Implement Starlette permissions per endpoint

[szelenka]

Is your feature request related to a problem? Please describe.
I want to be able to specify different permission levels on endpoints but I can't because FastAPI is not compatible with the default Starlette Authentication starlette.authentication.requires wrapper.

Describe the solution you'd like
The default Starlette starlette.authentication.requires looks for the request keyword in the argument list of the endpoint, but the @route wrappers of FastAPI strip this out. It would be great to have a keyword in the @route wrappers to allow a user to specify the list of strings (i.e. Permissions) following the original Starlette documentation. Something like:

from fastapi import Depends, FastAPI

@app = FastAPI()

@app.get(
    path='/',
    requires=['secret_permission']
)
def my_endpoint(current_user: User = Security(custom_auth_handler)) -> str:
    return f'{current_user} is authorized!'

When a given user does not have the specified permission, a 401 should be raised. Additionally, if the user is not authenticated, it should redirect to the login endpoint before throwing a 401.

Describe alternatives you've considered
When using FastAPI, manually specifying the request: starlette.requests.Request in the endpoint as a required input parameter in addition to the starlette.authentication.requires wrapper from Starlette results in an IndexError exception (because FastAPI is messing with the order of arguments when it attempts to parse out the other parameters). The only other solution is to strip out components of FastAPI for pydantic parsing and use the default Starlette library to have access to permissions per endpoint.

Additional context
Working sample in Starlette can be found in their documentation here.

[tiangolo]

Hmm, the authentication mechanisms included in Starlette are a little bit more fixed.

For example, it is assumed that there is a user entity with two fixed attributes:

• .is_authenticated
• .display_name

---

In FastAPI, to define different permission levels you would probably use the dependency injection system (and the security system): https://fastapi.tiangolo.com/tutorial/security/get-current-user/

With that, you can do any check you need in a dependency and handle permissions directly, you don't have to communicate the approval/disapproval using a list of strings. And it also would apply for authenticating things that are not necessarily users, like third party applications.

For a more complete example. for example, requiring that a there should be a user logged in, that the user should be active and that it should be a "superuser", see the project generators: https://fastapi.tiangolo.com/project-generation/

For example, this dependency in this line requires exactly that I just described: https://github.com/tiangolo/full-stack-fastapi-postgresql/blob/master/%7B%7Bcookiecutter.project_slug%7D%7D/backend/app/app/api/api_v1/endpoints/user.py#L38

Using the same mechanism, you could restrict with way more complex logic. For example, "only a user that has more than 100 items can access this endpoint".

[szelenka]

Thanks @tiangolo

I've managed to get a functional prototype using something similar to what you mentioned:

from typing import Set, List
import uuid
from pydantic import BaseModel, Schema
from starlette.requests import Request
from starlette.exceptions import HTTPException
from starlette.status import HTTP_401_UNAUTHORIZED, HTTP_403_FORBIDDEN
from fastapi import FastAPI, Depends, Header


# define User object
class User(BaseModel):
    id: uuid.UUID = Schema(
        default=...,
        description='Identifier for this User'
    )
    username: str = Schema(
        default=...,
        description='Username for this User'
    )
    roles: Set[str] = Schema(
        default=set(),
        description='Roles assigned to this User'
    )


# mock repository lookup
_db = dict(
    validUser=User(
        id=uuid.uuid4(),
        username='validUser',
        roles=set(['valid_role'])
    ),
    badUser=User(
        id=uuid.uuid4(),
        username='badUser',
        roles=set()
    )
)


# define header authentication
class HTTPHeaderAuthentication:
    def __init__(self, *, scopes: List[str]):
        self.scopes = set(scopes)

    async def __call__(self, request: Request, authuser: str = Header(None)) -> User:
        user = self.locate_user(username=authuser)
        if not user:
            raise HTTPException(
                status_code=HTTP_403_FORBIDDEN, detail="Not authenticated"
            )
        if not self.has_required_scope(user.roles):
            raise HTTPException(
                status_code=HTTP_401_UNAUTHORIZED, detail=f"{user.username} is not authorized to access this endpoint"
            )
        return user

    def locate_user(self, username: str) -> User:
        """Mock lookup in repository"""
        return _db.get(username, None)

    def has_required_scope(self, user_scopes: Set[str]) -> bool:
        """Verify the user has the desired auth scope"""
        for scope in self.scopes:
            if scope not in user_scopes:
                return False
        return True


# create FastAPI object
app = FastAPI()


# setup protected endpoint
@app.get(
    path='/'
)
def root(current_user: User = Depends(HTTPHeaderAuthentication(scopes=['valid_role']))) -> User:
    return current_user


if __name__ == '__main__':
    import uvicorn
    app.debug = True
    uvicorn.run(app, host="127.0.0.1", port=5000)

validUser has the required scope roles to access the endpoint, while badUser does not. Though it's not as elegant as the Starlette AuthenticationMiddleware.

Do you have any suggestion on how to write custom authentication handlers in FastAPI?

[tiangolo]

Cool!

---

Yes, I have suggestions, you don't have to create a full HTTPHeaderAuthentication class.

If you want to use something pre-defined, you can import and use the OAuth2 classes that do more or less the same things, but based on a standard: https://fastapi.tiangolo.com/tutorial/security/get-current-user/ ...and then you can also say your API is using a fancy security/authentication standard (OAuth2), the same used by Google, Facebook, etc. 😉

About having different requirements per path operation, check the project generators, for example, there are:

• get_current_user: get's the current user as authenticated by OAuth2 access token (using JWT)
• get_current_active_user: the same as above, but checks that the user is active
• get_current_active_superuser: the same as above, but ensures the user is a superuser.

You could do the same to ensure some specific roles. In fact, that's how it's done in the Couchbase project generator: https://github.com/tiangolo/full-stack-fastapi-couchbase

[tiangolo]

As I understand you were able to solve your problem I'll close this issue now. But feel free to add more comments or create new issues.

[Midnighter]

I just got bitten by the incompatibility, too. Implemented a nice authorization middleware and then realized it doesn't work. Would it be possible to add a section to the documentation that lists which features of Starlette FastAPI is not compatible with?

[tiangolo]

Just to clarify, you can use @app.route exactly the same as with Starlette. In fact, it's the same. FastAPI doesn't modify .route(). It only adds (additional) methods .get(), .post(), etc.

@Midnighter you should be able to use all the parts of the middleware exactly the same, even with .get(), .post(), etc. except for the decorator @requires, as it expects to alter the response that is passed to Starlette, but .get() is already a layer on top. But you should be able to use the same "Authentication middleware", "user handling", and "custom authentication error responses".

If you have something that expects to work with Starlette's @requires, you can modify it to be a dependency with Security, it can also receive scopes. And then it can check them using all the same tooling from Starlette. As a bonus, your app becomes OAuth 2.0 compatible (or at least has the base to do it).

If you have a specific case that you need to solve with @requires, please open a new issue, so that we can think about a solution, possibly adding docs explaining how to achieve the equivalent in FastAPI.

[Midnighter]

My apologies, I wasn't clear. I indeed meant the requires decorator which then lead me to the same IndexError as described above.

If you have a specific case that you need to solve with @requires, please open a new issue, so that we can think about a solution, possibly adding docs explaining how to achieve the equivalent in FastAPI.

I have already re-written it to use dependency injection and it works quite beautifully. I just wanted to suggest documenting the little parts that don't work well together such as requires. I haven't come across anything else but maybe other cases exist.

[tiangolo]

Thanks @Midnighter . I'm glad it's working well for you now.

I'm not sure what else might not work right away (I don't think there's much more). This is mainly becase Starlette also moves quite fast, for example, when FastAPI was created, there was not authentication middleware in Starlette...

Maybe something we could do is to document how to create a Security dependency that uses the rest of the authentication parts from Starlette and is equivalent to using @requires.

That's something that could also go in a blog post 😉

[apocalypse32]

I'm also facing same issue, I partially solved it with security but I think middleware approach is more elegant way to fix it.

My use case:

I'm using istio authentication. My token is already verified before it reaches to the application layer. In application layer all I need to check if authorization logic using scope.

Expected flow:

1. Create a custom starlette.authentication.AuthenticationBackend to use backend for starlette.middleware.authentication.AuthenticationMiddleware

app.add_middleware(AuthenticationMiddleware, backend=CustomAuthenticationBackend())

Which basically add a user(type of starlette.authentication.BaseUser) and auth object(starlette.authentication.AuthCredentials)

2. I'd like to define scopes to protect my path just as same as requires from starlette @requires('authenticated')

Implementing another get user or scope checking with Security function is unnecessary at this point. As user is already inside request and writing another function to validate scope manually is unnecessary.

2nd problem, When I'm using Security, openapi docs not reflecting the authentication mechanism in the UI

[apocalypse32]

@tiangolo Is this feature added to fastapi already? Since this issue was created more than a year ago. Not sure if a should create new issue!

Can you take a look at my comment please, if it makes sense and you have any suggestion?

[tiangolo]

@apocalypse32 I would suggest you use the integrated tools in FastAPI, that would work better: https://fastapi.tiangolo.com/tutorial/security/get-current-user/