Trusted host middleware in FastAPI - Mini Project: Build & Apply

Trusted Host Middleware in FastAPI

📖 Scenario: You are building a simple web API using FastAPI. To keep your API safe, you want to allow requests only from certain trusted hosts, like your own domain and localhost.

🎯 Goal: Build a FastAPI app that uses the TrustedHostMiddleware to accept requests only from example.com and localhost.

📋 What You'll Learn

Create a FastAPI app instance

Add TrustedHostMiddleware with allowed hosts example.com and localhost

Create a simple root endpoint that returns a welcome message

Run the app so it enforces trusted hosts

💡 Why This Matters

🌍 Real World

TrustedHostMiddleware helps protect your API by allowing requests only from known, safe domains. This prevents some types of attacks and misuse.

💼 Career

Understanding middleware and security features like trusted hosts is important for backend developers building secure web APIs.

1

Create a FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

# Import FastAPI and create app instance
# Your code here

Hint

Use from fastapi import FastAPI and then app = FastAPI().

2

Add TrustedHostMiddleware with allowed hosts

Import TrustedHostMiddleware from fastapi.middleware.trustedhost and add it to app with allowed_hosts set to ["example.com", "localhost"].

FastAPI

from fastapi import FastAPI

app = FastAPI()
# Add TrustedHostMiddleware with allowed_hosts ["example.com", "localhost"]
# Your code here

Hint

Use app.add_middleware(TrustedHostMiddleware, allowed_hosts=["example.com", "localhost"]).

3

Create a root endpoint

Define a GET endpoint at "/" on app that returns a JSON message {"message": "Welcome to the trusted host API!"}.

FastAPI

from fastapi import FastAPI
from fastapi.middleware.trustedhost import TrustedHostMiddleware

app = FastAPI()

app.add_middleware(TrustedHostMiddleware, allowed_hosts=["example.com", "localhost"])

# Create a GET endpoint at "/" returning JSON message
# Your code here

Hint

Use @app.get("/") decorator and define an async function root that returns the message.

4

Run the FastAPI app

Add the code to run the app with uvicorn when the script is executed directly. Use uvicorn.run(app, host="0.0.0.0", port=8000) inside if __name__ == "__main__".

FastAPI

from fastapi import FastAPI
from fastapi.middleware.trustedhost import TrustedHostMiddleware
import uvicorn

app = FastAPI()

app.add_middleware(TrustedHostMiddleware, allowed_hosts=["example.com", "localhost"])

@app.get("/")
async def root():
    return {"message": "Welcome to the trusted host API!"}

# Add code to run app with uvicorn when executed directly
# Your code here

Hint

Import uvicorn and use if __name__ == "__main__" to run the app.

Practice

1. What is the main purpose of the TrustedHostMiddleware in FastAPI?

easy

A. To block requests from hosts not in the allowed list

B. To speed up the response time of the app

C. To handle database connections securely

D. To manage user authentication tokens

Solution

1. Step 1: Understand middleware role

   The TrustedHostMiddleware is designed to filter incoming requests based on their host header.

2. Step 2: Identify its security purpose

   It blocks requests from hosts not explicitly allowed to protect against host header attacks.

3. Final Answer:

   To block requests from hosts not in the allowed list -> Option A

4. Quick Check:

   TrustedHostMiddleware blocks unknown hosts = D [OK]

Hint: Remember: Trusted hosts means allowed hosts only [OK]

Common Mistakes:

• Confusing it with authentication middleware
• Thinking it speeds up app performance
• Assuming it manages database connections

2. Which of the following is the correct way to add TrustedHostMiddleware to a FastAPI app?

easy

A. app.middleware(TrustedHostMiddleware, allowed=['example.com'])

B. app.add_middleware(TrustedHostMiddleware, allowed_hosts=['example.com'])

C. app.use(TrustedHostMiddleware, hosts=['example.com'])

D. app.add_middleware(TrustedHostMiddleware, hosts=['example.com'])

Solution

1. Step 1: Recall FastAPI middleware syntax

   FastAPI uses app.add_middleware() with the middleware class and keyword arguments.

2. Step 2: Check correct argument name

   The correct argument for allowed hosts is allowed_hosts, not hosts or allowed.

3. Final Answer:

   app.add_middleware(TrustedHostMiddleware, allowed_hosts=['example.com']) -> Option B

4. Quick Check:

   Use add_middleware with allowed_hosts = C [OK]

Hint: Use add_middleware and allowed_hosts keyword [OK]

Common Mistakes:

• Using wrong method like app.use()
• Passing 'hosts' instead of 'allowed_hosts'
• Incorrect argument names like 'allowed'

3. Given this FastAPI app code snippet, what will happen if a request comes from host 'malicious.com'?

from fastapi import FastAPI
from starlette.middleware.trustedhost import TrustedHostMiddleware

app = FastAPI()
app.add_middleware(TrustedHostMiddleware, allowed_hosts=['example.com', 'localhost'])

@app.get('/')
def read_root():
    return {'message': 'Hello World'}

medium

A. The request will be redirected to 'example.com'

B. The request will succeed and return 'Hello World'

C. The app will crash with an exception

D. The request will be blocked with a 400 Bad Request error

Solution

1. Step 1: Check allowed hosts list

   The allowed hosts are 'example.com' and 'localhost'. 'malicious.com' is not in this list.

2. Step 2: Understand middleware behavior on unknown hosts

   TrustedHostMiddleware blocks requests from hosts not in the allowed list by returning a 400 error.

3. Final Answer:

   The request will be blocked with a 400 Bad Request error -> Option D

4. Quick Check:

   Unknown host causes 400 error = A [OK]

Hint: Requests from hosts not allowed get 400 error [OK]

Common Mistakes:

• Assuming the request passes through
• Thinking the app crashes on unknown hosts
• Believing the request is redirected automatically

4. Identify the error in this FastAPI app setup using TrustedHostMiddleware:

from fastapi import FastAPI
from starlette.middleware.trustedhost import TrustedHostMiddleware

app = FastAPI()
app.add_middleware(TrustedHostMiddleware, allowed_hosts='example.com')

@app.get('/')
def home():
    return {'msg': 'Welcome'}

medium

A. The route function must be async

B. TrustedHostMiddleware is not imported correctly

C. allowed_hosts should be a list, not a string

D. Missing middleware initialization parameters

Solution

1. Step 1: Check allowed_hosts argument type

   The allowed_hosts parameter expects a list of strings, but a single string was given.

2. Step 2: Understand impact of wrong type

   Passing a string instead of a list will cause the middleware to treat each character as a host, leading to incorrect behavior or errors.

3. Final Answer:

   allowed_hosts should be a list, not a string -> Option C

4. Quick Check:

   allowed_hosts must be list = A [OK]

Hint: allowed_hosts always needs a list, not a string [OK]

Common Mistakes:

• Passing a single string instead of list
• Thinking route functions must be async
• Assuming import is incorrect without error

5. You want to allow requests from any subdomain of example.com and also from localhost. Which allowed_hosts list correctly configures TrustedHostMiddleware for this?

hard

A. ['*.example.com', 'localhost']

B. ['example.com', 'localhost']

C. ['example.com/*', 'localhost']

D. ['*example.com', 'localhost']

Solution

1. Step 1: Understand wildcard usage in allowed_hosts

   TrustedHostMiddleware supports wildcards like *.example.com to allow all subdomains.

2. Step 2: Check each option for correct wildcard syntax

   ['*.example.com', 'localhost'] uses '*.example.com' which correctly matches all subdomains; others use incorrect patterns.

3. Final Answer:

   ['*.example.com', 'localhost'] -> Option A

4. Quick Check:

   Use '*.example.com' for subdomains = B [OK]

Hint: Use '*.domain.com' to allow all subdomains [OK]

Common Mistakes:

• Using 'example.com/*' which is invalid
• Using '*example.com' missing dot after *
• Not using wildcard for subdomains