We are given a website that lets us add a format string that is passed to `date` command.
First what we see is a banner:
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We check address `http://220.127.116.11:5675/.index.php.swp` and get some version of `index.php` file. Here we see that there is a file `varflag.php` which holds a flag.
Back to the form. The command we send is escaped, but as a whole, so if we could interfere with outside world with `date` arguments only, we could probably do something. Reading manual tells that option `-f` allows to read format strings from file. We send ` -f varflag.php` (space is to separate format string from the `-f` argument), view source (so that `'
Please remove/blank the writeup, the CTF is still running.
Please explain how you have found it.