X.509 is published as ITU recommendation ITU-T X.509 (formerly CCITT X.509) and ISO/IEC/ITU 9594-8 which defines a standard certificate format for public key certificates and certification validation. With minor differences in dates and titles, these publications provide identical text in the defining of public-key and attribute certificates.

Internet X.509 Public Key Infrastructure Certificate and CRL Profile
This memo profiles the X.509 v3 certificate and X.509 v2 CRL for use in the Internet.