Chapter 7: Security Utilities

Hi there! 👋 In Chapter 6: Error Handling, we learned how to handle situations where things go wrong in our API, like when a user requests an item that doesn’t exist. Now, let’s talk about protecting our API endpoints.

Imagine our online store API. Anyone should be able to browse items (GET /items/). But maybe only registered, logged-in users should be allowed to create new items (POST /items/) or view their own profile (GET /users/me). How do we ensure only the right people can access certain parts of our API?

That’s where Security Utilities come in!

Our Goal Today: Learn how FastAPI provides ready-made tools to implement common security mechanisms like username/password checks or API keys, making it easy to protect your endpoints.

What Problem Does This Solve?

When you build an API, some parts might be public, but others need protection. You need a way to:

1. Identify the User: Figure out who is making the request. Are they logged in? Do they have a valid API key? This process is called Authentication (AuthN - proving who you are).
2. Check Permissions (Optional but related): Once you know who the user is, you might need to check if they have permission to do what they’re asking. Can user “Alice” delete user “Bob”? This is called Authorization (AuthZ - checking what you’re allowed to do). (We’ll focus mainly on Authentication in this beginner chapter).
3. Ask for Credentials: How does the user provide their identity? Common ways include:
   • HTTP Basic Authentication: Sending a username and password directly (encoded) in the request headers. Simple, but less secure over plain HTTP.
   • API Keys: Sending a secret key (a long string) in the headers, query parameters, or cookies. Common for server-to-server communication.
   • OAuth2 Bearer Tokens: Sending a temporary token (obtained after logging in) in the headers. Very common for web and mobile apps.
4. Document Security: How do you tell users of your API (in the /docs) that certain endpoints require authentication and how to provide it?

Implementing these security schemes from scratch can be complex and tricky. FastAPI gives you pre-built components (like different types of locks and keys) that handle the common patterns for asking for and receiving credentials.

Key Concepts

1. Security Schemes: These are the standard protocols or methods used for authentication, like HTTP Basic, API Keys (in different locations), and OAuth2. FastAPI provides classes that represent these schemes (e.g., HTTPBasic, APIKeyHeader, OAuth2PasswordBearer). Think of these as the type of lock mechanism you want to install on your door.

2. fastapi.security Module: This module contains all the pre-built security scheme classes. You’ll import things like HTTPBasic, APIKeyHeader, APIKeyQuery, APIKeyCookie, OAuth2PasswordBearer from here.

3. Credentials: The actual “secret” information the user provides to prove their identity (username/password, the API key string, the OAuth2 token string).

4. Verifier Dependency: A function you write (a dependency, like we learned about in Chapter 5: Dependency Injection) that takes the credentials extracted by the security scheme and checks if they are valid. It might check a username/password against a database or validate an API key. This function decides if the “key” fits the “lock”.

5. Security() Function: This is a special function imported from fastapi (from fastapi import Security). It works almost exactly like Depends(), but it’s specifically designed for security dependencies. You use it like this: user: Annotated[UserType, Security(your_verifier_dependency)].

   • Main Difference from Depends(): Using Security() tells FastAPI to automatically add the corresponding security requirements to your OpenAPI documentation (/docs). This means /docs will show a little lock icon on protected endpoints and provide UI elements for users to enter their credentials (like username/password or a token) when trying out the API.

Analogy:

• Security Scheme (HTTPBasic, APIKeyHeader): The type of lock on the door (e.g., a key lock, a combination lock).
• Scheme Instance (security = HTTPBasic()): Installing that specific lock on a particular door frame.
• Credentials (username/password, API key): The key or combination provided by the person trying to open the door.
• Verifier Dependency (get_current_user): The person or mechanism that takes the key/combination, checks if it’s correct, and decides whether to let the person in.
• Security(get_current_user): Declaring that the door requires the verifier to check the key/combination before allowing entry, and also putting a “Lock” sign on the door in the building map (/docs).

Using Security Utilities: HTTP Basic Auth Example

Let’s protect an endpoint using the simplest method: HTTP Basic Authentication. We’ll create an endpoint /users/me that requires a valid username and password.

Step 1: Import necessary tools

We need HTTPBasic (the scheme), HTTPBasicCredentials (a Pydantic model to hold the extracted username/password), Security (to declare the dependency), Annotated, and HTTPException (for errors).

# main.py (or your router file)
from typing import Annotated

from fastapi import Depends, FastAPI, HTTPException, status
from fastapi.security import HTTPBasic, HTTPBasicCredentials

Step 2: Create an instance of the security scheme

We create an instance of HTTPBasic. This object knows how to ask the browser/client for username/password via standard HTTP mechanisms.

# Right after imports
security = HTTPBasic()

app = FastAPI() # Or use your APIRouter

Step 3: Define the “Verifier” Dependency Function

This function will receive the credentials extracted by security and check if they are valid. For this beginner example, we’ll use hardcoded values. In a real app, you’d check against a database.

# Our "verifier" function
def get_current_username(credentials: Annotated[HTTPBasicCredentials, Depends(security)]):
    # NOTE: In a real app, NEVER hardcode credentials like this!
    #       Always use secure password hashing (e.g., with passlib)
    #       and check against a database.
    correct_username = "stanley"
    correct_password = "password123" # Don't do this in production!

    # Basic check (insecure comparison for demonstration)
    is_correct_username = credentials.username == correct_username
    is_correct_password = credentials.password == correct_password # Insecure!

    if not (is_correct_username and is_correct_password):
        # If credentials are bad, raise an exception
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Incorrect email or password",
            headers={"WWW-Authenticate": "Basic"}, # Required header for 401 Basic Auth
        )
    # If credentials are okay, return the username
    return credentials.username

Explanation:

• get_current_username is our dependency function.
• credentials: Annotated[HTTPBasicCredentials, Depends(security)]: It depends on our security object (HTTPBasic). FastAPI will run security first. security will extract the username and password from the Authorization: Basic ... header and provide them as an HTTPBasicCredentials object to this function.
• Inside, we perform a (very insecure, for demo only!) check against hardcoded values.
• If the check fails, we raise HTTPException with status 401 Unauthorized. The headers={"WWW-Authenticate": "Basic"} part is important; it tells the browser how it should ask for credentials (using the Basic scheme).
• If the check passes, we return the validated username.

Step 4: Use Security() in the Path Operation

Now, let’s create our protected endpoint /users/me. Instead of Depends, we use Security with our verifier function.

@app.get("/users/me")
async def read_current_user(
    # Use Security() with the verifier function
    username: Annotated[str, Security(get_current_username)]
):
    # If the code reaches here, get_current_username ran successfully
    # and returned the validated username.
    # 'username' variable now holds the result from get_current_username.
    return {"username": username}

Explanation:

• username: Annotated[str, Security(get_current_username)]: We declare that this path operation requires the get_current_username dependency, using Security.
  • FastAPI will first run get_current_username.
  • get_current_username will, in turn, trigger security (HTTPBasic) to get the credentials.
  • If get_current_username succeeds (doesn’t raise an exception), its return value (the username string) will be injected into the username parameter of read_current_user.
  • If get_current_username (or the underlying HTTPBasic) raises an HTTPException, the request stops, the error response is sent, and read_current_user is never called.
  • Crucially, Security() also adds the HTTP Basic security requirement to the OpenAPI schema for this endpoint.

How it Behaves:

1. Run the App: uvicorn main:app --reload
2. Visit /docs: Go to http://127.0.0.1:8000/docs.
   • You’ll see the /users/me endpoint now has a padlock icon 🔒 next to it.
   • Click the “Authorize” button (usually near the top right). A popup will appear asking for Username and Password for the “HTTPBasic” scheme.
   • Enter stanley and password123 and click Authorize.
   • Now, try out the /users/me endpoint. Click “Try it out”, then “Execute”. It should work and return {"username": "stanley"}. The browser automatically added the correct Authorization header because you authorized in the UI.
   • Click “Authorize” again and “Logout”. Now try executing /users/me again. You’ll get a 401 Unauthorized error with {"detail": "Not authenticated"} (this default comes from HTTPBasic when no credentials are provided).
3. Use curl (Command Line):
   • curl http://127.0.0.1:8000/users/me -> Returns {"detail":"Not authenticated"} (401).
   • curl -u wronguser:wrongpass http://127.0.0.1:8000/users/me -> Returns {"detail":"Incorrect email or password"} (401). The -u flag makes curl use HTTP Basic Auth.
   • curl -u stanley:password123 http://127.0.0.1:8000/users/me -> Returns {"username": "stanley"} (200 OK).

You’ve successfully protected an endpoint using HTTP Basic Auth!

Other Common Schemes (Briefly)

The pattern is very similar for other schemes.

API Key in Header

# --- Imports ---
from fastapi.security import APIKeyHeader

# --- Scheme Instance ---
api_key_header_scheme = APIKeyHeader(name="X-API-KEY") # Expect key in X-API-KEY header

# --- Verifier Dependency (Example) ---
async def get_api_key(
    api_key: Annotated[str, Security(api_key_header_scheme)] # Use Security() with the SCHEME instance here
):
    if api_key == "SECRET_API_KEY": # Check the key (use a secure way in real apps!)
        return api_key
    else:
        raise HTTPException(
            status_code=status.HTTP_403_FORBIDDEN, detail="Could not validate API KEY"
        )

# --- Path Operation ---
@app.get("/secure-data")
async def get_secure_data(
    # Inject the VALIDATED key using Depends() - no need for Security() again
    # if the get_api_key dependency already uses Security() internally.
    # Alternatively, if get_api_key just returned the key without raising errors,
    # you could use Security(get_api_key) here. Let's stick to the pattern:
    # the verifier dependency uses Security(scheme), the endpoint uses Depends(verifier)
    # or directly uses Security(verifier) if the verifier handles errors.
    # Let's adjust get_api_key to make it cleaner:
    api_key: Annotated[str, Security(api_key_header_scheme)] # Scheme extracts the key
):
    # Now, a separate check or use the key
    if api_key == "SECRET_API_KEY": # Re-checking here for simplicity, ideally done in a dependent function
         return {"data": "sensitive data", "api_key_used": api_key}
    else:
         # This path might not be reachable if auto_error=True in APIKeyHeader
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Invalid API Key provided")

# Let's refine the API Key example pattern to match the Basic Auth pattern:
# Scheme Instance
api_key_header_scheme = APIKeyHeader(name="X-API-KEY", auto_error=False) # auto_error=False lets verifier handle missing key

# Verifier Dependency
async def verify_api_key(api_key: Annotated[str | None, Security(api_key_header_scheme)]):
    if api_key is None:
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="X-API-KEY header missing")
    if api_key == "SECRET_API_KEY":
        return api_key # Return key or user info associated with the key
    else:
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Invalid API Key")

# Path Operation using the verifier
@app.get("/secure-data")
async def get_secure_data_v2(
    # Use Security() with the VERIFIER function
    verified_key: Annotated[str, Security(verify_api_key)]
):
    # verified_key holds the result from verify_api_key (the validated key)
    return {"data": "sensitive data", "key": verified_key}

OAuth2 Password Bearer Flow

This is common for user logins in web apps. It usually involves two endpoints: one to exchange username/password for a token (/token), and protected endpoints that require the token.

# --- Imports ---
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm

# --- Scheme Instance ---
# The 'tokenUrl' points to the path operation where users get the token
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

# --- Token Endpoint (Example) ---
@app.post("/token")
async def login_for_access_token(
    form_data: Annotated[OAuth2PasswordRequestForm, Depends()]
):
    # 1. Verify form_data.username and form_data.password (check DB)
    # 2. If valid, create an access token (e.g., a JWT)
    # 3. Return the token
    # (Skipping implementation details for brevity)
    access_token = f"token_for_{form_data.username}" # Fake token
    return {"access_token": access_token, "token_type": "bearer"}

# --- Verifier Dependency (Example: decode token and get user) ---
async def get_current_user(token: Annotated[str, Security(oauth2_scheme)]):
    # In a real app:
    # 1. Decode the token (e.g., JWT)
    # 2. Validate the token (check expiry, signature)
    # 3. Extract user identifier from token payload
    # 4. Fetch user from database
    # 5. Raise HTTPException if token is invalid or user doesn't exist
    if token == "token_for_stanley": # Fake check
        return {"username": "stanley", "email": "stanley@example.com"}
    else:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )

# --- Protected Path Operation ---
@app.get("/users/me/oauth")
async def read_users_me_oauth(
    # Use Security() with the user verifier function
    current_user: Annotated[dict, Security(get_current_user)]
):
    # current_user holds the dict returned by get_current_user
    return current_user

The core pattern remains: Instantiate the scheme -> Define a verifier dependency that uses the scheme -> Protect endpoints using Security(verifier_dependency).

How it Works Under the Hood (Simplified)

Let’s trace the HTTP Basic Auth example (GET /users/me requiring stanley/password123):

1. Request: Client sends GET /users/me with header Authorization: Basic c3RhbmxleTpwYXNzd29yZDEyMw== (where c3Rh... is base64(“stanley:password123”)).
2. Routing: FastAPI matches the request to read_current_user.
3. Dependency Analysis: FastAPI sees username: Annotated[str, Security(get_current_username)]. It knows it needs to resolve the get_current_username dependency using the Security mechanism.
4. Security Dependency Resolution:
   • FastAPI looks inside get_current_username and sees its dependency: credentials: Annotated[HTTPBasicCredentials, Depends(security)].
   • It needs to resolve security (our HTTPBasic() instance).
5. Scheme Execution (HTTPBasic.__call__):
   • FastAPI calls the security object (which is callable).
   • The HTTPBasic object’s __call__ method executes. It reads the Authorization header from the request.
   • It finds the Basic scheme and the parameter c3RhbmxleTpwYXNzd29yZDEyMw==.
   • It base64-decodes the parameter to get stanley:password123.
   • It splits this into username (stanley) and password (password123).
   • It creates and returns an HTTPBasicCredentials(username="stanley", password="password123") object.
   • (If the header was missing or malformed, HTTPBasic.__call__ would raise HTTPException(401) here, stopping the process).
6. Verifier Execution (get_current_username):
   • FastAPI now has the result from security. It calls get_current_username(credentials=<HTTPBasicCredentials object>).
   • Your verifier code runs. It compares the credentials. They match the hardcoded values.
   • The function returns the username "stanley".
   • (If the credentials didn’t match, your code would raise HTTPException(401) here, stopping the process).
7. Path Operation Execution (read_current_user):
   • FastAPI now has the result from get_current_username. It calls read_current_user(username="stanley").
   • Your path operation function runs and returns {"username": "stanley"}.
8. Response: FastAPI sends the 200 OK JSON response back to the client.
9. OpenAPI Generation: Separately, when generating /openapi.json, FastAPI sees Security(get_current_username) -> Depends(security) -> security is HTTPBasic. It adds the “HTTPBasic” security requirement definition to the global components.securitySchemes and references it in the security requirements for the /users/me path operation. This is what makes the lock icon appear in /docs.

Here’s a simplified diagram:

sequenceDiagram
    participant Client
    participant FastAPIApp as FastAPI App
    participant HTTPBasicInst as security (HTTPBasic Instance)
    participant VerifierFunc as get_current_username
    participant PathOpFunc as read_current_user

    Client->>+FastAPIApp: GET /users/me (Authorization: Basic ...)
    FastAPIApp->>FastAPIApp: Match route, see Security(get_current_username)
    FastAPIApp->>FastAPIApp: Resolve get_current_username dependencies: Depends(security)
    FastAPIApp->>+HTTPBasicInst: Call security(request)
    HTTPBasicInst->>HTTPBasicInst: Read header, decode base64, split user/pass
    HTTPBasicInst-->>-FastAPIApp: Return HTTPBasicCredentials(user="stanley", pass="...")
    FastAPIApp->>+VerifierFunc: Call get_current_username(credentials=...)
    VerifierFunc->>VerifierFunc: Check credentials -> OK
    VerifierFunc-->>-FastAPIApp: Return username "stanley"
    FastAPIApp->>+PathOpFunc: Call read_current_user(username="stanley")
    PathOpFunc-->>-FastAPIApp: Return {"username": "stanley"}
    FastAPIApp-->>-Client: Send 200 OK JSON Response

Code Connections

• fastapi.Security: The function you import and use. It’s a thin wrapper around fastapi.params.Security. (fastapi/param_functions.py)
• fastapi.params.Security: The class that signals a security dependency, inheriting from Depends but adding the scopes parameter. (fastapi/params.py)
• fastapi.security.*: This package contains the scheme implementations:
  • fastapi.security.http: Contains HTTPBase, HTTPBasic, HTTPBearer, HTTPDigest, and the HTTPBasicCredentials, HTTPAuthorizationCredentials models.
  • fastapi.security.api_key: Contains APIKeyHeader, APIKeyQuery, APIKeyCookie.
  • fastapi.security.oauth2: Contains OAuth2, OAuth2PasswordBearer, OAuth2AuthorizationCodeBearer, OAuth2PasswordRequestForm, SecurityScopes.
• Scheme __call__ methods: Each scheme class (e.g., HTTPBasic, APIKeyHeader, OAuth2PasswordBearer) implements async def __call__(self, request: Request) which contains the logic to extract credentials from the specific request location (headers, query, etc.).
• Dependency Injection System: The core system described in Chapter 5: Dependency Injection resolves the dependencies, calling the scheme instance and then your verifier function.
• OpenAPI Integration: FastAPI’s OpenAPI generation logic specifically checks for Security dependencies and uses the associated scheme model (security.model) to add the correct security requirements to the schema.

Conclusion

You’ve now learned the basics of securing your FastAPI endpoints!

• You understand the need for authentication (who is the user?).
• You know about common security schemes like HTTP Basic, API Keys, and OAuth2 Bearer tokens.
• You learned that FastAPI provides utility classes (e.g., HTTPBasic, APIKeyHeader, OAuth2PasswordBearer) in the fastapi.security module to handle these schemes.
• You saw how to use the Security() function (similar to Depends()) to integrate these schemes into your path operations via verifier dependencies.
• You understand that Security() automatically adds security requirements to your OpenAPI documentation (/docs).
• You grasped the core pattern: Scheme Instance -> Verifier Dependency -> Security(verifier).

Using these tools allows you to easily add robust security layers to your API without reinventing the wheel.

Sometimes, after handling a request and sending a response, you might need to perform some follow-up actions, like sending a notification email or processing some data, without making the user wait. How can we do that?

Ready to run tasks in the background? Let’s move on to Chapter 8: Background Tasks!

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