$Echo

The page looks like is passing our input to the echo command filtering some kind of characters. Looks like it wont parse ` so we can execute, for example, the ls command putting `ls`. Also we can use < so we can tell bash to pass a file content to echo.

Putting `ls ../` return flag.txt html as output, so there is the flag. Lets try this:

`<../flag.txt`

And we get the flag:

flag{1beadaf44586ea4aba2ea9a00c5b6d91}