Script injection is the unauthorized execution of code in a user’s browser; cross-site scripting (XSS) is a common form. If malicious code runs during an Entra ID sign-in, it could expose credentials or tokens, hijack a session, deliver malware, or undermine trust. Microsoft plans to enforce a browser-side Content Security Policy (CSP) for browser sign-ins at login.microsoftonline.com in mid-to-late October 2026. The change is intended as an additional layer of protection, not a claim that every sign-in flow or browser extension is unsafe.
What script injection means
Script or code injection occurs when malicious scripts run in a user’s browser without authorization. XSS is one common type. In a sign-in context, the risk is that code executing in the browser could interact with information or activity available during authentication.
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Possible consequences of a successful compromise include theft of credentials or tokens, session hijacking, malware delivery, and damage to user confidence or an organization’s reputation. These are potential outcomes, not evidence that a particular Entra tenant has been attacked.
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How Microsoft’s CSP is intended to help
A Content Security Policy tells the browser which scripts may run. Microsoft says the Entra sign-in experience will allow scripts from trusted Microsoft domains and trusted script nonces and origins, while blocking other scripts by default. Microsoft describes CSP as an added defense against injection attacks such as XSS, including in situations where another protection is bypassed.
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Microsoft reports that most CSP violations in its analysis stem from external browser extensions or scripts injected by third-party tools. That identifies a common source of violations; it does not mean every extension is malicious or that extensions are the only possible cause.
Which Entra sign-ins are in scope
Microsoft’s announced enforcement applies to browser-based sign-in at login.microsoftonline.com. The stated exclusions are MSAL/API flows that interact with Entra STS APIs and Microsoft Entra External ID sign-ins using custom or CIAM domains.
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Microsoft’s published start window is mid-to-late October 2026. This is its announced plan, not confirmation that global enforcement has already completed.
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What administrators should check
Microsoft says injected scripts will be blocked when enforcement applies. Users should still be able to sign in normally, but a sign-in or monitoring workflow that depends on injected code could be disrupted. Check actual sign-in scenarios and tools in your environment rather than assuming a product is affected or unaffected.
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- Exercise relevant sign-in scenarios. Test the browser-based flows your organization uses at
login.microsoftonline.com, including the associated sign-in and monitoring tools. - Review browser developer-console violations. Check for CSP violations in each relevant scenario and identify which extension or tool is associated with the injected script, if possible.
- Work with the tool vendor. Ask whether the product injects scripts into the sign-in page and whether a CSP-compliant version or alternative is available.
- Remove or migrate dependent tools. If a workflow relies on injected code, plan a replacement or migration before enforcement affects it.
- Retest after changes. Confirm that users can complete sign-in and that monitoring workflows still function.
Microsoft does not identify which specific third-party products inject scripts into an individual organization’s sign-in flow. That must be established by examining the organization’s own browser behavior and consulting the relevant vendor.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How this differs from Entra branding CSS changes
Microsoft’s separate company-branding changes concern tenant-configured appearance—such as logos, backgrounds, layout, text, and custom CSS. They are not the CSP rollout. CSP controls which executable scripts may run in the browser; branding CSS controls visual styling and layout. The documentation does not establish that custom CSS is equivalent to injected JavaScript.
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| Change | What it controls | Where it applies | Administrator action |
|---|---|---|---|
| Script-injection CSP | Executable browser scripts, allowing trusted Microsoft script origins and nonces while blocking other scripts by default. | Browser-based sign-in at login.microsoftonline.com; MSAL/API flows and External ID custom or CIAM domains are stated exclusions. |
Audit script-injecting extensions and tools, review developer-console violations, and coordinate with vendors. |
| Branding CSS restrictions | Visual styling and layout properties in tenant branding CSS. | Company-branding configuration; not the CSP sign-in enforcement. | Inspect downloaded CSS and branding localizations for affected properties, then test changes in a test tenant. |
Microsoft says tenants created after January 5, 2026, do not have custom CSS available. After July 21, 2026, older tenants that are not already using custom CSS cannot configure it. Microsoft is also retiring layout and positioning properties and says it eventually plans to retire custom CSS entirely.
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The CSS reference lists affected properties including position, margin, transform, opacity, overflow, display, and visibility. Microsoft says there is no supported migration or replacement for those properties. For branding changes, inspect downloaded CSS and branding localizations, remove affected properties, and test in a test tenant before changing production.
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