SharePoint Online: CSP Explained

Key insights

• Content Security Policy (CSP): CSP is a browser-enforced policy that controls which domains can load scripts, images, fonts, and iframes on modern SharePoint pages.
  It helps protect SharePoint Framework (SPFx) solutions from script injection and cross-site scripting (XSS).
• Modes and timeline: Microsoft runs CSP in report-only mode to log issues until February 28, 2026, then turns on full enforcement mode from March 1, 2026, with an optional 90-day delay available via tenant settings.
  Administrators can enable enforcement early for their tenant to test impact.
• What breaks & migration: Inline scripts (for example, inline onclick handlers or document.write scripts) are blocked under enforcement and must move to external files on trusted script sources.
  Developers should host scripts on approved domains and update SPFx components to avoid blocked behavior; you can test a page by appending ?csp=enforce to the URL.
• Admin tools: Use SharePoint Online cmdlets to manage CSP sources: Get-SPOContentSecurityPolicy, Add-SPOContentSecurityPolicy, Remove-SPOContentSecurityPolicy, and ResyncContentSecurityPolicyConfigurationEntries.
  The tenant property EnforceContentSecurityPolicyConfiguration controls early enforcement.
• Security benefits: CSP reduces attack surface by blocking untrusted scripts and lowers the risk of XSS and related exploits.
  Logged violations integrate with monitoring so teams can track issues in tools like Microsoft Purview.
• Testing and remediation workflow: Use report-only logs in the browser console and tenant logs to find violations, fix code or add trusted sources, then resync SPFx component entries (note manifests may take up to 24 hours to sync).
  Retest pages after fixes to confirm no remaining CSP violations before enforcement reaches your tenant.

Overview of the demo

Microsoft 365 published a demo-style presentation that explains the rollout of Content Security Policy in SharePoint Online, and the video clarifies why this change matters for organizations running custom pages and solutions. The recording comes from a Microsoft 365 community call and highlights practical steps developers and administrators can take to identify and remediate issues before enforcement begins. Presenters walk through how the policy behaves in both monitoring and blocking modes, while also showing tools and commands that support migration. Consequently, the session serves as a concise guide for teams that need to prepare their tenants and customizations.

The demo emphasizes that the current phase is report-only and that enforcement will begin on a scheduled date, which makes the session timely for administrators. It aims to reduce surprises by teaching how to detect violations and how to register safe script sources. At the same time, it stresses that CSP will help reduce common browser-based threats, such as cross-site scripting, by restricting where scripts and other resources may load from. Therefore, the message is both preventive and practical.

What CSP means for SharePoint Online

Content Security Policy is a browser security mechanism that restricts the origins from which a page may load resources like scripts, images, and frames. In SharePoint Online, the policy primarily targets script execution on modern pages, and it blocks inline scripts and resources from sources that are not explicitly allowed. This change affects custom SharePoint Framework (SPFx) solutions, embedded third-party widgets, and any other code that currently relies on inline execution or unapproved CDNs. As a result, tenants must ensure that legitimate resources are registered as trusted before enforcement to avoid runtime breakage.

Importantly, CSP in SharePoint operates in two useful modes: a report-only mode that logs violations without blocking, and an enforcement mode that actively blocks non-compliant resources. The report-only phase allows teams to audit real-world behavior and make targeted fixes without user disruption, while enforcement will raise the security baseline by preventing risky scripts from running. Thus, administrators can balance safety and continuity by using the monitoring window to remediate problems. In practice, migrating inline code to hosted files and registering trusted sources are common remediation steps.

How enforcement and testing work

SharePoint administrators manage trusted script sources through the admin center or by using PowerShell commands that surface and update the policy configuration. Commands such as Get-SPOContentSecurityPolicy, Add-SPOContentSecurityPolicy, Remove-SPOContentSecurityPolicy, and ResyncContentSecurityPolicyConfigurationEntries help list, add, remove, and synchronize entries respectively, and the tenant property EnforceContentSecurityPolicyConfiguration controls enforcement behavior. The demo shows how to test enforcement immediately by appending a query string like ?csp=enforce to a modern page URL and then examining the browser console for violations. Consequently, teams can simulate blocking and verify fixes before the global enforcement date.

Logging and auditing integrate with Microsoft Purview and browser consoles so that administrators do not rely only on end-user reports to find problems. The Resync operation is highlighted as useful when SPFx component manifests add new source entries, although synchronization can take time, often up to a day depending on tenant geography. Therefore, planning and staged rollouts are important to avoid missed updates. In short, the available tools enable a structured approach to discovery, remediation, and verification.

Benefits and tradeoffs

Adopting CSP in SharePoint Online offers clear security benefits: it reduces the attack surface for cross-site scripting and other injection risks by allowing scripts only from trusted origins. Consequently, tenants gain stronger protection without changing browser behavior outside SharePoint, and compliant third-party CDNs continue to work after registration. Additionally, report-only monitoring gives teams a low-risk window to measure impact and prioritize fixes, which helps preserve productivity during migration. This balance of prevention and observability is a major reason Microsoft recommends early audits.

However, the move also involves tradeoffs and operational challenges, primarily for legacy or highly customized environments. Inline scripts, event-handler attributes, or third-party embeds that expect unrestricted execution will stop working under enforcement, so developers must refactor such code into externally hosted files and update references. Synchronization delays and the need to maintain an accurate list of trusted origins add administrative overhead, and overly permissive source lists would undermine the security gains. Therefore, teams must weigh the costs of refactoring and ongoing governance against the security benefits and plan resources accordingly.

Timeline and recommended next steps

Microsoft provides a transition window in which CSP runs in report-only mode so tenants can audit and remediate violations. Enforcement is scheduled to start on the announced date, and organizations should treat the report-only phase as the final opportunity to find issues without impact. Administrators are advised to run audits, register legitimate sources, resync manifests when needed, and test pages with the enforcement query parameter to validate fixes. Taken together, these actions reduce the risk of service interruptions once strict blocking begins.

In practical terms, teams should inventory customizations, prioritize high-impact pages, and communicate with developers and stakeholders about required code changes. Because enforcement raises the security baseline, policymakers should keep trusted sources tight and document exceptions to prevent drift. Finally, monitoring and iterative remediation will make the migration manageable: by using the tools and techniques shown in the demo, organizations can move to enforcement with confidence while minimizing user disruption.

Keywords

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