The darwinssl_connect_step1 function in lib/vtls/curl_darwinssl.c in libcurl 7.31.0 through 7.39.0, when using the DarwinSSL (aka SecureTransport) back-end for TLS, does not check if a cached TLS session validated the certificate when reusing the session, which allows man-in-the-middle attackers to spoof servers via a crafted certificate.
http://kb.juniper.net/InfoCenter/index?page=content&id=JSA10743
http://lists.apple.com/archives/security-announce/2015/Aug/msg00001.html
http://secunia.com/advisories/61925