The (1) serf_ssl_cert_issuer, (2) serf_ssl_cert_subject, and (3) serf_ssl_cert_certificate functions in Serf 0.2.0 through 1.3.x before 1.3.7 does not properly handle a NUL byte in a domain name in the subject's Common Name (CN) field of an X.509 certificate, which allows man-in-the-middle attackers to spoof arbitrary SSL servers via a crafted certificate issued by a legitimate Certification Authority.