Don't check the `at_hash` (access token hash) in OIDC ID Tokens if we don't use the access token (#18374)

Co-authored-by: Eric Eastwood <erice@element.io>

1 files changed, 27 insertions, 2 deletions

diff --git a/synapse/handlers/oidc.py b/synapse/handlers/oidc.py

index c4cf0636a3..fb759172b3 100644
--- a/synapse/handlers/oidc.py
+++ b/synapse/handlers/oidc.py
@@ -587,6 +587,24 @@ class OidcProvider:
         )
 
     @property
+    def _uses_access_token(self) -> bool:
+        """Return True if the `access_token` will be used during the login process.
+
+        This is useful to determine whether the access token
+        returned by the identity provider, and
+        any related metadata (such as the `at_hash` field in
+        the ID token), should be validated.
+        """
+        # Currently, Synapse only uses the access_token to fetch user metadata
+        # from the userinfo endpoint. Therefore we only have a single criteria
+        # to check right now but this may change in the future and this function
+        # should be updated if more usages are introduced.
+        # 
+        # For example, if we start to use the access_token given to us by the
+        # IdP for more things, such as accessing Resource Server APIs.
+        return self._uses_userinfo
+
+    @property
     def issuer(self) -> str:
         """The issuer identifying this provider."""
         return self._config.issuer
@@ -957,9 +975,16 @@ class OidcProvider:
             "nonce": nonce,
             "client_id": self._client_auth.client_id,
         }
-        if "access_token" in token:
+        if self._uses_access_token and "access_token" in token:
             # If we got an `access_token`, there should be an `at_hash` claim
-            # in the `id_token` that we can check against.
+            # in the `id_token` that we can check against. Setting this
+            # instructs authlib to check the value of `at_hash` in the
+            # ID token.
+            #
+            # We only need to verify the access token if we actually make
+            # use of it. Which currently only happens when we need to fetch
+            # the user's information from the userinfo_endpoint. Thus, this
+            # check is also gated on self._uses_userinfo.
             claims_params["access_token"] = token["access_token"]
 
         claims_options = {"iss": {"values": [metadata["issuer"]]}}