Cause: Microsoft modified their implementation of PKINIT in order to support SHA-2 checksum of the pre-authentication request. They made a non-standard addition to the protocol which is now required by Windows Server 2025. Consequence: MIT krb5 clients (in their default configuration) are currently not able complete PKINIT pre-authentication against an Active Directory instance running on Windows Server 2025. Workaround: It was confirmed by Microsoft that (for now) the requirement for the new PKINIT element is effective only when using finite field Difffie-Hellman (FFDH) for key agreement. Elliptic curve Diffie-Hellman (ECDH) is not affected by this new requirement. As a consequence, setting an elliptic curve as default minimal DH parameter can work around this issue. To do so, the "pkinit_dh_min_bits" setting should be set with "P-256", "P-384", or "P-521". As an example, to use the P-256 curve, create the file "/etc/krb5.conf.d/pkinit_use_ecdh.conf" with the following content: [libdefaults] pkinit_dh_min_bits = P-256 Result: The workaround will allow completing PKINIT pre-authentication until MIT krb5 supports the change required by Active Directory (which will allow FFDH-based PKINIT to work again).