E.Tree was a Python Flask application that used XPATH to parse XML files. We were presented with an example XML file from where we could see that some users have an additional selfDestructCode element set. Knowing this, we were able to do an error-based XPATH injection to determine the flag.

Original writeup (https://rfc-infosec.com/ctf/cyber%20apocalypse%202021/E.Tree-Challenge-Write-up-Cyber-Apocalypse-2021/).