Add default protection for SSRF

src/pretix/helpers/monkeypatching.py

@@ -19,12 +19,26 @@
 # You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
 # <https://www.gnu.org/licenses/>.
 #
+import ipaddress
+import socket
+import sys
 import types
 from datetime import datetime
 from http import cookies
 
+from django.conf import settings
 from PIL import Image
 from requests.adapters import HTTPAdapter
+from urllib3.connection import HTTPConnection, HTTPSConnection
+from urllib3.connectionpool import HTTPConnectionPool, HTTPSConnectionPool
+from urllib3.exceptions import (
+    ConnectTimeoutError, HTTPError, LocationParseError, NameResolutionError,
+    NewConnectionError,
+)
+from urllib3.util.connection import (
+    _TYPE_SOCKET_OPTIONS, _set_socket_options, allowed_gai_family,
+)
+from urllib3.util.timeout import _DEFAULT_TIMEOUT
 
 
 def monkeypatch_vobject_performance():
@@ -89,6 +103,123 @@ def monkeypatch_requests_timeout():
     HTTPAdapter.send = httpadapter_send
 
 
+def monkeypatch_urllib3_ssrf_protection():
+    """
+    pretix allows HTTP requests to untrusted URLs, e.g. through webhooks or external API URLs. This is dangerous since
+    it can allow access to private networks that should not be reachable by users ("server-side request forgery", SSRF).
+    Validating URLs at submission is not sufficient, since with DNS rebinding an attacker can make a domain name pass
+    validation and then resolve to a private IP address on actual execution. Unfortunately, there seems no clean solution
+    to this in Python land, so we monkeypatch urllib3's connection management to check the IP address to be external
+    *after* the DNS resolution.
+
+    This does not work when a global http(s) proxy is used, but in that scenario the proxy can perform the validation.
+    """
+    if settings.ALLOW_HTTP_TO_PRIVATE_NETWORKS:
+        # Settings are not supposed to change during runtime, so we can optimize performance and complexity by skipping
+        # this if not needed.
+        return
+
+    def create_connection(
+        address: tuple[str, int],
+        timeout=_DEFAULT_TIMEOUT,
+        source_address: tuple[str, int] | None = None,
+        socket_options: _TYPE_SOCKET_OPTIONS | None = None,
+    ) -> socket.socket:
+        # This is copied from urllib3.util.connection v2.3.0
+        host, port = address
+        if host.startswith("["):
+            host = host.strip("[]")
+        err = None
+
+        # Using the value from allowed_gai_family() in the context of getaddrinfo lets
+        # us select whether to work with IPv4 DNS records, IPv6 records, or both.
+        # The original create_connection function always returns all records.
+        family = allowed_gai_family()
+
+        try:
+            host.encode("idna")
+        except UnicodeError:
+            raise LocationParseError(f"'{host}', label empty or too long") from None
+
+        for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
+            af, socktype, proto, canonname, sa = res
+
+            if not settings.ALLOW_HTTP_TO_PRIVATE_NETWORKS:
+                ip_addr = ipaddress.ip_address(sa[0])
+                if ip_addr.is_multicast:
+                    raise HTTPError(f"Request to multicast address {sa[0]} blocked")
+                if ip_addr.is_loopback or ip_addr.is_link_local:
+                    raise HTTPError(f"Request to local address {sa[0]} blocked")
+                if ip_addr.is_private:
+                    raise HTTPError(f"Request to private address {sa[0]} blocked")
+
+            sock = None
+            try:
+                sock = socket.socket(af, socktype, proto)
+
+                # If provided, set socket level options before connecting.
+                _set_socket_options(sock, socket_options)
+
+                if timeout is not _DEFAULT_TIMEOUT:
+                    sock.settimeout(timeout)
+                if source_address:
+                    sock.bind(source_address)
+                sock.connect(sa)
+                # Break explicitly a reference cycle
+                err = None
+                return sock
+
+            except OSError as _:
+                err = _
+                if sock is not None:
+                    sock.close()
+
+        if err is not None:
+            try:
+                raise err
+            finally:
+                # Break explicitly a reference cycle
+                err = None
+        else:
+            raise OSError("getaddrinfo returns an empty list")
+
+    class ProtectionMixin:
+        def _new_conn(self) -> socket.socket:
+            # This is 1:1 the version from urllib3.connection.HTTPConnection._new_conn v2.3.0
+            # just with a call to our own create_connection
+            try:
+                sock = create_connection(
+                    (self._dns_host, self.port),
+                    self.timeout,
+                    source_address=self.source_address,
+                    socket_options=self.socket_options,
+                )
+            except socket.gaierror as e:
+                raise NameResolutionError(self.host, self, e) from e
+            except socket.timeout as e:
+                raise ConnectTimeoutError(
+                    self,
+                    f"Connection to {self.host} timed out. (connect timeout={self.timeout})",
+                ) from e
+
+            except OSError as e:
+                raise NewConnectionError(
+                    self, f"Failed to establish a new connection: {e}"
+                ) from e
+
+            sys.audit("http.client.connect", self, self.host, self.port)
+            return sock
+
+    class ProtectedHTTPConnection(ProtectionMixin, HTTPConnection):
+        pass
+
+    class ProtectedHTTPSConnection(ProtectionMixin, HTTPSConnection):
+        pass
+
+    HTTPConnectionPool.ConnectionCls = ProtectedHTTPConnection
+    HTTPSConnectionPool.ConnectionCls = ProtectedHTTPSConnection
+
+
 def monkeypatch_cookie_morsel():
     # See https://code.djangoproject.com/ticket/34613
     cookies.Morsel._flags.add("partitioned")
@@ -99,4 +230,5 @@ def monkeypatch_all_at_ready():
     monkeypatch_vobject_performance()
     monkeypatch_pillow_safer()
     monkeypatch_requests_timeout()
+    monkeypatch_urllib3_ssrf_protection()
     monkeypatch_cookie_morsel()

src/pretix/settings.py

@@ -223,6 +223,7 @@ CSRF_TRUSTED_ORIGINS = [urlparse(SITE_URL).scheme + '://' + urlparse(SITE_URL).h
 
 TRUST_X_FORWARDED_FOR = config.getboolean('pretix', 'trust_x_forwarded_for', fallback=False)
 USE_X_FORWARDED_HOST = config.getboolean('pretix', 'trust_x_forwarded_host', fallback=False)
+ALLOW_HTTP_TO_PRIVATE_NETWORKS = config.getboolean('pretix', 'allow_http_to_private_networks', fallback=False)
 
 
 REQUEST_ID_HEADER = config.get('pretix', 'request_id_header', fallback=False)