In Clojure before 1.9.0, classes can be used to construct a serialized object that executes arbitrary code upon deserialization. This is relevant if a server deserializes untrusted objects.
https://security.snyk.io/vuln/SNYK-JAVA-ORGCLOJURE-5740378
https://github.com/frohoff/ysoserial/pull/68/files
https://github.com/clojure/clojure/commit/271674c9b484d798484d134a5ac40a6df15d3ac3