Microsoft to Retire Exchange ActiveSync Certificate-Based Authentication

Microsoft is phasing out an old authentication method for mobile email, which signals another major step toward stricter, modern security in Exchange Online. Organizations relying on certificate-based access via ActiveSync must now prepare for a transition to Entra ID–based authentication ahead of the 2026 deadline.

Direct Exchange ActiveSync certificate-based authentication is being phased out because it is classified as a legacy approach that does not align with modern security standards. Unlike newer methods, it does not rely on OAuth tokens and instead depends on internal validation within Exchange, which limits the ability to enforce advanced controls such as Conditional Access. Microsoft is retiring it as part of a broader effort to strengthen security and move away from outdated authentication practices.

“Certificate-based authentication for EAS was introduced as a way for organizations to allow mobile device access without passwords, using client certificates for a highly secure, passwordless sign-in experience. With CBA, each user has a certificate verified by the tenant’s root certificate authority, and the user can authenticate via a TLS handshake using the public key of that certificate – meaning no private key or password is ever sent over the network, providing a more secure alternative to basic authentication,” the Exchange team explained.

According to Microsoft, organizations are required to transition to Microsoft Entra ID-based certificate authentication, which replaces the older direct connection method. In this new process, the client first sends its certificate to Entra ID for verification; once validated, Microsoft Entra issues an OAuth access token that the client then uses to securely access Exchange Online. This aligns EAS with modern authentication standards and centralized policy enforcement.

Security benefits of the new authentication model

The new model delivers several security and management advantages by allowing organizations to apply consistent policies across all applications and access methods. It also supports passwordless, phishing‑resistant authentication while removing dependence on highly privileged internal processes, which results in a more secure and streamlined authentication framework.

According to Microsoft, this change might impact organizations using certificate authentication instead of OAuth in mobile device setups. Admins can also verify usage through Microsoft Entra sign-in logs.

Steps admins should take before the 2026 deadline

Microsoft advises organizations to begin the migration process as early as possible to ensure a smooth transition. This involves setting up certificate authorities within Entra ID, verifying that user certificates contain accurate identity details such as email addresses in the Subject Alternative Name field, updating device or mobile device management configurations, and conducting pilot testing while monitoring sign-in logs.

This change reflects Microsoft’s broader strategy to modernize and strengthen the security framework of Exchange Online by eliminating outdated authentication methods and aligning all access with modern identity standards. For administrators, this means taking a proactive role in assessing current authentication setups, identifying devices and users still relying on legacy EAS certificate-based authentication, and carefully planning the migration to the Entra ID-based model. They should also ensure early preparation, clear communication with users, coordination with MDM or client vendors, and thorough testing to ensure a seamless transition with minimal user impact and uninterrupted service.