fix(api_server): add optional ADK_API_TOKEN Bearer auth middleware

[ghost]

Summary

Adds an opt-in Bearer-token authentication middleware to ApiServer. The middleware is a no-op when ADK_API_TOKEN is unset (current behavior preserved for every existing deployment) and requires a matching Authorization: Bearer <token> header on every non-public request when ADK_API_TOKEN is set.

/health and /version remain open so liveness probes do not need credentials.

Why

ApiServer registers the following routes without per-route authentication:

• POST /run, POST /run_sse — invoke the agent runner, which can reach in-process Python code execution via UnsafeLocalCodeExecutor.execute_code if the loaded agent wires that executor.
• POST/GET/PUT/DELETE /apps/{app_name}/users/{user_id}/sessions/{session_id} — per-user session state CRUD, where the user_id is taken from the URL with no caller-identity binding.
• All other routes registered by _register_production_endpoints and the dev-server endpoints in dev_server.py.

The only middleware in front today is CORSMiddleware + _OriginCheckMiddleware + _DefaultAppRewriteMiddleware — none of which authenticate the caller. When an operator binds the server to a network-reachable address (--host 0.0.0.0, container host, LAN demo, etc.) and the deployment has no upstream auth layer, any network-reachable caller hits those routes unauthenticated.

This PR provides the smallest opt-in mitigation that turns the existing unauth surface into authenticated endpoints without breaking any current deployment.

How

New _BearerAuthMiddleware in src/google/adk/cli/api_server.py mirrors the shape of the existing _OriginCheckMiddleware and _DefaultAppRewriteMiddleware ASGI middlewares in the same file.

Behavior matrix:

ADK_API_TOKEN env	Path	Authorization header	Result
unset	any	any	passes through (current behavior)
set	/health or /version	any	passes through
set	other	absent	401
set	other	Bearer <wrong-token>	401
set	other	Bearer <correct-token>	passes through
set	non-HTTP (websocket, lifespan)	any	passes through

The middleware is wired in ApiServer.get_fast_api_app() right after _DefaultAppRewriteMiddleware. No CLI, public-API, or config-schema changes.

Tests

tests/unittests/cli/test_bearer_auth_middleware.py covers each row of the matrix above by exercising the middleware directly through the ASGI protocol (no FastAPI test client needed). Token-unset, token-set-no-header, token-set-wrong-bearer, token-set-correct-bearer, token-set-public-paths (parameterised over /health + /version), non-HTTP scopes, and session-route guard.

Backward compatibility

ADK_API_TOKEN is unset by default, in which case the middleware is a pass-through. Every existing deployment continues to behave exactly as before. Adopting the new behavior is a one-line environment variable change at deploy time.

Reference

Filed as a patch against Google bug-tracker issue 509219988 (the reviewing team there asked for a public patch alongside the report).

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[adk-bot]

Response from ADK Triaging Agent

Hello @Cross-Trade-XT, thank you for creating this PR!

It looks like this PR is not fully following the contribution guidelines:

1. Contributor License Agreement (CLA): The CLA check has failed. Please sign the CLA at https://cla.developers.google.com/ to proceed.
2. Logs or Screenshots: For features or bug fixes, please provide logs or screenshots after the fix is applied to help reviewers understand the changes.
3. Testing Plan & E2E Evidence: Please include a dedicated testing plan section in your PR description. Since this is an API server change (Runner), please provide the testing setup (e.g., how you tested it manually), commands used, and the console output/logs showing successful requests with and without the token.

This information will help reviewers to review your PR more efficiently. Thanks!

[f4x0rz]

Closing in favor of #5981 which is opened from my CLA-linked account. Same commit content, with a Testing Plan section and live integration evidence added per @adk-bot's request. Sorry for the noise.

[ghost]

Closing in favor of #5981, opened from my CLA-linked account. Same commit content, plus a Testing Plan section and live integration evidence added per @adk-bot's request. Sorry for the noise.