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Simple OAuth2 with Password and Bearer

Now let's build from the previous chapter and add the missing parts to have a complete security flow.

Get the username and password

We are going to use FastAPI security utilities to get the username and password.

OAuth2 specifies that when using the "password flow" (that we are using) the client/user must send a username and password fields as form data.

And the spec says that the fields have to be named like that. So user-name or email wouldn't work.

But don't worry, you can show it as you wish to your final users in the frontend.

And your database models can use any other names you want.

But for the login path operation, we need to use these names to be compatible with the spec (and be able to, for example, use the integrated API documentation system).

The spec also states that the username and password must be sent as form data (so, no JSON here).

scope

The spec also says that the client can send another form field "scope".

The form field name is scope (in singular), but it is actually a long string with "scopes" separated by spaces.

Each "scope" is just a string (without spaces).

They are normally used to declare specific security permissions, for example:

  • "users:read" or "users:write" are common examples.
  • instagram_basic is used by Facebook / Instagram.
  • https://www.googleapis.com/auth/drive is used by Google.

Info

In OAuth2 a "scope" is just a string that declares a specific permission required.

It doesn't matter if it has other characters like :, or if it is a URL.

Those details are implementation specific.

For OAuth2 they are just strings.

Code to get the username and password

Now let's use the utilities provided by FastAPI to handle this.

OAuth2PasswordRequestForm

First, import OAuth2PasswordRequestForm, and use it as a dependency with Depends for the path /token:

from fastapi import Depends, FastAPI, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel
from starlette.status import HTTP_401_UNAUTHORIZED

fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "fakehashedsecret",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Wonderson",
        "email": "alice@example.com",
        "hashed_password": "fakehashedsecret2",
        "disabled": True,
    },
}

app = FastAPI()


def fake_hash_password(password: str):
    return "fakehashed" + password


oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def fake_decode_token(token):
    # This doesn't provide any security at all
    # Check the next version
    user = get_user(fake_users_db, token)
    return user


async def get_current_user(token: str = Depends(oauth2_scheme)):
    user = fake_decode_token(token)
    if not user:
        raise HTTPException(
            status_code=HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user


async def get_current_active_user(current_user: User = Depends(get_current_user)):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token")
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    user_dict = fake_users_db.get(form_data.username)
    if not user_dict:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    user = UserInDB(**user_dict)
    hashed_password = fake_hash_password(form_data.password)
    if not hashed_password == user.hashed_password:
        raise HTTPException(status_code=400, detail="Incorrect username or password")

    return {"access_token": user.username, "token_type": "bearer"}


@app.get("/users/me")
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user

OAuth2PasswordRequestForm is a class dependency that declares a form body with:

  • The username.
  • The password.
  • An optional scope field as a big string, composed of strings separated by spaces.
  • An optional grant_type.

Tip

The OAuth2 spec actually requires a field grant_type with a fixed value of password, but OAuth2PasswordRequestForm doesn't enforce it.

If you need to enforce it, use OAuth2PasswordRequestFormStrict instead of OAuth2PasswordRequestForm.

  • An optional client_id (we don't need it for our example).
  • An optional client_secret (we don't need it for our example).

Info

The OAuth2PasswordRequestForm is not a special class for FastAPI as is OAuth2PasswordBearer.

OAuth2PasswordBearer makes FastAPI know that it is a security scheme. So it is added that way to OpenAPI.

But OAuth2PasswordRequestForm is just a class dependency that you could have written yourself, or you could have declared Form parameters directly.

But as it's a common use case, it is provided by FastAPI directly, just to make it easier.

Use the form data

Tip

The instance of the dependency class OAuth2PasswordRequestForm won't have an attribute scope with the long string separated by spaces, instead, it will have a scopes attribute with the actual list of strings for each scope sent.

We are not using scopes in this example, but the functionality is there if you need it.

Now, get the user data from the (fake) database, using the username from the form field.

If there is no such user, we return an error saying "incorrect username or password".

For the error, we use the exception HTTPException:

from fastapi import Depends, FastAPI, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel
from starlette.status import HTTP_401_UNAUTHORIZED

fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "fakehashedsecret",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Wonderson",
        "email": "alice@example.com",
        "hashed_password": "fakehashedsecret2",
        "disabled": True,
    },
}

app = FastAPI()


def fake_hash_password(password: str):
    return "fakehashed" + password


oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def fake_decode_token(token):
    # This doesn't provide any security at all
    # Check the next version
    user = get_user(fake_users_db, token)
    return user


async def get_current_user(token: str = Depends(oauth2_scheme)):
    user = fake_decode_token(token)
    if not user:
        raise HTTPException(
            status_code=HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user


async def get_current_active_user(current_user: User = Depends(get_current_user)):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token")
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    user_dict = fake_users_db.get(form_data.username)
    if not user_dict:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    user = UserInDB(**user_dict)
    hashed_password = fake_hash_password(form_data.password)
    if not hashed_password == user.hashed_password:
        raise HTTPException(status_code=400, detail="Incorrect username or password")

    return {"access_token": user.username, "token_type": "bearer"}


@app.get("/users/me")
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user

Check the password

At this point we have the user data from our database, but we haven't checked the password.

Let's put that data in the Pydantic UserInDB model first.

You should never save plaintext passwords, so, we'll use the (fake) password hashing system.

If the passwords don't match, we return the same error.

Password hashing

"Hashing" means: converting some content (a password in this case) into a sequence of bytes (just a string) that looks like gibberish.

Whenever you pass exactly the same content (exactly the same password) you get exactly the same gibberish.

But you cannot convert from the gibberish back to the password.

What for?

If your database is stolen, the thief won't have your users' plaintext passwords, only the hashes.

So, the thief won't be able to try to use that password in another system (as many users use the same password everywhere, this would be dangerous).

from fastapi import Depends, FastAPI, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel
from starlette.status import HTTP_401_UNAUTHORIZED

fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "fakehashedsecret",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Wonderson",
        "email": "alice@example.com",
        "hashed_password": "fakehashedsecret2",
        "disabled": True,
    },
}

app = FastAPI()


def fake_hash_password(password: str):
    return "fakehashed" + password


oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def fake_decode_token(token):
    # This doesn't provide any security at all
    # Check the next version
    user = get_user(fake_users_db, token)
    return user


async def get_current_user(token: str = Depends(oauth2_scheme)):
    user = fake_decode_token(token)
    if not user:
        raise HTTPException(
            status_code=HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user


async def get_current_active_user(current_user: User = Depends(get_current_user)):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token")
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    user_dict = fake_users_db.get(form_data.username)
    if not user_dict:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    user = UserInDB(**user_dict)
    hashed_password = fake_hash_password(form_data.password)
    if not hashed_password == user.hashed_password:
        raise HTTPException(status_code=400, detail="Incorrect username or password")

    return {"access_token": user.username, "token_type": "bearer"}


@app.get("/users/me")
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user

About **user_dict

UserInDB(**user_dict) means:

Pass the keys and values of the user_dict directly as key-value arguments, equivalent to:

UserInDB(
    username = user_dict["username"],
    email = user_dict["email"],
    full_name = user_dict["full_name"],
    disabled = user_dict["disabled"],
    hashed_password = user_dict["hashed_password"],
)

Info

For a more complete explanation of **user_dict check back in the documentation for Extra Models.

Return the token

The response of the token endpoint must be a JSON object.

It should have a token_type. In our case, as we are using "Bearer" tokens, the token type should be "bearer".

And it should have an access_token, with a string containing our access token.

For this simple example, we are going to just be completely insecure and return the same username as the token.

Tip

In the next chapter, you will see a real secure implementation, with password hashing and JWT tokens.

But for now, let's focus on the specific details we need.

from fastapi import Depends, FastAPI, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel
from starlette.status import HTTP_401_UNAUTHORIZED

fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "fakehashedsecret",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Wonderson",
        "email": "alice@example.com",
        "hashed_password": "fakehashedsecret2",
        "disabled": True,
    },
}

app = FastAPI()


def fake_hash_password(password: str):
    return "fakehashed" + password


oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def fake_decode_token(token):
    # This doesn't provide any security at all
    # Check the next version
    user = get_user(fake_users_db, token)
    return user


async def get_current_user(token: str = Depends(oauth2_scheme)):
    user = fake_decode_token(token)
    if not user:
        raise HTTPException(
            status_code=HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user


async def get_current_active_user(current_user: User = Depends(get_current_user)):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token")
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    user_dict = fake_users_db.get(form_data.username)
    if not user_dict:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    user = UserInDB(**user_dict)
    hashed_password = fake_hash_password(form_data.password)
    if not hashed_password == user.hashed_password:
        raise HTTPException(status_code=400, detail="Incorrect username or password")

    return {"access_token": user.username, "token_type": "bearer"}


@app.get("/users/me")
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user

Tip

By the spec, you should return a JSON with an access_token and a token_type, the same as in this example.

This is something that you have to do yourself in your code, and make sure you use those JSON keys.

It's almost the only thing that you have to remember to do correctly yourself, to be compliant with the specifications.

For the rest, FastAPI handles it for you.

Update the dependencies

Now we are going to update our dependencies.

We want to get the current_user only if this user is active.

So, we create an additional dependency get_current_active_user that in turn uses get_current_user as a dependency.

Both of these dependencies will just return an HTTP error if the user doesn't exists, or if is inactive.

So, in our endpoint, we will only get a user if the user exists, was correctly authenticated, and is active:

from fastapi import Depends, FastAPI, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel
from starlette.status import HTTP_401_UNAUTHORIZED

fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "fakehashedsecret",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Wonderson",
        "email": "alice@example.com",
        "hashed_password": "fakehashedsecret2",
        "disabled": True,
    },
}

app = FastAPI()


def fake_hash_password(password: str):
    return "fakehashed" + password


oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def fake_decode_token(token):
    # This doesn't provide any security at all
    # Check the next version
    user = get_user(fake_users_db, token)
    return user


async def get_current_user(token: str = Depends(oauth2_scheme)):
    user = fake_decode_token(token)
    if not user:
        raise HTTPException(
            status_code=HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user


async def get_current_active_user(current_user: User = Depends(get_current_user)):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token")
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    user_dict = fake_users_db.get(form_data.username)
    if not user_dict:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    user = UserInDB(**user_dict)
    hashed_password = fake_hash_password(form_data.password)
    if not hashed_password == user.hashed_password:
        raise HTTPException(status_code=400, detail="Incorrect username or password")

    return {"access_token": user.username, "token_type": "bearer"}


@app.get("/users/me")
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user

Info

The additional header WWW-Authenticate with value Bearer we are returning here is also part of the spec.

Any HTTP (error) status code 401 "UNAUTHORIZED" is supposed to also return a WWW-Authenticate header.

In the case of bearer tokens (our case), the value of that header should be Bearer.

You can actually skip that extra header and it would still work.

But it's provided here to be compliant with the specifications.

Also, there might be tools that expect and use it (now or in the future) and that might be useful for you or your users, now or in the future.

That's the benefit of standards...

See it in action

Open the interactive docs: http://127.0.0.1:8000/docs.

Authenticate

Click the "Authorize" button.

Use the credentials:

User: johndoe

Password: secret

After authenticating in the system, you will see it like:

Get your own user data

Now use the operation GET with the path /users/me.

You will get your user's data, like:

{
  "username": "johndoe",
  "email": "johndoe@example.com",
  "full_name": "John Doe",
  "disabled": false,
  "hashed_password": "fakehashedsecret"
}

If you click the lock icon and logout, and then try the same operation again, you will get an HTTP 401 error of:

{
  "detail": "Not authenticated"
}

Inactive user

Now try with an inactive user, authenticate with:

User: alice

Password: secret2

And try to use the operation GET with the path /users/me.

You will get an "inactive user" error, like:

{
  "detail": "Inactive user"
}

Recap

You now have the tools to implement a complete security system based on username and password for your API.

Using these tools, you can make the security system compatible with any database and with any user or data model.

The only detail missing is that it is not actually "secure" yet.

In the next chapter you'll see how to use a secure password hashing library and JWT tokens.