fix(auth): disambiguate auth-denied vs missing component messages (#4165)

fastmcp_slim/fastmcp/server/middleware/authorization.py

@@ -136,11 +136,15 @@ class AuthMiddleware(Middleware):
                 f"Authorization failed for tool '{tool_name}': missing context"
             )
 
-        # Get tool (component auth is checked in get_tool, raises if unauthorized)
+        # get_tool returns None both when the tool does not exist and when
+        # component-level auth denied access, so the two cases are
+        # indistinguishable here. Keep the message ambiguous to avoid
+        # disclosing existence of tools the caller is not authorized to see.
         tool = await fastmcp.fastmcp.get_tool(tool_name)
         if tool is None:
             raise AuthorizationError(
-                f"Authorization failed for tool '{tool_name}': tool not found"
+                f"Authorization failed for tool '{tool_name}': "
+                "not found or not authorized"
             )
 
         # Global auth check
@@ -204,13 +208,17 @@ class AuthMiddleware(Middleware):
                 f"Authorization failed for resource '{uri}': missing context"
             )
 
-        # Get resource/template (component auth is checked in get_*, raises if unauthorized)
+        # get_resource/get_resource_template return None both when the resource
+        # does not exist and when component-level auth denied access, so the two
+        # cases are indistinguishable here. Keep the message ambiguous to avoid
+        # disclosing existence of resources the caller is not authorized to see.
         component = await fastmcp.fastmcp.get_resource(str(uri))
         if component is None:
             component = await fastmcp.fastmcp.get_resource_template(str(uri))
         if component is None:
             raise AuthorizationError(
-                f"Authorization failed for resource '{uri}': resource not found"
+                f"Authorization failed for resource '{uri}': "
+                "not found or not authorized"
             )
 
         # Global auth check
@@ -303,11 +311,15 @@ class AuthMiddleware(Middleware):
                 f"Authorization failed for prompt '{prompt_name}': missing context"
             )
 
-        # Get prompt (component auth is checked in get_prompt, raises if unauthorized)
+        # get_prompt returns None both when the prompt does not exist and when
+        # component-level auth denied access, so the two cases are
+        # indistinguishable here. Keep the message ambiguous to avoid
+        # disclosing existence of prompts the caller is not authorized to see.
         prompt = await fastmcp.fastmcp.get_prompt(prompt_name)
         if prompt is None:
             raise AuthorizationError(
-                f"Authorization failed for prompt '{prompt_name}': prompt not found"
+                f"Authorization failed for prompt '{prompt_name}': "
+                "not found or not authorized"
             )
 
         # Global auth check

tests/server/auth/test_authorization.py

@@ -810,3 +810,77 @@ class TestAuthMiddlewareCallTool:
                 )
         finally:
             auth_context_var.reset(tok)
+
+
+# =============================================================================
+# Tests for component-level auth denial messaging (issue #4054 bug 1)
+# =============================================================================
+
+
+def _allow_all(ctx: AuthContext) -> bool:
+    """Global auth check that always passes (component-level auth still applies)."""
+    return True
+
+
+class TestComponentAuthDenialMessage:
+    """When component-level auth denies access, get_tool/get_resource/get_prompt
+    return None, so the middleware cannot distinguish "missing" from "denied".
+
+    The message must stay ambiguous ("not found or not authorized") rather than
+    asserting the component does not exist (misleading) or that it exists but is
+    forbidden (leaks existence to unauthorized callers).
+    """
+
+    async def test_call_tool_denied_by_component_auth(self):
+        mcp = FastMCP(middleware=[AuthMiddleware(auth=_allow_all)])
+
+        @mcp.tool(auth=require_scopes("admin"))
+        def secret_tool() -> str:
+            return "secret"
+
+        token = make_token(scopes=["read"])
+        tok = set_token(token)
+        try:
+            async with Client(mcp) as client:
+                with pytest.raises(Exception) as exc_info:
+                    await client.call_tool("secret_tool", {})
+            message = str(exc_info.value)
+            assert "not found or not authorized" in message
+        finally:
+            auth_context_var.reset(tok)
+
+    async def test_read_resource_denied_by_component_auth(self):
+        mcp = FastMCP(middleware=[AuthMiddleware(auth=_allow_all)])
+
+        @mcp.resource("data://secret", auth=require_scopes("admin"))
+        def secret_resource() -> str:
+            return "secret"
+
+        token = make_token(scopes=["read"])
+        tok = set_token(token)
+        try:
+            async with Client(mcp) as client:
+                with pytest.raises(Exception) as exc_info:
+                    await client.read_resource("data://secret")
+            message = str(exc_info.value)
+            assert "not found or not authorized" in message
+        finally:
+            auth_context_var.reset(tok)
+
+    async def test_get_prompt_denied_by_component_auth(self):
+        mcp = FastMCP(middleware=[AuthMiddleware(auth=_allow_all)])
+
+        @mcp.prompt(auth=require_scopes("admin"))
+        def secret_prompt() -> str:
+            return "secret"
+
+        token = make_token(scopes=["read"])
+        tok = set_token(token)
+        try:
+            async with Client(mcp) as client:
+                with pytest.raises(Exception) as exc_info:
+                    await client.get_prompt("secret_prompt")
+            message = str(exc_info.value)
+            assert "not found or not authorized" in message
+        finally:
+            auth_context_var.reset(tok)