curl and libcurl 7.27.0 through 7.35.0, when using the SecureTransport/Darwinssl backend, as used in in Apple OS X 10.9.x before 10.9.2, does not verify that the server hostname matches a domain name in the subject's Common Name (CN) or subjectAltName field of the X.509 certificate when accessing a URL that uses a numerical IP address, which allows man-in-the-middle attackers to spoof servers via an arbitrary valid certificate.