External sharing is one of Microsoft 365’s greatest strengths — and one of its most misunderstood risks.
In most organisations, external sharing begins as a productivity enabler rather than a security decision. Teams need to work quickly with suppliers, partners, and customers, and Microsoft 365 removes many of the traditional barriers to collaboration. The problem is not that sharing exists, but that it often expands organically without clear boundaries, ownership, or review. Over time, what began as controlled access turns into persistent exposure.
Done well, it enables collaboration at scale. Done poorly, it creates silent data exposure that persists long after the original business need has passed.
This article explores how external sharing creates risk in Microsoft 365, where organisations typically lose control, and what secure sharing actually looks like.
Modern organisations do not operate in isolation. Suppliers, partners, and customers all require access to data.
Microsoft 365 makes this easy — sometimes too easy.
Default sharing configurations prioritise usability. Without explicit design decisions, tenants drift toward over-sharing. These defaults are designed to reduce friction for a global user base with widely varying risk appetites. They assume a level of trust, visibility, and governance that many organisations have not yet established. When left unchanged, default configurations encourage behaviours that feel natural to users but quietly erode control over where data flows and who retains access.
Two common exposure models:
Anonymous access represents a fundamental loss of control because it severs the relationship between data and identity. Once a link is created and distributed, there is no reliable way to understand who is accessing the data, from where, or for what purpose. Revocation becomes reactive rather than preventative, and visibility is reduced to inference rather than evidence.
Anonymous links present particular challenges:
Once distributed, control is effectively lost.
SharePoint and OneDrive serve different purposes and introduce different risks:
The risk profiles of SharePoint and OneDrive differ not only in scale, but in intent. SharePoint is typically designed for shared access, which makes oversharing a governance issue. OneDrive, by contrast, is personal by design, which makes unintentional exposure more likely. Links created for speed or convenience often bypass formal review entirely, turning personal storage into an unmonitored distribution channel.
OneDrive links created for convenience frequently bypass governance entirely.
External sharing is not a misfeature — it is a deliberate response to how modern organisations operate. Supply chains are distributed, projects are collaborative, and data must move beyond organisational boundaries to deliver value. Any realistic security strategy must therefore accommodate sharing rather than attempt to eliminate it. The challenge lies in making those exchanges intentional and accountable.
Common leakage scenarios include:
These leakage patterns rarely trigger immediate alarms because they often involve legitimate access rather than obvious misuse. Data is shared through approved mechanisms, accessed by known external parties, and consumed in ways that appear benign. Without explicit monitoring and review, these scenarios blend into normal activity, allowing exposure to persist unnoticed for months or years.
Even well-configured tenants often lack:
When monitoring is absent or incomplete, organisations lose the ability to distinguish between expected collaboration and risky behaviour. Investigations become difficult because there is no baseline to compare against, and incident response teams are left reconstructing events without sufficient data. In these conditions, even minor sharing mistakes can escalate into significant exposure before anyone realises there is a problem.
Mature organisations recognise that secure collaboration is a strategic capability, not a technical toggle. They invest time in understanding where data flows, who depends on it, and how access should evolve as relationships change. This intentionality is what separates collaboration that empowers the business from sharing that quietly undermines it.
Secure external sharing requires:
These elements are interdependent. Policy without enforcement is ignored, enforcement without monitoring is blind, and monitoring without review quickly loses relevance. Secure sharing only emerges when all three are treated as part of an ongoing process rather than a one-time configuration exercise.
Most importantly, it requires treating sharing as a security design decision, not a convenience feature.
The mistake many organisations make is framing external sharing as a binary choice between openness and security. In reality, the issue is not whether sharing occurs, but whether it is governed with the same care as internal access. Once data crosses organisational boundaries, the cost of poor decisions increases dramatically. External sharing is not inherently dangerous — unmanaged sharing is.
The risk does not come from collaboration itself, but from the absence of boundaries, ownership, and review. Once data leaves the organisation’s control, the consequences are rarely immediate but often severe.
Mature organisations recognise that secure sharing is not about restriction, but intentionality. Knowing what is shared, with whom, and why is the difference between enablement and exposure.
Trusted Microsoft Cloud Security Advisor with 27 years experience | Empowering Businesses to Embrace Cloud Innovation with Confidence
