Due to a vulnerability in the SSLv3 (Secure Socket Layer) protocol, support for this protocol is deprecated in z/OS Explorer. However, SSL was the default protocol up until the deprecation, which implies that existing host and client setups require updates to switch to TLS (Transport Layer Security).
The recommended action for the host is to explicitly disable the usage of SSL by adding GSK_PROTOCOL_SSLV3=OFF to rse.env (support for TLS is already enabled by default).
During the transition period in which older clients are updated to use TLS, the z/OS Explorer host must be able to support both SSL and TLS. Depending on your service level of System SSL and Java™, this might be a more involved process, as outlined here:
$ cp /usr/lpp/java/J7.0/lib/security/java.security /etc/zexpl/ssl.java.security
$ oedit /etc/zexpl/ssl.java.security
-> comment: #jdk.tls.disabledAlgorithms=SSLv3
GSK_PROTOCOL_SSLV3=ON
GSK_PROTOCOL_TLSV1=ON
_RSE_JAVAOPTS="$_RSE_JAVAOPTS -Dcom.ibm.jsse2.disableSSLv3=false"
_RSE_JAVAOPTS="$_RSE_JAVAOPTS -Djava.security.properties==/etc/zexpl/ssl.java.security"