Add OIDC config to add extra parameters to the authorize URL (#16971)

changelog.d/16971.feature

@@ -0,0 +1 @@
+Add an OIDC config to specify extra parameters for the authorization grant URL. IT can be useful to pass an ACR value for example.

docs/usage/configuration/config_documentation.md

@@ -3349,6 +3349,9 @@ Options for each entry include:
    not included in `scopes`. Set to `userinfo_endpoint` to always use the
    userinfo endpoint.
 
+* `additional_authorization_parameters`: String to string dictionary that will be passed as
+   additional parameters to the authorization grant URL.
+
 * `allow_existing_users`: set to true to allow a user logging in via OIDC to
    match a pre-existing account instead of failing. This could be used if
    switching from password logins to OIDC. Defaults to false.
@@ -3473,6 +3476,8 @@ oidc_providers:
     token_endpoint: "https://accounts.example.com/oauth2/token"
     userinfo_endpoint: "https://accounts.example.com/userinfo"
     jwks_uri: "https://accounts.example.com/.well-known/jwks.json"
+    additional_authorization_parameters:
+      acr_values: 2fa
     skip_verification: true
     enable_registration: true
     user_mapping_provider:

synapse/config/oidc.py

@@ -342,6 +342,9 @@ def _parse_oidc_config_dict(
         user_mapping_provider_config=user_mapping_provider_config,
         attribute_requirements=attribute_requirements,
         enable_registration=oidc_config.get("enable_registration", True),
+        additional_authorization_parameters=oidc_config.get(
+            "additional_authorization_parameters", {}
+        ),
     )
 
 
@@ -444,3 +447,6 @@ class OidcProviderConfig:
 
     # Whether automatic registrations are enabled in the ODIC flow. Defaults to True
     enable_registration: bool
+
+    # Additional parameters that will be passed to the authorization grant URL
+    additional_authorization_parameters: Mapping[str, str]

synapse/handlers/oidc.py

@@ -453,6 +453,10 @@ def __init__(
         # optional brand identifier for this auth provider
         self.idp_brand = provider.idp_brand
 
+        self.additional_authorization_parameters = (
+            provider.additional_authorization_parameters
+        )
+
         self._sso_handler = hs.get_sso_handler()
         self._device_handler = hs.get_device_handler()
 
@@ -1006,17 +1010,21 @@ async def handle_redirect_request(
 
         metadata = await self.load_metadata()
 
+        additional_authorization_parameters = dict(
+            self.additional_authorization_parameters
+        )
         # Automatically enable PKCE if it is supported.
-        extra_grant_values = {}
         if metadata.get("code_challenge_methods_supported"):
             code_verifier = generate_token(48)
 
             # Note that we verified the server supports S256 earlier (in
             # OidcProvider._validate_metadata).
-            extra_grant_values = {
-                "code_challenge_method": "S256",
-                "code_challenge": create_s256_code_challenge(code_verifier),
-            }
+            additional_authorization_parameters.update(
+                {
+                    "code_challenge_method": "S256",
+                    "code_challenge": create_s256_code_challenge(code_verifier),
+                }
+            )
 
         cookie = self._macaroon_generaton.generate_oidc_session_token(
             state=state,
@@ -1055,7 +1063,7 @@ async def handle_redirect_request(
             scope=self._scopes,
             state=state,
             nonce=nonce,
-            **extra_grant_values,
+            **additional_authorization_parameters,
         )
 
     async def handle_oidc_callback(