A critical vulnerability in Microsoft Entra could have let hackers access any user account. The flaw affected key authentication processes, allowing normal security checks to be bypassed. Microsoft released a patch quickly, but accounts that are not updated remain exposed.
Experts urge immediate action to protect sensitive information. Applying the latest updates promptly ensures personal and business accounts stay secure and prevents unauthorized access or potential data breaches.
How the flaw was discovered?
Security researcher Dirk-jan Mollema privately reported the flaw to Microsoft in July 2025; the issue was later tracked as CVE-2025-55241 and involved a token-validation weakness in legacy Entra/Azure AD flows that could let crafted ‘actor’ tokens bypass authentication checks.
Microsoft deployed mitigations and a patch in the weeks that followed; Microsoft said it had found no evidence of widespread exploitation at the time of disclosure.
Any tenant still using the affected legacy token validation paths that had not applied Microsoft’s mitigations remained at risk.
Microsoft published guidance and fixes for affected Entra/Azure AD configurations, organizations must apply Microsoft’s security update or configuration mitigations immediately and verify remediation in their tenant.
Individuals should confirm account protection and monitor activity. Applying updates immediately remains the most effective defense against unauthorized access and helps safeguard sensitive personal and organizational data.
What the flaw targeted?
The vulnerability involved undocumented ‘actor’ tokens issued by Microsoft’s retired Access Control Service (ACS) and a validation gap in the legacy Azure AD Graph API.
Crafted tokens could be accepted across tenants, effectively allowing attackers to impersonate users (including admins) without normal verification checks in certain configurations. Awareness of the flaw ensures better preparedness for similar threats in the future.