TL;DR:

  • Microsoft 365 Copilot inherits the permissions of the user who activates it — if someone can access a file, Copilot can too, meaning overly permissive sharing creates a Copilot data exposure risk
  • The most common issue is “overshared” SharePoint and OneDrive documents — files shared with “Everyone” or “Anyone with the link” become searchable and summarisable by Copilot for any user in your organisation
  • Before enabling Copilot, run a permissions audit on SharePoint and OneDrive, restrict broad sharing policies, and apply sensitivity labels to documents containing personal data, financial information, or HR records

Microsoft 365 Copilot is now available on Business Standard and Business Premium plans, which means it’s landing in UK small businesses who haven’t necessarily thought through the security implications. It’s a useful tool. It’s also a tool that can surface sensitive information to the wrong people if your Microsoft 365 data permissions aren’t in good shape.

This guide is for small business owners and IT managers who want to use Copilot safely, without creating an accidental data exposure problem.

What Copilot Can Access

Microsoft 365 Copilot works by connecting to the data your Microsoft 365 tenant contains and making it searchable, summarisable, and usable in prompts. Specifically, it can access:

  • Emails in Exchange Online (including old emails and shared mailboxes you’ve been added to)
  • Documents in OneDrive and SharePoint (any file the user can access)
  • Teams messages and meeting transcripts
  • Calendar entries
  • Loop pages and notebooks

The important principle: Copilot doesn’t bypass permissions. It can only access what the currently logged-in user can access. If you can’t open a file, Copilot can’t read it for you.

The problem is not that Copilot breaks your permissions. The problem is that it amplifies them. If a user has access to sensitive files they’d never normally bother to open, Copilot can surface that content automatically in response to a relevant prompt — without the user consciously choosing to look at it.

The Oversharing Problem

The most common data risk in Microsoft 365 environments is oversharing. It accumulates gradually: a document gets shared with “Everyone in the organisation” for convenience, a SharePoint site inherits broad permissions from its parent, a shared drive gets added to a Teams channel, and suddenly a large portion of your company’s documents are technically accessible to all staff.

In a normal Microsoft 365 environment without Copilot, this matters but the practical impact is limited — most people don’t go looking for files they don’t know exist. With Copilot, the dynamic changes. A staff member could ask “what are our margins on the Jones account?” and Copilot might surface a financial analysis document that was shared too broadly, that the person asking would never have found manually but technically has permission to access.

This is not a Copilot bug. It’s a permissions problem that Copilot makes visible.

The Pre-Copilot Checklist

Before enabling Microsoft 365 Copilot for your team, these are the steps worth taking:

1. Audit SharePoint sharing settings

In the Microsoft 365 Admin Centre, go to SharePoint admin centre > Policies > Sharing. Review your external and internal sharing settings. For most small businesses, the right settings are:

  • External sharing: “Only people already in your organisation” (restrict external sharing to explicit invitations, not public links)
  • Default link type: “Only specific people” rather than “Anyone” or “People in your organisation”

2. Find broadly-shared documents

Use the SharePoint admin centre’s content search or the Microsoft Purview portal to identify documents shared with “Everyone” or “Everyone except external users.” These are the high-priority files to review — they are accessible to all Copilot users in your organisation.

3. Apply sensitivity labels to sensitive documents

Microsoft Purview sensitivity labels let you mark documents as Confidential, Highly Confidential, or similar, and enforce access controls based on those labels. Documents labelled as Confidential can be configured to only appear in Copilot results for users who have explicit access, adding a layer of control beyond standard SharePoint permissions.

For small businesses without a dedicated IT function, the priority categories to label first are: HR and personnel records, financial statements and management accounts, client contracts, and any documents containing personal data under GDPR.

4. Review shared mailboxes

Copilot can read shared mailboxes the user is a member of. If your business has shared inboxes (sales@, info@, accounts@) that contain sensitive customer or financial information, check who is a member of those mailboxes and whether Copilot access to that content is appropriate.

5. Teams meeting transcripts

If your organisation uses Teams meeting transcription (enabled by default in some plans), Copilot can summarise and reference those transcripts. Review who has access to meeting recordings and transcripts in your Teams admin settings.

GDPR Considerations

Under GDPR, you remain the data controller for any personal data in your Microsoft 365 tenant. When Copilot processes that data to respond to prompts, Microsoft acts as a data processor.

Key points for UK GDPR compliance:

Data residency: Microsoft 365 Copilot processes data in Microsoft’s EU and UK data centres. If you’re using the standard Microsoft 365 Business plans, your data stays within the EU/UK data region.

Prompts and responses: Microsoft does not use your Copilot prompts or responses to train its AI models (they are covered by the commercial data protection agreement). Your staff’s Copilot conversations are treated as organisational data.

Subject access requests: If a person’s personal data is present in documents that Copilot has processed, this could theoretically be referenced in a data subject access request. Having clear data retention policies for documents containing personal data — and applying sensitivity labels — helps manage this.

Consent: Staff using Copilot are submitting prompts that may reference personal data about colleagues, clients, or employees. A brief internal policy clarifying what types of queries are appropriate (don’t ask Copilot to summarise a specific employee’s performance records without HR involvement, for example) is good practice.

Is the Risk Worth It?

Copilot is genuinely useful for small businesses — drafting emails, summarising documents, preparing meeting notes, pulling together information from across your systems. The security risks are manageable with a bit of preparation.

The mistake to avoid is enabling Copilot without first cleaning up your permissions. If your SharePoint environment has years of overshared documents and nobody has thought about who can access what, Copilot will surface that mess in a way that makes it visible. Fix the permissions first, then enable the AI. The order matters.