There's a little hint in http://tasks.open.kksctf.ru:8001/robots.txt
On sending a POST request just as below to http://tasks.open.kksctf.ru:8001/postbox we get the flag.
```curl -X POST 'http://tasks.open.kksctf.ru:8001/postbox' -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0' -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8' -H 'Accept-Language: en-US,en;q=0.5' --compressed -H 'Content-Type: application/x-www-form-urlencoded' -H 'Origin: http://tasks.open.kksctf.ru:8001' -H 'Connection: keep-alive' -H 'Referer: http://tasks.open.kksctf.ru:8001' -H 'Upgrade-Insecure-Requests: 1' -H 'Pragma: no-cache' -H 'Cache-Control: no-cache'```