It's a simple challenge with 300 points :)

command "ls" Display files available on the server.

**index.php shell.jpg flag.txt **

Then through the description we pass the command "**history**" and we found

1 ZXh0c
2 mFfc2
3 VjdXJ
4 lX2Zp
5 bGVfc
6 mVhZG
7 Vy

Decode base 64 we found (**extra_secure_file_reader)**

Finally to read the file (**flag.txt)** we just type **extra_secure_file_reader flag.txt** and flag is displayed.

flag is : YUBITSEC{shello_shello_are_you_thereo}