Custom lambda authorizer for websocket api not working

Question

Hi,

I am trying to secure a websocket api using cognito. I created a lambda authorizer following the documentation and forums posts, and I deploy everything using AWS SAM CLI. My template looks like this (I ommited some stuff for clarity).

```
  AuthDependenciesLayer:
    Type: AWS::Serverless::LayerVersion
    Properties:
      LayerName: AuthDependenciesLayer
      ContentUri: auth_dependencies/
      CompatibleRuntimes:
        - python3.12

AuthFunction:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: !Sub '${AWS::StackName}-auth-fn'
      CodeUri: authorizer/
      Architectures:
        - x86_64
      Environment:
        Variables:
          COGNITO_USER_POOL_ID: !Ref 'CognitoUserPoolId'
          COGNITO_USER_POOL_CLIENT_ID: !Ref 'CognitoUserPoolClientId'
          COGNITO_REGION: !Ref 'AWS::Region'
      Layers:
        - !Ref AuthDependenciesLayer

AuthPermission:
    Type: AWS::Lambda::Permission
    DependsOn:
      - AuthFunction
      - Api
    Properties:
      Action: lambda:InvokeFunction
      FunctionName: !Ref AuthFunction
      Principal: apigateway.amazonaws.com

Authorizer:
    Type: AWS::ApiGatewayV2::Authorizer
    Properties:
      Name: !Sub '${AWS::StackName}-authorizer'
      ApiId: !Ref Api
      AuthorizerType: REQUEST
      AuthorizerUri: !Sub 'arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/${AuthFunction.Arn}/invocations'
      IdentitySource:
        - route.request.querystring.token
```

I also make sure my connection function uses the authorizer

```
      AuthorizationType: CUSTOM
      AuthorizerId: !Ref LambdaAuthorizer
```

On the console I can clearly see my authorizer being connected to the route. I also tested the lambda with a valid JWT, and it worked as expected. However whenever I try to connect to my websocket using wscat and the correct websocket url (wss://MY_API.execute-api.REGION.amazonaws.com/STAGE/) i can connect without giving a token. No cloudwatch logs are generated either.

Is there something I missed, something not written in the docs or is it a bug ?

I am on a time constraint so I would love the help. Alternatively I can implement the auth logic inside the connect lambda.
Thanks a lot

Varunprakash Shanmugam · Answer

1. **Authorizer not triggered on `$connect` route**: For WebSocket APIs, the custom authorizer only runs on the `$connect` route. Make sure you have explicitly set the `AuthorizationType` and `AuthorizerId` **on the `$connect` route**. If it's missing or misconfigured, the authorizer won’t be triggered.

2. **`route.request.querystring.token` not passed in test**: When using `wscat` to connect, you must include the token in the URL query string exactly as defined in `IdentitySource`. For example:
   ```
   wscat -c "wss://MY_API.execute-api.REGION.amazonaws.com/STAGE?token=eyJraWQiOi..."
   ```
   If the query param is missing or misnamed, the authorizer won't be called and the connection may be allowed.

3. **IAM permissions for `InvokeFunction`**: You have the `AWS::Lambda::Permission` block set up, but double-check that `!Ref AuthFunction` is actually resolving to the correct function name and that it includes the full ARN (especially if you're referencing it from another region or stack).

4. **Check deployment and stage configuration**: Sometimes changes don’t propagate unless you redeploy both the API and the stage. Try running:
   ```
   sam deploy --guided
   ```
   and ensure the deployment updates are actually applied to the correct stage.

5. **CloudWatch logging not showing anything**: This suggests that the authorizer lambda isn't being invoked at all. Reconfirm the `$connect` route is configured with the authorizer and redeploy. Also, verify that logging is enabled for the WebSocket API stage.

6. **Fallback approach**: If you're still running into a wall and are short on time, placing the token verification logic inside the `$connect` lambda function is a reasonable workaround. You can manually decode and validate the JWT there using a library like `python-jose`.

Custom lambda authorizer for websocket api not working

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