Trusted host middleware in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of Trusted Host Middleware in FastAPI?

Trusted Host Middleware helps protect your app by allowing requests only from specific hostnames you trust. It blocks requests from unknown or suspicious hosts.

beginner

How do you add Trusted Host Middleware in a FastAPI app?

You import TrustedHostMiddleware from starlette.middleware.trustedhost and add it to your app with app.add_middleware(TrustedHostMiddleware, allowed_hosts=[...]).

beginner

What happens if a request comes from a host not in the allowed_hosts list?

The middleware returns a 400 Bad Request response and blocks the request from reaching your app.

intermediate

Why is it important to include both domain names and localhost in allowed_hosts during development?

Including localhost allows testing on your machine, while domain names protect your app in production by only accepting trusted hosts.

intermediate

Can you use wildcards in allowed_hosts with Trusted Host Middleware?

Yes, you can use patterns like '*.example.com' to allow all subdomains of example.com.

What status code does Trusted Host Middleware return for disallowed hosts?

A 400 Bad Request

B 404 Not Found

C 403 Forbidden

D 500 Internal Server Error

Trusted Host Middleware returns 400 Bad Request when the host is not in the allowed list.

Which FastAPI method is used to add Trusted Host Middleware?

A app.use_middleware()

B app.include_middleware()

C app.add_middleware()

D app.register_middleware()

You use app.add_middleware() to add middleware like TrustedHostMiddleware in FastAPI.

Which import is correct to use Trusted Host Middleware in FastAPI?

A from fastapi.middleware.trustedhost import TrustedHostMiddleware

B from starlette.middleware.trustedhost import TrustedHostMiddleware

C from fastapi.middleware import TrustedHostMiddleware

D from starlette.middleware import TrustedHostMiddleware

TrustedHostMiddleware is imported from starlette.middleware.trustedhost.

What should you include in allowed_hosts for local testing?

A localhost and 127.0.0.1

B Only your domain name

C Only IP addresses

D No hosts needed

Include 'localhost' and '127.0.0.1' in allowed_hosts to test locally.

Can you allow all subdomains of example.com using Trusted Host Middleware?

A Only exact hostnames are allowed

B No, wildcards are not supported

C Yes, by adding 'example.*' to allowed_hosts

D Yes, by adding '*.example.com' to allowed_hosts

You can use '*.example.com' to allow all subdomains of example.com.

Explain how Trusted Host Middleware protects a FastAPI application and how to configure it.

Think about which hosts your app accepts and what happens if a host is not trusted.

Describe why including localhost in allowed_hosts is important during development with Trusted Host Middleware.

Consider how you test your app on your own computer.

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