The Apache Qpid Proton-J transport includes an optional wrapper layer to perform TLS, enabled by use of the "transport.ssl" methods. Unless a verification mode was explicitly configured, client and server modes previously defaulted as documented to not verifying a peer certificate, with options to configure this explicitly or select a certificate verification mode with or without hostname verification being performed. The latter hostname verifying mode was not implemented in Apache Qpid Proton-J versions 0.3 to 0.29.0, with attempts to use it resulting in an exception. This left only the option to verify the certificate is trusted, leaving such a client vulnerable to Man In The Middle attack. Uses of the Proton-J protocol engine which do not utilise the optional transport TLS wrapper are not impacted, e.g. usage within Qpid JMS. Uses of Proton-J utilising the optional transport TLS wrapper layer that wish to enable hostname verification must be upgraded to version 0.30.0 or later and utilise the VerifyMode#VERIFY_PEER_NAME configuration, which is now the default for client mode usage unless configured otherwise.