OpenSSL before 0.9.8za 1.0.0 before 1.0.0m and 1.0.1 before 1.0.1h does not properly restrict processing of ChangeCipherSpec messages which allows man-in-the-middle attackers to trigger use of a zero-length master key in certain OpenSSL-to-OpenSSL communications and consequently hijack sessions or obtain sensitive information via a crafted TLS handshake aka the \CCS Injection\ vulnerability.