Series: The AI Governance Series — Blog 4 of 4

TL;DR — Key Takeaways

• Microsoft 365 Copilot doesn’t create new data risks — it surfaces the ones that already exist. Overpermissioned files become instantly searchable by anyone.
• 70% of Fortune 500 companies have adopted Copilot, but most enterprises remain in pilot mode with immature governance, per Forrester.
• 47% of IT leaders lack confidence in their ability to manage Copilot’s security and access risks, per Gartner.
• Security researchers have demonstrated live Copilot attacks including prompt injection, data exfiltration, and ASCII smuggling — some with zero user interaction required.
• Gartner’s top Copilot risk: SharePoint oversharing. The average enterprise has 802,000 overshared files.
• Classie’s M365 supervision platform gives security and compliance teams full-context visibility into how Copilot is actually being used.

Most Copilot rollouts follow the same arc. IT gets the licenses. Pilots run in a few teams. Leadership pushes for broader deployment. Someone asks if the data governance is ready. Nobody has a clean answer. The deployment happens anyway.

Six months later, someone starts asking uncomfortable questions. Which files can Copilot see? Who asked it what? What did it summarize? Can we prove it didn’t surface something it shouldn’t have?

Those questions don’t have easy answers if you didn’t build the oversight layer before you scaled.

We covered the broader AI governance gap in the first post in this series. This post focuses on the Copilot-specific version of that gap — which is real, well-documented, and showing up in Gartner briefings and Black Hat presentations alike.

What are the biggest Microsoft Copilot security risks after deployment?

The short answer is that Copilot amplifies whatever data governance problems you already had. It doesn’t introduce new risks so much as make existing ones impossible to ignore.

Gartner’s Dennis Xu presented “Mitigating the Top 5 Microsoft 365 Copilot Security Risks” at the Gartner Security & Risk Management Summit in March 2026. His number one risk, which he spent 20 of his 30 minutes on: SharePoint oversharing. His framing was direct: “Copilot makes over-shared documents more accessible. This is not a net new risk, but a known risk amplified by AI.”

The other four risks Gartner identified were remote code execution through malicious prompts, sensitive data access through third-party SaaS plugins (the web content plugin is on by default), prompt injection attacks, and inaccurate or toxic output that employees act on without verifying.

The data behind the oversharing risk is stark. Research from Metomic and Varonis found 16% of business-critical data is overshared across an average of 802,000 files per organization, and 83% of sensitive business files are overshared with users inside the company. Copilot doesn’t change who technically has access to those files. It makes them findable through plain English questions — which is a very different thing.

Is Copilot actually being exploited, or is this theoretical?

It’s not theoretical. Security researchers have demonstrated real, working attacks against Copilot deployments, and several have been patched only after public disclosure.

Michael Bargury, former Microsoft Azure Security CTO and now CTO of Zenity, presented at Black Hat USA 2024 under the title “Living off Microsoft Copilot.” He demonstrated five live attack scenarios: manipulating Copilot to change vendor banking details to redirect payments, exfiltrating data ahead of earnings reports, turning Copilot into an automated phishing machine that mimics a user’s writing style, directing users to phishing sites, and pulling passwords from Teams chat history. His assessment: “When you give AI access to data, that data is now an attack surface for prompt injection. This is the equivalent of remote code execution in the world of Copilot.” Microsoft acknowledged his work and thanked him for responsible disclosure.

Johann Rehberger discovered the ASCII Smuggling vulnerability in early 2024 — a technique combining prompt injection with special Unicode characters invisible in the Copilot UI to silently exfiltrate data including sales records and MFA codes. Reported in January 2024, it wasn’t patched until July 2024.

More recently, Varonis Threat Labs discovered Reprompt (CVE-2026-21521), a single-click attack that exploits a Copilot URL parameter to silently exfiltrate user data in three stages. Dolev Taler of Varonis described it plainly: “AI assistants have become trusted companions — but trust can be easily exploited, and an AI assistant can turn into a data exfiltration weapon with a single click.” It was reported in August 2025 and patched in January 2026.

And in January 2025, a code defect allowed Copilot to summarize emails marked as confidential even when DLP policies were in place — meaning the DLP label was present, but Copilot ignored it.

Mid-deployment and not sure if your governance is keeping pace? CI can run an assessment.

→ Book a discovery call with CI

Who is responsible for Copilot security — Microsoft or the customer?

Microsoft operates a shared responsibility model, and the division of responsibility matters here because most organizations assume Microsoft handles more than it does.

Microsoft’s responsibilities cover the model lifecycle, application infrastructure, content filtering, prompt injection blocking, platform security, and safety systems. The customer’s responsibilities cover AI governance and oversight — usage policies, review processes, responsible AI training — along with data governance, identity and access management configuration, sensitivity labeling, DLP policy configuration, permissions management, and plugin governance.

Microsoft has published an Oversharing Deployment Blueprint acknowledging that “gaps in governance, such as over-permissioned sites, inherited access, or lack of sensitivity label protections, become amplified” by Copilot.

The permissions problem is the one that catches most organizations off guard. The average employee can access 17 million files on their first day of work, according to Varonis research. Copilot makes all of those files instantly queryable through natural language. An employee can ask “What’s our current salary band for senior engineers?” and if that data exists in a file they technically have access to, Copilot will answer.

— Nikki Chapple, Microsoft MVP

Why are most Copilot deployments still operating without real governance?

Because most enterprises are further behind than they think.

A 2026 industry report from 2toLead and Cybersecurity Insiders surveying 1,253 security professionals found 73% of organizations have deployed AI tools, but only 7% govern them with real-time policy enforcement. 94% report gaps in AI activity visibility. And 88% cannot distinguish personal AI accounts from corporate instances — which matters enormously for Copilot, where licensed users interact differently than unlicensed ones using free-tier access.

Gartner’s January 2025 analysis found 47% of IT leaders are either not very confident or have no confidence at all in their ability to manage Copilot’s security and access risks. Forrester reports that most enterprises are 12–18 months from scaled Copilot deployment, with data readiness and ROI measurement cited as the primary blockers.

The pattern we see repeatedly: organizations deploy Copilot on the productivity timeline, not the security timeline. The licenses are bought. The rollout happens. The governance conversation starts six months later when someone asks a question Copilot answered that it probably shouldn’t have.

— Dennis Xu, Research VP, Gartner

What does Copilot governance actually require?

It starts with permissions. Before Copilot goes anywhere near production at scale, someone needs to audit SharePoint and Teams permissions, apply sensitivity labels to executive, HR, financial, and legal documents, and enforce Conditional Access for all users including contractors. This isn’t optional — it’s the foundation Copilot’s safety depends on.

From there, governance means being able to answer specific questions: What is Copilot doing with sensitive data? Which documents is it summarizing, for whom? Are there patterns of use that fall outside approved workflows? Is the web content plugin surfacing data it shouldn’t?

Those questions require visibility that standard Microsoft logs don’t provide. Classie’s M365 supervision platform [REPLACE: CLASSIE-M365-URL] is built specifically for this gap. It delivers full-context visibility into Copilot interactions — not just activity logs, but complete prompt and response capture, document-aware context that links interactions to specific files, and intent classification that distinguishes legitimate workflows from risky usage patterns. It integrates with Microsoft Purview sensitivity labels, so you can correlate AI activity against the files that matter most.

If you’ve already worked through the broader AI governance questions, the CISO questions post [REPLACE: BLOG2-URL] earlier in this series covers the framework-level decisions that should be in place before you scale any AI deployment.

Ready to close the Copilot governance gap? CI Digital runs the assessment and implementation.

→ Talk to CI about Copilot governance

Frequently Asked Questions

What are the main Microsoft Copilot security risks?

Gartner identifies five: SharePoint oversharing (the dominant risk), remote code execution through malicious prompts, sensitive data access through third-party SaaS plugins, prompt injection attacks, and inaccurate or toxic output. The full Gartner briefing from March 2026 is worth reviewing for any organization mid-deployment.

What is Copilot data exposure and how does it happen?

Copilot data exposure occurs when employees use natural language queries to surface files or data they technically have access to but were never intended to find easily. Because Copilot searches across all permissioned content, over-permissioned files — salary data, HR documents, M&A information, legal correspondence — become retrievable by anyone with Copilot access in the tenant.

Is Microsoft responsible for Copilot security incidents?

Microsoft and the customer share responsibility. Microsoft covers the model, infrastructure, safety systems, and content filtering. The customer is responsible for data governance, permissions management, sensitivity labeling, DLP policy configuration, and AI usage oversight. Microsoft’s own Oversharing Blueprint acknowledges that governance gaps become amplified by Copilot.

Has Microsoft Copilot actually been hacked or exploited?

Yes. Security researchers have demonstrated live attacks including prompt injection via ASCII smuggling (discovered 2024, patched July 2024), the Reprompt vulnerability CVE-2026-21521 (patched January 2026), a cross-prompt injection in the email summarization feature (patched March 2026), and a DLP bypass bug that allowed Copilot to summarize confidential emails despite active policies.

What should IT leaders do before scaling Copilot deployment?

Audit SharePoint and Teams permissions, apply Microsoft Purview sensitivity labels to all sensitive documents, enforce Conditional Access for all users, disable or restrict the web content plugin, and deploy a supervision layer that gives security teams visibility into what Copilot is accessing and summarizing in real time. Microsoft recommends a phased Pilot → Deploy → Operate approach using SharePoint Advanced Management, Purview, and Data Security Posture Management for AI.

What is Microsoft 365 Copilot governance and why does it matter?

Copilot governance is the set of policies, controls, and monitoring systems that ensure Copilot operates within approved boundaries — accessing only what it should, summarizing only what’s appropriate, and leaving an auditable trail that compliance and legal teams can use. Without it, organizations face data exposure risks, compliance gaps, and limited ability to investigate incidents after the fact.

This post completes The AI Governance Series