Building Secure APIs with FastAPI and JWT

What is JWT?

JSON Web Token (JWT) is an open standard (RFC 7519) for securely transmitting information between parties as a JSON object. It is compact, self-contained, and digitally signed, meaning that it can be trusted and verified.

A typical JWT consists of three parts:

1. Header – Specifies the signing algorithm.

2. Payload – Contains the claims (data).

3. Signature – Verifies that the token hasn’t been tampered with.

Example token format:

xxxxx.yyyyy.zzzzz

These tokens are widely used in authentication systems because they enable stateless communication between clients and servers, meaning the server doesn’t need to store session info — the token carries all the necessary data.

Setting Up FastAPI for JWT Auth

First, let’s install the required dependencies:

pip install fastapi uvicorn python-jose[cryptography] passlib[bcrypt] python-multipart

These packages enable FastAPI, handle password hashing, JWT encoding/decoding, and parsing form data.

JWT Authentication Setup

Step 1: Basic Configuration

Start by importing required libraries and setting up constants:

from datetime import datetime, timedelta
from typing import Optional
from jose import JWTError, jwt
from passlib.context import CryptContext
from fastapi import Depends, FastAPI, HTTPException, status
from fastapi.security import OAuth2PasswordBearer

# JWT Configuration
SECRET_KEY = "your-secret-key"  # Use a secure key in production
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30

Step 2: Password Hashing and OAuth2 Setup

# Password hashing
pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

# OAuth2 Bearer Token URL
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

# Hash a password
def get_password_hash(password):
    return pwd_context.hash(password)

# Verify password
def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)

Step 3: Creating & Verifying JWT Tokens

def create_access_token(data: dict, expires_delta: Optional[timedelta] = None):
    to_encode = data.copy()
    expire = datetime.utcnow() + (expires_delta or timedelta(minutes=15))
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt

async def get_current_user(token: str = Depends(oauth2_scheme)):
    credentials_exception = HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail="Could not validate credentials",
        headers={"WWW-Authenticate": "Bearer"},
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
    except JWTError:
        raise credentials_exception
    return {"username": username}  # In real apps, fetch from DB

Login Endpoint

Let’s allow users to log in and receive JWT tokens.

from fastapi.security import OAuth2PasswordRequestForm
from pydantic import BaseModel

app = FastAPI()

# Mock database
fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW",  # 'secret'
        "disabled": False,
    }
}

class Token(BaseModel):
    access_token: str
    token_type: str

@app.post("/token", response_model=Token)
async def login_for_access_token(form_data: OAuth2PasswordRequestForm = Depends()):
    user = fake_users_db.get(form_data.username)
    if not user or not verify_password(form_data.password, user["hashed_password"]):
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Incorrect username or password",
            headers={"WWW-Authenticate": "Bearer"},
        )

    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user["username"]},
        expires_delta=access_token_expires
    )
    return {"access_token": access_token, "token_type": "bearer"}

Protecting Routes with JWT

Now let’s add some routes that require authentication.

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

@app.get("/users/me", response_model=User)
async def read_users_me(current_user: dict = Depends(get_current_user)):
    user = fake_users_db.get(current_user["username"])
    return user

@app.get("/protected-resource")
async def protected_resource(current_user: dict = Depends(get_current_user)):
    return {"message": "This is a protected resource", "user": current_user}

Conclusion

With this setup, you've created a full JWT authentication flow using FastAPI. This approach is:

• ✅ Stateless — no server-side session storage required.

• ✅ Scalable — easy to deploy on distributed systems.

• ✅ Secure — uses industry-standard cryptographic algorithms.

Final Tips

• Always use HTTPS in production to prevent token interception.

• Store your SECRET_KEY securely — ideally in environment variables or a secrets manager.

• Consider implementing:

• Refresh tokens for longer sessions

• Role-based access control (RBAC)

• Integration with OAuth providers like Google, GitHub, etc.

Now you're ready to build secure APIs with FastAPI and JWT authentication. Happy coding!