WebSocket authentication in FastAPI - Mini Project: Build & Apply

WebSocket Authentication with FastAPI

📖 Scenario: You are building a chat application where users connect through WebSocket. To keep the chat secure, you want to check if users are authenticated before allowing them to join the chat.

🎯 Goal: Build a FastAPI WebSocket endpoint that authenticates users by checking a token before accepting the connection.

📋 What You'll Learn

Create a dictionary called valid_tokens with exact token-user pairs

Create a variable called auth_token to hold the token from the WebSocket query

Use a for loop with variables token and user to check if auth_token is valid

Accept the WebSocket connection only if the token is valid, otherwise close it

💡 Why This Matters

🌍 Real World

WebSocket authentication is essential for real-time apps like chat, live notifications, or games to ensure only authorized users connect.

💼 Career

Understanding WebSocket authentication is important for backend developers working with real-time communication and security.

1

Create valid tokens dictionary

Create a dictionary called valid_tokens with these exact entries: 'token123': 'alice', 'token456': 'bob', 'token789': 'carol'.

FastAPI

from fastapi import FastAPI, WebSocket, WebSocketDisconnect

app = FastAPI()

# Create valid_tokens dictionary here

Hint

Use curly braces {} to create the dictionary with the exact keys and values.

2

Extract token from WebSocket query

Inside the websocket_endpoint function, create a variable called auth_token that gets the token from websocket.query_params.get('token').

FastAPI

from fastapi import FastAPI, WebSocket, WebSocketDisconnect

app = FastAPI()

valid_tokens = {'token123': 'alice', 'token456': 'bob', 'token789': 'carol'}

@app.websocket('/ws')
async def websocket_endpoint(websocket: WebSocket):
    # Extract auth_token from query params
    # Your code here

Hint

Use websocket.query_params.get('token') to get the token from the URL query.

3

Check if token is valid

Use a for loop with variables token and user to iterate over valid_tokens.items(). Inside the loop, check if auth_token == token. If yes, accept the WebSocket connection with await websocket.accept() and break the loop.

FastAPI

from fastapi import FastAPI, WebSocket, WebSocketDisconnect

app = FastAPI()

valid_tokens = {'token123': 'alice', 'token456': 'bob', 'token789': 'carol'}

@app.websocket('/ws')
async def websocket_endpoint(websocket: WebSocket):
    auth_token = websocket.query_params.get('token')
    # Check token validity and accept connection
    # Your code here

Hint

Use for token, user in valid_tokens.items(): to loop through tokens and users.

4

Close connection if token invalid

After the for loop, add an else clause that closes the WebSocket connection with await websocket.close() if no valid token was found.

FastAPI

from fastapi import FastAPI, WebSocket, WebSocketDisconnect

app = FastAPI()

valid_tokens = {'token123': 'alice', 'token456': 'bob', 'token789': 'carol'}

@app.websocket('/ws')
async def websocket_endpoint(websocket: WebSocket):
    auth_token = websocket.query_params.get('token')
    for token, user in valid_tokens.items():
        if auth_token == token:
            await websocket.accept()
            break
    # Close connection if token invalid
    # Your code here

Hint

Use the else clause on the for loop to close the connection if no token matches.

Practice

1. What is the main purpose of WebSocket authentication in FastAPI?

easy

A. To speed up the WebSocket connection process

B. To confirm the user's identity before allowing the WebSocket connection

C. To encrypt the WebSocket messages automatically

D. To log all WebSocket messages for debugging

Solution

1. Step 1: Understand WebSocket authentication role

   Authentication ensures only authorized users can connect to the WebSocket.

2. Step 2: Identify the main goal in FastAPI context

   FastAPI uses authentication to confirm user identity before opening the connection.

3. Final Answer:

   To confirm the user's identity before allowing the WebSocket connection -> Option B

4. Quick Check:

   Authentication = Confirm identity [OK]

Hint: Authentication means confirming user identity before connection [OK]

Common Mistakes:

• Confusing authentication with encryption
• Thinking authentication speeds up connection
• Assuming authentication logs messages

2. Which of the following is the correct way to accept a WebSocket connection with token authentication in FastAPI?

easy

A. await websocket.accept() without checking token

B. Call await websocket.close() before checking token

C. Use await websocket.send_text() before accepting

D. Check token, then call await websocket.accept() if valid

Solution

1. Step 1: Understand WebSocket connection flow

   You must check the token first to decide if connection is allowed.

2. Step 2: Accept connection only if token is valid

   Call await websocket.accept() only after confirming token validity.

3. Final Answer:

   Check token, then call await websocket.accept() if valid -> Option D

4. Quick Check:

   Accept after token check = C [OK]

Hint: Accept connection only after validating token [OK]

Common Mistakes:

• Accepting connection before token check
• Closing connection before checking token
• Sending messages before accepting connection

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

async def websocket_endpoint(websocket: WebSocket):
    token = websocket.query_params.get('token')
    if token != 'secret123':
        await websocket.close(code=1008)
    else:
        await websocket.accept()
        await websocket.send_text('Welcome!')

medium

A. The connection closes immediately with code 1008

B. The connection accepts and sends 'Welcome!' anyway

C. The server raises a runtime error

D. The connection stays open but no message is sent

Solution

1. Step 1: Check token validation logic

   If token is not 'secret123', the code calls await websocket.close(code=1008).

2. Step 2: Understand WebSocket close behavior

   Calling close immediately ends the connection with the given code.

3. Final Answer:

   The connection closes immediately with code 1008 -> Option A

4. Quick Check:

   Invalid token triggers close(1008) [OK]

Hint: Invalid token triggers immediate close with code 1008 [OK]

Common Mistakes:

• Assuming connection accepts anyway
• Expecting a runtime error
• Thinking connection stays open silently

4. Identify the error in this FastAPI WebSocket authentication code:

async def websocket_endpoint(websocket: WebSocket):
    token = websocket.headers.get('Authorization')
    if token != 'Bearer secret123':
        await websocket.accept()
        await websocket.send_text('Unauthorized')
        await websocket.close()
    else:
        await websocket.accept()
        await websocket.send_text('Welcome')

medium

A. Using headers.get instead of query_params.get

B. Not closing connection on invalid token

C. Accepting connection before sending 'Unauthorized' message

D. Missing await before websocket.close()

Solution

1. Step 1: Analyze connection flow on invalid token

   The code calls await websocket.accept() before sending 'Unauthorized' and closing.

2. Step 2: Understand best practice for unauthorized access

   It is better to close connection immediately without accepting it to avoid opening unauthorized connection.

3. Final Answer:

   Accepting connection before sending 'Unauthorized' message -> Option C

4. Quick Check:

   Unauthorized should close without accept [OK]

Hint: Do not accept connection before rejecting unauthorized users [OK]

Common Mistakes:

• Accepting connection before rejecting
• Ignoring connection close on invalid token
• Confusing headers and query parameters

5. You want to authenticate WebSocket connections in FastAPI using a token passed as a query parameter. Which approach correctly handles authentication and rejects unauthorized users cleanly?

hard

A. Check token from query params; if invalid, call await websocket.close(1008) before accept; else accept connection

B. Accept connection first, then check token; if invalid, send 'Unauthorized' message and close

C. Ignore token and accept all connections; handle authorization in message handlers

D. Check token in headers only; ignore query parameters

Solution

1. Step 1: Understand token location and validation

   Token is passed as query parameter, so check websocket.query_params for token.

2. Step 2: Handle unauthorized users before accepting connection

   If token is invalid, close connection immediately with code 1008 to reject cleanly.

3. Step 3: Accept connection only if token is valid

   Only call await websocket.accept() after confirming token validity.

4. Final Answer:

   Check token from query params; if invalid, call await websocket.close(1008) before accept; else accept connection -> Option A

5. Quick Check:

   Reject before accept with close(1008) = A [OK]

Hint: Reject unauthorized before accept using close(1008) [OK]

Common Mistakes:

• Accepting connection before token check
• Ignoring query parameters for token
• Handling authorization after connection open