Remove form-action from default consent CSP, forward consent_csp_policy in all providers

Drop form-action from the default Content Security Policy on the OAuth
consent page. Chrome enforces form-action across the entire redirect
chain, which breaks flows where an HTTPS callback internally redirects
to a custom scheme (e.g. claude://, cursor://). Since the form posts
to itself and all redirects are server-controlled, form-action adds
no security value here.

Also forward the consent_csp_policy parameter through all concrete
OAuth providers (Auth0, Azure, Google, GitHub, Discord, WorkOS, AWS
Cognito, OCI) so users can override the CSP without accessing private
attributes.

src/fastmcp/server/auth/oauth_proxy/ui.py

@@ -5,8 +5,6 @@ This module contains HTML generation functions for consent and error pages.
 
 from __future__ import annotations
 
-from urllib.parse import urlparse
-
 from fastmcp.utilities.ui import (
     BUTTON_STYLES,
     DETAIL_BOX_STYLES,
@@ -198,20 +196,13 @@ def create_consent_html(
     # If csp_policy is empty string, CSP will be disabled entirely in create_page
     # If csp_policy is a non-empty string, use it as-is
     if csp_policy is None:
-        # Need to allow form-action for form submission
-        # Chrome requires explicit scheme declarations in CSP form-action when redirect chains
-        # end in custom protocol schemes (e.g., cursor://). Parse redirect_uri to include its scheme.
-        parsed_redirect = urlparse(redirect_uri)
-        redirect_scheme = parsed_redirect.scheme.lower()
-
-        # Build form-action directive with standard schemes plus custom protocol if present
-        form_action_schemes = ["https:", "http:"]
-        if redirect_scheme and redirect_scheme not in ("http", "https"):
-            # Custom protocol scheme (e.g., cursor:, vscode:, etc.)
-            form_action_schemes.append(f"{redirect_scheme}:")
-
-        form_action_directive = " ".join(form_action_schemes)
-        csp_policy = f"default-src 'none'; style-src 'unsafe-inline'; img-src https: data:; base-uri 'none'; form-action {form_action_directive}"
+        # The consent form posts to itself (action="") and all subsequent redirects
+        # are server-controlled. Chrome enforces form-action across the entire redirect
+        # chain (Chromium issue #40923007), which breaks flows where an HTTPS callback
+        # internally redirects to a custom scheme (e.g., claude:// or cursor://).
+        # Since the form target is same-origin and we control the redirect chain,
+        # omitting form-action is safe and avoids these browser-specific CSP issues.
+        csp_policy = "default-src 'none'; style-src 'unsafe-inline'; img-src https: data:; base-uri 'none'"
 
     return create_page(
         content=content,

src/fastmcp/server/auth/providers/auth0.py

@@ -70,6 +70,7 @@ class Auth0Provider(OIDCProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
     ) -> None:
         """Initialize Auth0 OAuth provider.
 
@@ -114,6 +115,7 @@ class Auth0Provider(OIDCProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
         )
 
         logger.debug(

src/fastmcp/server/auth/providers/aws.py

@@ -109,6 +109,7 @@ class AWSCognitoProvider(OIDCProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
     ):
         """Initialize AWS Cognito OAuth provider.
 
@@ -161,6 +162,7 @@ class AWSCognitoProvider(OIDCProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
         )
 
         logger.debug(

src/fastmcp/server/auth/providers/azure.py

@@ -108,6 +108,7 @@ class AzureProvider(OAuthProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
         base_authority: str = "login.microsoftonline.com",
         http_client: httpx.AsyncClient | None = None,
     ) -> None:
@@ -233,6 +234,7 @@ class AzureProvider(OAuthProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
             valid_scopes=parsed_required_scopes,
         )
 

src/fastmcp/server/auth/providers/discord.py

@@ -193,6 +193,7 @@ class DiscordProvider(OAuthProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
         http_client: httpx.AsyncClient | None = None,
     ):
         """Initialize Discord OAuth provider.
@@ -253,6 +254,7 @@ class DiscordProvider(OAuthProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
         )
 
         logger.debug(

src/fastmcp/server/auth/providers/github.py

@@ -192,6 +192,7 @@ class GitHubProvider(OAuthProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
         http_client: httpx.AsyncClient | None = None,
     ):
         """Initialize GitHub OAuth provider.
@@ -247,6 +248,7 @@ class GitHubProvider(OAuthProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
         )
 
         logger.debug(

src/fastmcp/server/auth/providers/google.py

@@ -207,6 +207,7 @@ class GoogleProvider(OAuthProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
         extra_authorize_params: dict[str, str] | None = None,
         http_client: httpx.AsyncClient | None = None,
     ):
@@ -283,6 +284,7 @@ class GoogleProvider(OAuthProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
             extra_authorize_params=extra_authorize_params_final,
         )
 

src/fastmcp/server/auth/providers/oci.py

@@ -129,6 +129,7 @@ class OCIProvider(OIDCProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
     ) -> None:
         """Initialize OCI OIDC provider.
 
@@ -161,6 +162,7 @@ class OCIProvider(OIDCProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
         )
 
         logger.debug(

src/fastmcp/server/auth/providers/workos.py

@@ -157,6 +157,7 @@ class WorkOSProvider(OAuthProxy):
         client_storage: AsyncKeyValue | None = None,
         jwt_signing_key: str | bytes | None = None,
         require_authorization_consent: bool = True,
+        consent_csp_policy: str | None = None,
         http_client: httpx.AsyncClient | None = None,
     ):
         """Initialize WorkOS OAuth provider.
@@ -218,6 +219,7 @@ class WorkOSProvider(OAuthProxy):
             client_storage=client_storage,
             jwt_signing_key=jwt_signing_key,
             require_authorization_consent=require_authorization_consent,
+            consent_csp_policy=consent_csp_policy,
         )
 
         logger.debug(

tests/server/auth/test_oauth_consent_page.py

@@ -289,8 +289,8 @@ class TestConsentPageServerIcon:
 class TestConsentCSPPolicy:
     """Tests for Content Security Policy customization on consent page."""
 
-    async def test_default_csp_includes_form_action(self):
-        """Test that default CSP includes form-action directive."""
+    async def test_default_csp_omits_form_action(self):
+        """Test that default CSP omits form-action to avoid Chrome redirect chain issues."""
 
         verifier = Mock(spec=TokenVerifier)
         verifier.required_scopes = ["read"]
@@ -335,9 +335,9 @@ class TestConsentCSPPolicy:
             response = client.get(f"/consent?txn_id={txn_id}")
 
             assert response.status_code == 200
-            # Default CSP should be present with form-action
+            # Default CSP should be present but WITHOUT form-action
             assert 'http-equiv="Content-Security-Policy"' in response.text
-            assert "form-action" in response.text
+            assert "form-action" not in response.text
 
     async def test_empty_csp_disables_csp_meta_tag(self):
         """Test that empty string CSP disables CSP meta tag entirely."""