Custom authentication challenge Lambda triggers
As you build out your authentication flows for your Amazon Cognito user pool, you might find that you want to extend your authentication model beyond the built-in flows. One common use case for the custom challenge triggers is to implement additional security checks beyond username, password, and multi-factor authentication (MFA). A custom challenge is any question and response you can generate in a Lambda-supported programming language. For example, you might want to require users to solve a CAPTCHA or answer a security question before being allowed to authenticate. Another potential need is to integrate with specialized authentication factors or devices. Or you might have already developed software that authenticates users with a hardware security key or a biometric device. The definition of authentication success for a custom challenge is whatever answer your Lambda function accepts as correct: a fixed string, for example, or a satisfactory response from an external API.
You can start authentication with your custom challenge and control the authentication process entirely, or you can perform username-password authentication before your application receives your custom challenge.
The custom authentication challenge Lambda trigger:
- Defines
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Initiates a challenge sequence. Determines whether you want to initiate a new challenge, mark authentication as complete, or halt the authentication attempt.
- Creates
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Issues the question to your application that the user must answer. This function might present a security question or a link to a CAPTCHA that your application should display to your user.
- Verifies
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Knows the expected answer and compares it to the answer your application provides in the challenge response. The function might call the API of your CAPTCHA service to retrieve the expected results of your user's attempted solution.
These three Lambda functions chain together to present an authentication mechanism that is completely within your control and of your own design. Because custom authentication requires application logic in your client and in the Lambda functions, you can't process custom authentication within the hosted UI. This authentication system requires additional developer effort. Your application must perform the authentication flow with the user pools API and handle the resulting challenge with a custom UI that renders the question at the center of the custom authentication challenge.
For more information about implementing custom authentication, see Custom authentication flow and challenges
Authentication between the API operations InitiateAuth or AdminInitiateAuth, and RespondToAuthChallenge or AdminRespondToAuthChallenge. In this flow, a user authenticates by answering
successive challenges until authentication either fails or the user is issued tokens. A
challenge response might be a new challenge. In this case, your application responds as many
times as necessary to new challenges. Successful authentication happens when the define auth
challenge function analyzes the results so far, determines all challenges have been
answered, and returns IssueTokens.
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