Emdee five for life (HTB Web Challenge)

main.go

package main

import (
	"crypto/md5"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"net/url"
	"regexp"
	"strconv"
	"strings"
)

func parseHash(body string) string {
	re := regexp.MustCompile(`h3 align='center'>(.*)<\/h3>`)
	rs := re.FindStringSubmatch(body)
	return fmt.Sprintf("%x", md5.Sum([]byte(rs[1])))
}

func parseFlag(body string) string {
	re := regexp.MustCompile(`p align='center'>(.*)<\/p>`)
	rs := re.FindStringSubmatch(body)
	return rs[1]
}

func checkErr(err error) {
	if err != nil {
		log.Fatalln(err)
	}
}

func main() {
	targetURL := "http://docker.hackthebox.eu:31093/"

	res, err := http.Get(targetURL)
	checkErr(err)
	defer res.Body.Close()

	body, err := ioutil.ReadAll(res.Body)
	checkErr(err)

	cookies := res.Cookies()
	hash := parseHash(string(body))
	formData := url.Values{"hash": {hash}}

	client := &http.Client{}
	req, err := http.NewRequest("POST", targetURL, strings.NewReader(formData.Encode()))
	for i := range cookies {
		req.AddCookie(cookies[i])
	}
	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Add("Content-Length", strconv.Itoa(len(formData.Encode())))
	res, err = client.Do(req)
	checkErr(err)
	defer res.Body.Close()

	body, err = ioutil.ReadAll(res.Body)
	checkErr(err)
	flag := parseFlag(string(body))

	fmt.Println(flag)
}