Implement MSC3383: include destination in X-Matrix auth header (#11398)

Co-authored-by: Jan Christian Grünhage <jan.christian@gruenhage.xyz>
Co-authored-by: Marcus Hoffmann <bubu@bubu1.eu>

4 files changed, 48 insertions, 11 deletions

diff --git a/changelog.d/11398.feature b/changelog.d/11398.feature
new file mode 100644
index 0000000000..a910f4da14
--- /dev/null
+++ b/changelog.d/11398.feature
@@ -0,0 +1 @@
+Implement [MSC3383](https://github.com/matrix-org/matrix-spec-proposals/pull/3383) for including the destination in server-to-server authentication headers. Contributed by @Bubu and @jcgruenhage for Famedly GmbH.
diff --git a/scripts-dev/federation_client.py b/scripts-dev/federation_client.py
index c72e19f61d..079d2f5ed0 100755
--- a/scripts-dev/federation_client.py
+++ b/scripts-dev/federation_client.py
@@ -124,7 +124,12 @@ def request(
     authorization_headers = []
 
     for key, sig in signed_json["signatures"][origin_name].items():
-        header = 'X-Matrix origin=%s,key="%s",sig="%s"' % (origin_name, key, sig)
+        header = 'X-Matrix origin=%s,key="%s",sig="%s",destination="%s"' % (
+            origin_name,
+            key,
+            sig,
+            destination,
+        )
         authorization_headers.append(header.encode("ascii"))
         print("Authorization: %s" % header, file=sys.stderr)
 
diff --git a/synapse/federation/transport/server/_base.py b/synapse/federation/transport/server/_base.py
index 2529dee613..d629a3ecb5 100644
--- a/synapse/federation/transport/server/_base.py
+++ b/synapse/federation/transport/server/_base.py
@@ -16,7 +16,8 @@ import functools
 import logging
 import re
 import time
-from typing import TYPE_CHECKING, Any, Awaitable, Callable, Optional, Tuple, cast
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Any, Awaitable, Callable, Dict, Optional, Tuple, cast
 
 from synapse.api.errors import Codes, FederationDeniedError, SynapseError
 from synapse.api.urls import FEDERATION_V1_PREFIX
@@ -86,15 +87,24 @@ class Authenticator:
 
         if not auth_headers:
             raise NoAuthenticationError(
-                401, "Missing Authorization headers", Codes.UNAUTHORIZED
+                HTTPStatus.UNAUTHORIZED,
+                "Missing Authorization headers",
+                Codes.UNAUTHORIZED,
             )
 
         for auth in auth_headers:
             if auth.startswith(b"X-Matrix"):
-                (origin, key, sig) = _parse_auth_header(auth)
+                (origin, key, sig, destination) = _parse_auth_header(auth)
                 json_request["origin"] = origin
                 json_request["signatures"].setdefault(origin, {})[key] = sig
 
+                # if the origin_server sent a destination along it needs to match our own server_name
+                if destination is not None and destination != self.server_name:
+                    raise AuthenticationError(
+                        HTTPStatus.UNAUTHORIZED,
+                        "Destination mismatch in auth header",
+                        Codes.UNAUTHORIZED,
+                    )
         if (
             self.federation_domain_whitelist is not None
             and origin not in self.federation_domain_whitelist
@@ -103,7 +113,9 @@ class Authenticator:
 
         if origin is None or not json_request["signatures"]:
             raise NoAuthenticationError(
-                401, "Missing Authorization headers", Codes.UNAUTHORIZED
+                HTTPStatus.UNAUTHORIZED,
+                "Missing Authorization headers",
+                Codes.UNAUTHORIZED,
             )
 
         await self.keyring.verify_json_for_server(
@@ -142,13 +154,14 @@ class Authenticator:
             logger.exception("Error resetting retry timings on %s", origin)
 
 
-def _parse_auth_header(header_bytes: bytes) -> Tuple[str, str, str]:
+def _parse_auth_header(header_bytes: bytes) -> Tuple[str, str, str, Optional[str]]:
     """Parse an X-Matrix auth header
 
     Args:
         header_bytes: header value
 
     Returns:
+        origin, key id, signature, destination.
         origin, key id, signature.
 
     Raises:
@@ -157,7 +170,9 @@ def _parse_auth_header(header_bytes: bytes) -> Tuple[str, str, str]:
     try:
         header_str = header_bytes.decode("utf-8")
         params = header_str.split(" ")[1].split(",")
-        param_dict = {k: v for k, v in (kv.split("=", maxsplit=1) for kv in params)}
+        param_dict: Dict[str, str] = {
+            k: v for k, v in [param.split("=", maxsplit=1) for param in params]
+        }
 
         def strip_quotes(value: str) -> str:
             if value.startswith('"'):
@@ -172,7 +187,15 @@ def _parse_auth_header(header_bytes: bytes) -> Tuple[str, str, str]:
 
         key = strip_quotes(param_dict["key"])
         sig = strip_quotes(param_dict["sig"])
-        return origin, key, sig
+
+        # get the destination server_name from the auth header if it exists
+        destination = param_dict.get("destination")
+        if destination is not None:
+            destination = strip_quotes(destination)
+        else:
+            destination = None
+
+        return origin, key, sig, destination
     except Exception as e:
         logger.warning(
             "Error parsing auth header '%s': %s",
@@ -180,7 +203,7 @@ def _parse_auth_header(header_bytes: bytes) -> Tuple[str, str, str]:
             e,
         )
         raise AuthenticationError(
-            400, "Malformed Authorization header", Codes.UNAUTHORIZED
+            HTTPStatus.BAD_REQUEST, "Malformed Authorization header", Codes.UNAUTHORIZED
         )
 
 
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 5097b3ca57..e686445955 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -704,6 +704,9 @@ class MatrixFederationHttpClient:
         Returns:
             A list of headers to be added as "Authorization:" headers
         """
+        if destination is None and destination_is is None:
+            raise ValueError("destination and destination_is cannot both be None!")
+
         request: JsonDict = {
             "method": method.decode("ascii"),
             "uri": url_bytes.decode("ascii"),
@@ -726,8 +729,13 @@ class MatrixFederationHttpClient:
         for key, sig in request["signatures"][self.server_name].items():
             auth_headers.append(
                 (
-                    'X-Matrix origin=%s,key="%s",sig="%s"'
-                    % (self.server_name, key, sig)
+                    'X-Matrix origin=%s,key="%s",sig="%s",destination="%s"'
+                    % (
+                        self.server_name,
+                        key,
+                        sig,
+                        request.get("destination") or request["destination_is"],
+                    )
                 ).encode("ascii")
             )
         return auth_headers