metube/app/url_guard.py

"""Lightweight SSRF guard for user-submitted URLs.

MeTube hands user-submitted URLs to yt-dlp, whose generic extractor will fetch
any ``http(s)`` URL. Without a guard, an attacker can make the server fetch
internal endpoints (cloud metadata services, loopback, RFC1918 hosts, etc.) and
have the response saved to the download directory and served back.

This module provides two layers:

* ``validate_url`` — a cheap validator applied at every URL ingress.
* ``install_socket_guard`` — a connect-time ``getaddrinfo`` guard installed in
  the download subprocess, which re-validates every resolved address and so
  covers redirects, DNS rebinding, and media URLs yt-dlp derives from remote
  metadata — for any backend that resolves through Python's socket module.

Known limitations — network isolation (e.g. Docker) remains the backstop for
all of these:

* The socket guard is installed only in the download subprocess. Metadata
  extraction (``ytdl.DownloadQueue.__extract_info``) runs in the main process,
  where installing a process-wide guard would reject the server's own bind on
  ``HOST=0.0.0.0``. So extraction — which also follows redirects — is covered
  only by ``validate_url`` at ingress, not at connect time; a redirect from an
  allowed host to an internal one during extraction is not blocked (a lower-
  impact, blind SSRF, since the extraction response is not written to disk).
* Native resolvers (curl_cffi/libcurl via ``--impersonate``) resolve outside
  Python's socket module and bypass the connect-time guard entirely.
"""

import ipaddress
import logging
import socket
from urllib.parse import urlsplit

log = logging.getLogger('url_guard')

_ALLOWED_SCHEMES = ('http', 'https')

# Hostnames that must be blocked without needing a lookup. ``localhost`` and any
# subdomain of it are conventionally loopback, and the GCP metadata name is a
# well-known SSRF target that may resolve via a resolver we don't control.
_BLOCKED_HOSTNAMES = ('localhost', 'metadata.google.internal')


def _hostname_is_blocked(hostname: str) -> bool:
    host = hostname.rstrip('.').lower()
    for blocked in _BLOCKED_HOSTNAMES:
        if host == blocked or host.endswith('.' + blocked):
            return True
    return False


def _normalise_ip(addr: str):
    """Parse *addr*, unwrapping IPv4-mapped IPv6 (e.g. ``::ffff:169.254.169.254``)
    so the embedded IPv4 address is judged on its own merits. Returns ``None``
    when *addr* is not a valid IP literal."""
    try:
        ip = ipaddress.ip_address(addr)
    except ValueError:
        return None
    if isinstance(ip, ipaddress.IPv6Address) and ip.ipv4_mapped is not None:
        ip = ip.ipv4_mapped
    return ip


def _address_is_global(addr: str) -> bool:
    ip = _normalise_ip(addr)
    return ip is not None and ip.is_global


def _address_allowed_at_connect(addr: str) -> bool:
    """True if *addr* may be connected to at download time.

    Permits global addresses and loopback — loopback so that locally-configured
    proxies (e.g. ``proxy: http://127.0.0.1:9050``) keep working. Blocks the SSRF
    targets that matter: link-local (cloud metadata at 169.254.169.254), private
    (RFC1918), unique-local and every other non-global, non-loopback range.
    """
    ip = _normalise_ip(addr)
    return ip is not None and (ip.is_global or ip.is_loopback)


# Captured at import so re-installing the guard never wraps the wrapper.
_real_getaddrinfo = socket.getaddrinfo


def _guarded_getaddrinfo(host, *args, **kwargs):
    results = _real_getaddrinfo(host, *args, **kwargs)
    allowed = [r for r in results if _address_allowed_at_connect(r[4][0])]
    if not allowed:
        raise socket.gaierror(f'Refusing to connect to non-global address for host {host!r}')
    return allowed


def install_socket_guard() -> None:
    """Enforce the no-internal-hosts policy at actual connection time.

    ``validate_url`` only checks the *submitted* URL string; yt-dlp then follows
    HTTP redirects and resolves media URLs from remote metadata without
    re-validating them. Installing this in the download subprocess re-checks
    every resolved address at connect time, covering redirects and DNS rebinding
    for any networking backend that resolves through Python's socket module
    (urllib, requests). Native resolvers — notably curl_cffi/libcurl used by
    ``--impersonate`` — bypass this and rely on network isolation as the backstop.
    """
    socket.getaddrinfo = _guarded_getaddrinfo


def validate_url(url: str) -> str | None:
    """Return an error message if the URL is disallowed, else ``None``.

    Inputs without a ``://`` scheme separator (bare video IDs, ``ytsearch:``
    and other yt-dlp search/extractor prefixes) are allowed unchanged so that
    non-URL entries keep working.
    """
    if not isinstance(url, str):
        return 'Invalid URL'

    candidate = url.strip()
    if '://' not in candidate:
        # Not an absolute URL: bare video IDs, ytsearch: prefixes, etc.
        return None

    parts = urlsplit(candidate)
    scheme = parts.scheme.lower()
    if scheme not in _ALLOWED_SCHEMES:
        return f'URL scheme "{parts.scheme}" is not allowed (only http and https)'

    hostname = parts.hostname
    if not hostname:
        return 'URL is missing a host'

    if _hostname_is_blocked(hostname):
        return f'Refusing to fetch internal host "{hostname}"'

    try:
        addrinfo = socket.getaddrinfo(hostname, parts.port, proto=socket.IPPROTO_TCP)
    except socket.gaierror:
        # Fail closed: a host we cannot resolve is a host we cannot verify as
        # non-internal, so refuse it rather than letting the download proceed
        # to a target that may resolve differently at fetch time.
        return f'Could not resolve host "{hostname}"'
    except (UnicodeError, ValueError):
        return f'Invalid host "{hostname}"'

    for family, _type, _proto, _canonname, sockaddr in addrinfo:
        addr = sockaddr[0]
        if not _address_is_global(addr):
            return f'Refusing to fetch internal address "{addr}" for host "{hostname}"'

    return None