Certificate validation in Secure Socket Layer or Transport Layer Security protocol (SSL/TLS) is critical to Internet security. Thus, it is significant to check whether certificate validation in SSL/TLS is correctly implemented. With this motivation, we propose a novel differential testing approach which is directed by the standard Request For Comments (RFC). First, rules of certificates are extracted automatically from RFCs. Second, low-level test cases are generated through dynamic symbolic execution. Third, high-level test cases, i.e. certificates, are assembled automatically. Finally, with the assembled certificates being test cases, certificate validations in SSL/TLS implementations are tested to reveal latent vulnerabilities or bugs. Our approach named RFCcert has the following advantages: (1) certificates of RFCcert are discrepancy-targeted since they are assembled according to standards instead of genetics; (2) with the obtained certificates, RFCcert not only reveals the invalidity of traditional differential testing but also is able to conduct testing that traditional differential testing cannot do; and (3) the supporting tool of RFCcert has been implemented and extensive experiments show that the approach is effective in finding bugs of SSL/TLS implementations.