206 lines
7.8 KiB
Python
206 lines
7.8 KiB
Python
"""Security policy for credential-bearing stdlib urllib requests."""
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from __future__ import annotations
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import copy
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import logging
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import os
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import ssl
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import sys
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import urllib.parse
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import urllib.request
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from collections.abc import Callable, Iterable
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from pathlib import Path
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from typing import Any
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logger = logging.getLogger(__name__)
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# Headers safe to forward to a different origin. Everything else is dropped:
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# custom provider headers routinely carry credentials under arbitrary names.
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_CROSS_ORIGIN_SAFE_HEADERS = frozenset({"accept", "user-agent"})
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_DEFAULT_PORTS = {"http": 80, "https": 443}
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_CA_BUNDLE_ENV_VARS = (
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"HERMES_CA_BUNDLE",
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"SSL_CERT_FILE",
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"REQUESTS_CA_BUNDLE",
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"CURL_CA_BUNDLE",
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)
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def url_origin(url: str) -> tuple[str, str, int | None]:
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"""Return a normalized (scheme, hostname, effective port) origin."""
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parsed = urllib.parse.urlparse(url)
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scheme = (parsed.scheme or "").lower()
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# Accessing ``parsed.port`` validates malformed/non-numeric ports. Let the
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# ValueError fail the request closed instead of collapsing it to a default.
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port = parsed.port
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return (
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scheme,
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(parsed.hostname or "").lower().rstrip("."),
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port if port is not None else _DEFAULT_PORTS.get(scheme),
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)
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def _strip_headers(request, keep: frozenset[str]) -> None:
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"""Drop every header on *request* whose lowercased name is not in *keep*."""
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for name, _value in list(request.header_items()):
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if name.lower() not in keep:
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request.remove_header(name)
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class SafeCredentialRedirectHandler(urllib.request.HTTPRedirectHandler):
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"""Preserve request headers only while redirects stay on one origin."""
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def __init__(
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self,
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original_url: str,
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*,
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cross_origin_safe_headers: Iterable[str] = _CROSS_ORIGIN_SAFE_HEADERS,
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) -> None:
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self._original_origin = url_origin(original_url)
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self._cross_origin_safe_headers = frozenset(
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str(name).lower() for name in cross_origin_safe_headers
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)
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def redirect_request(self, req, fp, code, msg, headers, newurl):
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# Let urllib enforce status/method semantics first (notably 307/308).
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redirected = super().redirect_request(req, fp, code, msg, headers, newurl)
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if redirected is None:
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return None
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resolved_url = urllib.parse.urljoin(req.full_url, newurl)
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if url_origin(resolved_url) != self._original_origin:
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# Use an allowlist rather than guessing credential header names.
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# normalize_extra_headers permits arbitrary secret-bearing names.
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_strip_headers(redirected, self._cross_origin_safe_headers)
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return redirected
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class _CrossOriginRequestSanitizer(urllib.request.BaseHandler):
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"""Strip headers after installed request processors have run."""
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# Request processors run in ascending order. Keep this last so an installed
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# cookie/auth/instrumentation processor cannot re-add a secret after the
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# redirect handler sanitizes the new Request.
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# Infinity is greater than every finite handler order. If an installed
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# processor also uses infinity, stable sorting keeps this appended handler
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# after it, so sanitization still owns the final request boundary.
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handler_order = float("inf") # type: ignore[assignment]
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def __init__(self, original_url: str) -> None:
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self._original_origin = url_origin(original_url)
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def _sanitize(self, request: urllib.request.Request):
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if url_origin(request.full_url) != self._original_origin:
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_strip_headers(request, _CROSS_ORIGIN_SAFE_HEADERS)
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return request
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http_request = _sanitize
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https_request = _sanitize
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def _resolved_https_context() -> ssl.SSLContext | None:
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"""Return the explicit CA context for Hermes-owned urllib openers."""
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ca_bundle = next(
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(value for name in _CA_BUNDLE_ENV_VARS if (value := os.getenv(name, "").strip())), "")
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if ca_bundle:
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ca_path = Path(ca_bundle).expanduser()
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if ca_path.is_file():
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try:
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return ssl.create_default_context(cafile=str(ca_path))
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except (OSError, ssl.SSLError) as exc:
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logger.warning(
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"CA bundle could not be loaded from %s: %s — falling back to default certificates",
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ca_bundle,
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exc,
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)
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else:
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logger.warning(
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"CA bundle path does not exist: %s — falling back to default certificates",
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ca_bundle,
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)
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if sys.platform != "darwin":
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return None
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try:
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import certifi
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return ssl.create_default_context(cafile=certifi.where())
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except (ImportError, OSError, ssl.SSLError) as exc:
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logger.warning(
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"Could not load certifi for urllib HTTPS verification: %s — falling back to default certificates",
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exc,
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)
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return None
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def _secure_opener_from_installed_policy(original_url: str, *, ssl_context=None):
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"""Clone the installed opener's handlers, replacing redirect policy only.
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With ``ssl_context`` the cloned HTTPS handler is bound to it so per-provider TLS settings
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(``ssl_ca_cert``/``ssl_verify``) apply. With None, Hermes-owned openers get an explicit CA
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default via ``_resolved_https_context``; an application-installed opener's TLS policy is
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preserved.
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"""
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installed = getattr(urllib.request, "_opener", None)
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if installed is None:
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context = _resolved_https_context()
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installed = urllib.request.build_opener(
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*([] if context is None else [urllib.request.HTTPSHandler(context=context)]))
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_https_handler_cls = getattr(urllib.request, "HTTPSHandler", None)
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replace_https = ssl_context is not None and _https_handler_cls is not None
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handlers = [
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copy.copy(handler)
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for handler in getattr(installed, "handlers", ())
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if not isinstance(handler, urllib.request.HTTPRedirectHandler)
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and not (replace_https and isinstance(handler, _https_handler_cls))
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]
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if replace_https:
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handlers.append(_https_handler_cls(context=ssl_context))
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handlers.append(SafeCredentialRedirectHandler(original_url))
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handlers.append(_CrossOriginRequestSanitizer(original_url))
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secured = urllib.request.build_opener(*handlers)
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# OpenerDirector injects addheaders after request processors, which would
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# bypass the sanitizer on redirects. Carry them on the initial request
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# instead, then leave the rebuilt opener's late-injection list empty.
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secured._hermes_initial_addheaders = list(getattr(installed, "addheaders", ()))
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secured.addheaders = []
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return secured
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def open_credentialed_url(
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request: urllib.request.Request,
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*,
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timeout: float,
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opener_factory: Callable[..., Any] | None = None,
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ssl_context=None,
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):
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"""Open a request without forwarding credentials across origins.
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The default preserves an application-installed opener's proxy, TLS, cookies, custom protocol
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handlers, and instrumentation while replacing its redirect handler. ``opener_factory`` is an
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explicit test seam; security is never disabled based on global ``urlopen`` identity.
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``ssl_context`` (an ``ssl.SSLContext``) overrides the HTTPS handler's TLS policy for this
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request only.
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"""
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if opener_factory is None:
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opener = _secure_opener_from_installed_policy(
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request.full_url, ssl_context=ssl_context
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)
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for name, value in getattr(opener, "_hermes_initial_addheaders", ()):
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if not request.has_header(name):
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request.add_header(name, value)
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else:
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opener = opener_factory(SafeCredentialRedirectHandler(request.full_url))
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return opener.open(request, timeout=timeout)
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__all__ = [
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"SafeCredentialRedirectHandler",
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"open_credentialed_url",
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"url_origin",
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]
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