The version used of Log4j, the library used for logging by PowerNukkit, is subject to a remote code execution vulnerability via the ldap JNDI parser.
It's well detailed at CVE-2021-44228 and CVE-2021-45105(https://github.com/advisories/GHSA-p6xc-xr62-6r2g).
Impact
Malicious client code could be used to send messages and cause remote code execution on the server.
Patches
PowerNukkit 1.5.2.1 is a patch-release that only updates the Log4j version to 2.17.0 and should be used instead of 1.5.2.0.
All versions prior to 1.5.2.1 are affected and are not patched.
Workarounds
If you can't upgrade, you can use the -Dlog4j2.formatMsgNoLookups=true startup argument as remediation, as this prevents the vulnerability from happening.