JWT token verification in FastAPI - Mini Project: Build & Apply

JWT Token Verification with FastAPI

📖 Scenario: You are building a simple API that needs to check if a user is allowed access by verifying a JWT token sent with the request.This is like checking a ticket at a concert entrance to make sure it is valid before letting someone in.

🎯 Goal: Build a FastAPI app that verifies a JWT token from the request header and allows access only if the token is valid.

📋 What You'll Learn

Create a secret key variable for signing tokens

Create a function to decode and verify the JWT token

Use FastAPI dependency injection to check the token on a protected route

Return a success message if the token is valid

💡 Why This Matters

🌍 Real World

APIs often need to verify user identity securely using JWT tokens to protect sensitive data and actions.

💼 Career

Understanding JWT token verification is essential for backend developers working with modern web APIs and authentication.

1

Set up the secret key

Create a variable called SECRET_KEY and set it to the string "mysecretkey123".

FastAPI

# Your code here

Hint

The secret key is like a password used to sign and check tokens.

2

Create a function to verify the JWT token

Write a function called verify_token that takes a parameter token. Inside, use jwt.decode(token, SECRET_KEY, algorithms=["HS256"]) to decode the token and return the decoded data.

FastAPI

SECRET_KEY = "mysecretkey123"
# Your code here

Hint

Use the jose library's jwt.decode to check the token. Handle errors with a try-except block.

3

Add a FastAPI dependency to check the token

Import Depends and HTTPException from fastapi. Create a function called get_current_user that takes token: str = Depends(). Inside, call verify_token(token). If it returns None, raise HTTPException(status_code=401, detail="Invalid token"). Otherwise, return the decoded token data.

FastAPI

SECRET_KEY = "mysecretkey123"

from jose import jwt, JWTError
from fastapi import Depends, HTTPException

# Your code here

Hint

Use FastAPI's Depends to get the token automatically from the request.

4

Create a protected route using the token verification

Import FastAPI. Create an app instance called app. Add a route @app.get("/protected") that uses get_current_user as a dependency. The route function should return a dictionary with {"message": "Access granted", "user": user} where user is the decoded token data.

FastAPI

SECRET_KEY = "mysecretkey123"

from jose import jwt, JWTError
from fastapi import Depends, HTTPException, FastAPI

# Your code here

Hint

The route uses the get_current_user dependency to check the token before allowing access.

Practice

1. What is the main purpose of JWT token verification in a FastAPI application?

easy

A. To check if the user token is valid and trusted

B. To encrypt the user's password

C. To store user data in the database

D. To generate HTML pages dynamically

Solution

1. Step 1: Understand JWT token role

   JWT tokens are used to prove a user's identity securely.

2. Step 2: Identify verification purpose

   Verification checks if the token is valid and trusted before allowing access.

3. Final Answer:

   To check if the user token is valid and trusted -> Option A

4. Quick Check:

   JWT verification = check token validity [OK]

Hint: JWT verification means confirming token is valid [OK]

Common Mistakes:

• Confusing verification with encryption
• Thinking JWT stores user data permanently
• Mixing token verification with UI rendering

2. Which FastAPI dependency is commonly used to extract and verify a JWT token from the request header?

easy

A. Depends()

B. Form()

C. RequestBody()

D. OAuth2PasswordBearer

Solution

1. Step 1: Identify FastAPI dependency for JWT

   OAuth2PasswordBearer is designed to extract bearer tokens from headers.

2. Step 2: Confirm usage for JWT verification

   This dependency helps get the token string to verify it in your code.

3. Final Answer:

   OAuth2PasswordBearer -> Option D

4. Quick Check:

   OAuth2PasswordBearer extracts JWT token [OK]

Hint: OAuth2PasswordBearer extracts token from header [OK]

Common Mistakes:

• Using Depends() alone without OAuth2PasswordBearer
• Confusing Form() with header token extraction
• Using RequestBody() which reads body, not headers

3. Given this FastAPI code snippet, what will happen if the JWT token is invalid?

async def get_current_user(token: str = Depends(oauth2_scheme)):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
    except JWTError:
        raise HTTPException(status_code=401, detail="Invalid token")
    return payload

medium

A. The function returns the payload even if token is invalid

B. The server crashes with an unhandled exception

C. An HTTP 401 error is raised with 'Invalid token' message

D. The token is ignored and user is treated as anonymous

Solution

1. Step 1: Analyze try-except block

   If jwt.decode fails, it raises JWTError which is caught by except.

2. Step 2: Check except block behavior

   It raises HTTPException with status 401 and message 'Invalid token'.

3. Final Answer:

   An HTTP 401 error is raised with 'Invalid token' message -> Option C

4. Quick Check:

   Invalid token triggers HTTP 401 error [OK]

Hint: Invalid JWT triggers HTTPException 401 [OK]

Common Mistakes:

• Assuming function returns payload on invalid token
• Thinking server crashes without handling error
• Believing token is ignored silently

4. Identify the error in this FastAPI JWT verification code:

from fastapi import Depends, HTTPException
from jose import jwt, JWTError

def verify_token(token: str):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
    except:
        HTTPException(status_code=401, detail="Invalid token")
    return payload

medium

A. HTTPException is raised but not returned or raised properly

B. Missing import for HTTPException

C. jwt.decode is called with wrong parameters

D. The function should not return payload

Solution

1. Step 1: Check exception handling

   HTTPException is created but not raised or returned, so error is ignored.

2. Step 2: Correct usage of HTTPException

   Must use 'raise HTTPException(...)' to properly stop execution and send error.

3. Final Answer:

   HTTPException is raised but not returned or raised properly -> Option A

4. Quick Check:

   Use 'raise' keyword with HTTPException [OK]

Hint: Always 'raise' HTTPException to trigger error [OK]

Common Mistakes:

• Forgetting 'raise' before HTTPException
• Catching too broad exceptions without logging
• Returning payload even on error

5. How can you protect a FastAPI route so that only requests with a valid JWT token can access it?

hard

A. Check the token manually inside the route function without dependencies

B. Use a dependency that verifies the JWT token and include it in the route

C. Add a middleware that ignores JWT tokens

D. Use a global variable to store token validity

Solution

1. Step 1: Understand FastAPI dependencies

   Dependencies can run code before route logic and reject invalid requests.

2. Step 2: Use dependency to verify JWT

   Including a JWT verification dependency ensures only valid tokens allow access.

3. Final Answer:

   Use a dependency that verifies the JWT token and include it in the route -> Option B

4. Quick Check:

   Dependency verifies JWT before route runs [OK]

Hint: Protect routes with JWT verification dependency [OK]

Common Mistakes:

• Checking token inside route instead of dependency
• Ignoring token verification in middleware
• Using global variables for token state