XStream is a Java library to serialize objects to XML and back again. In XStream before version 1.4.16, there is a vulnerability which may allow a remote attacker to load and execute arbitrary code from a remote host only by manipulating the processed input stream. No user is affected, who followed the recommendation to setup XStream's security framework with a whitelist limited to the minimal required types. If you rely on XStream's default blacklist of the Security Framework, you will have to use at least version 1.4.16.
Unauthenticated RCE is possible when JMeter is used in distributed mode (-r or -R command line options). Attacker can establish a RMI connection to a jmeter-server using RemoteJMeterEngine and proceed with an attack using untrusted data deserialization. This only affect tests running in Distributed mode. Note that versions before 4.0 are not able to encrypt traffic between the nodes, nor authenticate the participating nodes so upgrade to JMeter 5.1 is also advised.