have been documented beyond just Reprompt:

Indirect prompt injections can allow attackers to hide malicious instructions in documents, files, or even diagrams that Copilot processes. One example showed how attackers used hidden content inside a spreadsheet to cause Copilot to retrieve sensitive emails and send them to an external server.

Security teams and researchers have discussed similar risks at events like Black Hat, where experts demonstrated how hidden prompt content in emails could alter Copilot behavior or extract information without clear user actions.

This makes clear why organizations need AI threat modeling and session security for AI tools as part of any deployment of Copilot or similar systems.

What Microsoft Is Doing to Improve Copilot Security

Microsoft has built several layers of defense into Microsoft 365 Copilot, including content filtering and classifiers that detect suspicious prompt patterns. These defenses help mitigate prompt injection risks, validate suspicious activity, and contain potential threats within the user’s identity or tenant context.

Some of these defenses include:

Spam and scam filtering: Blocks content that looks like phishing or malicious prompts.
Malicious prompt classifiers: These try to identify and ignore hidden or harmful instructions.
Session hardening: Techniques that limit how much access an attacker could gain even if some exploit occurred.

Microsoft also integrates its extended detection and response systems, like Microsoft Defender, to help security teams see prompt injection attempts and respond to them in context with broader activity across an enterprise environment.

Gaps in Current Copilot Security

Even with these protections, there are areas that require refinement:

1. User Education: Not all prompt injection risks are visible to casual users. People may not realize a seemingly innocuous link could contain hidden commands. That highlights the need for user training on how to spot suspicious links and verify content before interacting with AI tools.

2. Tool Integration Risks: When Copilot interacts with apps like SharePoint, Teams, or Outlook, any misconfiguration could create indirect prompt attack surfaces. That is why copilot security best practices include checking access controls and permissions across connected services.

3. Enterprise Monitoring: Enterprises should not rely solely on built‑in protections. Tools like Copilot session hijack protection and enterprise AI threat mitigation often require external security monitoring, logging, and alerting to catch unusual activity quickly.

Best Practices to Protect Against Reprompt‑Style Threats

Here are practical steps to reduce risk:

Stay Updated: Ensure systems always install security patches quickly. The Reprompt vulnerability was addressed in January 2026 updates. Without updates, even known exploits can be dangerous.

Monitor Sessions: Use security tools that track AI session behavior so you can detect anomalies, especially multiple background requests without user interaction. This supports Copilot session hijack protection.

Train Users: Teach employees about phishing tactics, suspicious links, and how prompt manipulation works. People who know the risks are harder for attackers to trick.

Apply Access Controls: Strict permissions prevent data exposure. Limit what Copilot can access and ensure only authorized users can retrieve sensitive data.

Audit and Logging: Maintain logs for all Copilot interactions and review them regularly. This can highlight unusual data requests or prompt behavior.

These steps improve AI assistant data theft prevention and reduce the broader risk profile for AI deployments.

Reprompt vs Other Prompt‑Related Attacks

FAQ

What is a Reprompt attack?
A Reprompt attack is a threat method that lets attackers insert hidden instructions into a legitimate Copilot link, hijack a session, and silently extract data. Microsoft has patched this vulnerability; however, awareness matters because similar techniques could appear again.

Can prompt attacks lead to data theft?
Yes, prompt attacks exploit how AI interprets instructions. They can make the assistant perform unauthorized actions, including retrieving sensitive information.

How to secure Microsoft Copilot sessions from manipulation?
Best practices include applying patches, using strict access controls, monitoring session behavior, and training users to recognize suspicious activity.

What are prompt injection attacks?
Prompt injection attacks occur when hidden or malicious instructions alter the behavior of an AI assistant beyond what the user intended, potentially leading to leaks or unauthorized tasks.

Final Thoughts

The Reprompt attack was a wake‑up call. It showed that even when Microsoft Copilot cybersecurity services include multiple safeguards, attackers will look for paths that bypass initial defenses and exploit assumptions about session continuity. While Microsoft responded with a patch, ongoing improvements in Copilot prompt manipulation mitigation and active monitoring remain critical.

By combining user education, up‑to‑date security tools, and strong governance policies, organizations can better defend against prompt injection risks and strengthen session security for AI tools.

Microsoft Copilot Security Risk Protection: How to Prevent Reprompt Attacks and Data Theft