Microsoft Enforces CSP to Block Entra ID Script Injection Attacks
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Microsoft is proactively enforcing Content Security Policy (CSP) on Entra ID sign-in pages to eliminate the risk of cross-site scripting (XSS) attacks, which can allow attackers to steal credentials or hijack authentication sessions. The root issue is that browser extensions and third-party scripts injected into authentication pages create an uncontrolled execution environment, bypassing identity security controls. This matters because authentication pages are high-value targets — a compromised sign-in flow can expose entire enterprise environments. Organizations that have customized sign-in experiences with external scripts or browser extensions must act before mid-October 2026 or risk breaking their own authentication workflows.
Tactical Insight
Immediate Actions
- Audit all browser extensions deployed across the organization to identify any that inject scripts into Microsoft Entra ID or other identity provider sign-in pages.
- Test existing sign-in workflows in a staging environment against the new CSP policy before the October 2026 deadline.
- Remove or replace any non-compliant browser extensions that modify authentication page behavior.
Long-Term Improvements
- Establish a policy that prohibits unapproved browser extensions on devices used to access sensitive enterprise applications.
- Maintain an approved software and extension inventory, reviewed quarterly, to ensure only vetted tools interact with identity systems.
- Adopt a zero-trust posture for authentication flows by restricting client-side script execution to explicitly trusted sources.
Detection Measures
- Implement endpoint monitoring to detect unauthorized browser extensions installed on managed devices.
- Enable Entra ID sign-in diagnostic logs and alert on authentication failures that may indicate CSP policy violations post-enforcement.