Use API Gateway Lambda authorizers - Amazon API Gateway
Lambda authorizer authorization workflowChoosing a type of Lambda authorizerExample REQUEST authorizer Lambda functionExample TOKEN authorizer Lambda functionAdditional examples of Lambda authorizer functions

Use API Gateway Lambda authorizers

Use a Lambda authorizer (formerly known as a custom authorizer) to control access to your API. When a client makes a request your API's method, API Gateway calls your Lambda authorizer. The Lambda authorizer takes the caller's identity as the input and returns an IAM policy as the output.

Use a Lambda authorizer to implement a custom authorization scheme. Your scheme can use request parameters to determine the caller's identity or use a bearer token authentication strategy such as OAuth or SAML. Create a Lambda authorizer in the API Gateway REST API console, using the AWS CLI, or an AWS SDK.

Lambda authorizer authorization workflow

The following diagram shows the authorization workflow for a Lambda authorizer.

API Gateway Lambda authorization workflow
API Gateway Lambda authorization workflow

  1. The client calls a method on an API Gateway API, passing a bearer token or request parameters.

  2. API Gateway checks if the method request is configured with a Lambda authorizer. If it is, API Gateway calls the Lambda function.

  3. The Lambda function authenticates the caller. The function can authenticate in the following ways:

    • By calling out to an OAuth provider to get an OAuth access token.

    • By calling out to a SAML provider to get a SAML assertion.

    • By generating an IAM policy based on the request parameter values.

    • By retrieving credentials from a database.

  4. The Lambda function returns an IAM policy and a principal identifier. If the Lambda function does not return that information, the call fails.

  5. API Gateway evaluates the IAM policy.

    • If access is denied, API Gateway returns a suitable HTTP status code, such as 403 ACCESS_DENIED.

    • If access is allowed, API Gateway invokes the method.

      If you enable authorization caching, API Gateway caches the policy so that the Lambda authorizer function isn't invoked again.

You can customize the 403 ACCESS_DENIED or the 401 UNAUTHORIZED gateway responses. To learn more, see Gateway responses for REST APIs in API Gateway.

Choosing a type of Lambda authorizer

There are two types of Lambda authorizers:

Request parameter-based Lambda authorizer (REQUEST authorizer)

A REQUEST authorizer receives the caller's identity in a combination of headers, query string parameters, stageVariables, and $context variables. You can use a REQUEST authorizer to create fine-grained policies based on the information from multiple identity sources, such as the $context.path and $context.httpMethod context variables.

If you turn on authorization caching for a REQUEST authorizer, API Gateway verifies that all specified identity sources are present in the request. If a specified identify source is missing, null, or empty, API Gateway returns a 401 Unauthorized HTTP response without calling the Lambda authorizer function. When multiple identity sources are defined, they are all used to derive the authorizer's cache key, with the order preserved. You can define a fine-grained cache key by using multiple identity sources.

If you change any of the cache key parts, and redeploy your API, the authorizer discards the cached policy document and generates a new one.

If you turn off authorization caching for a REQUEST authorizer, API Gateway directly passes the request to the Lambda function.

Token-based Lambda authorizer (TOKEN authorizer)

A TOKEN authorizer receives the caller's identity in a bearer token, such as a JSON Web Token (JWT) or an OAuth token.

If you turn on authorization caching for a TOKEN authorizer, the header name specified in the token source becomes the cache key.

Additionally, you can use token validation to enter a RegEx statement. API Gateway performs initial validation of the input token against this expression and invokes the Lambda authorizer function upon successful validation. This helps reduce calls to your API.

The IdentityValidationExpression property is supported for TOKEN authorizers only. For more information, see x-amazon-apigateway-authorizer object.

Note

We recommend that you use a REQUEST authorizer to control access to your API. You can control access to your API based on multiple identity sources when using a REQUEST authorizer, compared to a single identity source when using a TOKEN authorizer. In addition, you can separate cache keys using multiple identity sources for a REQUEST authorizer.

Example REQUEST authorizer Lambda function

The following example code creates a Lambda authorizer function that allows a request if the client-supplied HeaderAuth1 header, QueryString1 query parameter, and stage variable of StageVar1 all match the specified values of headerValue1, queryValue1, and stageValue1, respectively.

Node.js
// A simple request-based authorizer example to demonstrate how to use request 
// parameters to allow or deny a request. In this example, a request is  
// authorized if the client-supplied HeaderAuth1 header, QueryString1
// query parameter, and stage variable of StageVar1 all match
// specified values of 'headerValue1', 'queryValue1', and 'stageValue1',
// respectively.
    
export const handler = function(event, context, callback) {
    console.log('Received event:', JSON.stringify(event, null, 2));
    
    // Retrieve request parameters from the Lambda function input:
    var headers = event.headers;
    var queryStringParameters = event.queryStringParameters;
    var pathParameters = event.pathParameters;
    var stageVariables = event.stageVariables;
        
    // Parse the input for the parameter values
    var tmp = event.methodArn.split(':');
    var apiGatewayArnTmp = tmp[5].split('/');
    var awsAccountId = tmp[4];
    var region = tmp[3];
    var restApiId = apiGatewayArnTmp[0];
    var stage = apiGatewayArnTmp[1];
    var method = apiGatewayArnTmp[2];
    var resource = '/'; // root resource
    if (apiGatewayArnTmp[3]) {
        resource += apiGatewayArnTmp[3];
    }
        
    // Perform authorization to return the Allow policy for correct parameters and 
    // the 'Unauthorized' error, otherwise.

     
    if (headers.HeaderAuth1 === "headerValue1"
        && queryStringParameters.QueryString1 === "queryValue1"
        && stageVariables.StageVar1 === "stageValue1") {
        callback(null, generateAllow('me', event.methodArn));
    }  else {
        callback("Unauthorized");
    }
}
     
// Help function to generate an IAM policy
var generatePolicy = function(principalId, effect, resource) {
    // Required output:
    var authResponse = {};
    authResponse.principalId = principalId;
    if (effect && resource) {
        var policyDocument = {};
        policyDocument.Version = '2012-10-17'; // default version
        policyDocument.Statement = [];
        var statementOne = {};
        statementOne.Action = 'execute-api:Invoke'; // default action
        statementOne.Effect = effect;
        statementOne.Resource = resource;
        policyDocument.Statement[0] = statementOne;
        authResponse.policyDocument = policyDocument;
    }
    // Optional output with custom properties of the String, Number or Boolean type.
    authResponse.context = {
        "stringKey": "stringval",
        "numberKey": 123,
        "booleanKey": true
    };
    return authResponse;
}
     
var generateAllow = function(principalId, resource) {
    return generatePolicy(principalId, 'Allow', resource);
}
     
var generateDeny = function(principalId, resource) {
    return generatePolicy(principalId, 'Deny', resource);
}
Python
# A simple request-based authorizer example to demonstrate how to use request
# parameters to allow or deny a request. In this example, a request is
# authorized if the client-supplied headerauth1 header, QueryString1
# query parameter, and stage variable of StageVar1 all match
# specified values of 'headerValue1', 'queryValue1', and 'stageValue1',
# respectively.

import json


def lambda_handler(event, context):
    print(event)

    # Retrieve request parameters from the Lambda function input:
    headers = event['headers']
    queryStringParameters = event['queryStringParameters']
    pathParameters = event['pathParameters']
    stageVariables = event['stageVariables']

    # Parse the input for the parameter values
    tmp = event['methodArn'].split(':')
    apiGatewayArnTmp = tmp[5].split('/')
    awsAccountId = tmp[4]
    region = tmp[3]
    restApiId = apiGatewayArnTmp[0]
    stage = apiGatewayArnTmp[1]
    method = apiGatewayArnTmp[2]
    resource = '/'

    if (apiGatewayArnTmp[3]):
        resource += apiGatewayArnTmp[3]

    # Perform authorization to return the Allow policy for correct parameters
    # and the 'Unauthorized' error, otherwise.

    if (headers['HeaderAuth1'] == "headerValue1" and queryStringParameters['QueryString1'] == "queryValue1" and stageVariables['StageVar1'] == "stageValue1"):
        response = generateAllow('me', event['methodArn'])
        print('authorized')
        return response
    else:
        print('unauthorized')
        raise Exception('Unauthorized') # Return a 401 Unauthorized response

    # Help function to generate IAM policy


def generatePolicy(principalId, effect, resource):
    authResponse = {}
    authResponse['principalId'] = principalId
    if (effect and resource):
        policyDocument = {}
        policyDocument['Version'] = '2012-10-17'
        policyDocument['Statement'] = []
        statementOne = {}
        statementOne['Action'] = 'execute-api:Invoke'
        statementOne['Effect'] = effect
        statementOne['Resource'] = resource
        policyDocument['Statement'] = [statementOne]
        authResponse['policyDocument'] = policyDocument

    authResponse['context'] = {
        "stringKey": "stringval",
        "numberKey": 123,
        "booleanKey": True
    }

    return authResponse


def generateAllow(principalId, resource):
    return generatePolicy(principalId, 'Allow', resource)


def generateDeny(principalId, resource):
    return generatePolicy(principalId, 'Deny', resource)

In this example, the Lambda authorizer function checks the input parameters and acts as follows:

  • If all the required parameter values match the expected values, the authorizer function returns a 200 OK HTTP response and an IAM policy that looks like the following, and the method request succeeds:

    {
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "execute-api:Invoke",
      "Effect": "Allow",
      "Resource": "arn:aws:execute-api:us-east-1:123456789012:ivdtdhp7b5/ESTestInvoke-stage/GET/"
    }
  ]
}

  • Otherwise, the authorizer function returns a 401 Unauthorized HTTP response, and the method request fails.

In addition to returning an IAM policy, the Lambda authorizer function must also return the caller's principal identifier. Optionally, it can return a context object containing additional information that can be passed into the integration backend. For more information, see Output from an API Gateway Lambda authorizer.

In production code, you might need to authenticate the user before granting authorization. You can add authentication logic in the Lambda function by calling an authentication provider as directed in the documentation for that provider.

Example TOKEN authorizer Lambda function

The following example code creates a TOKEN Lambda authorizer function that allows a caller to invoke a method if the client-supplied token value is allow. The caller is not allowed to invoke the request if the token value is deny. If the token value is unauthorized or an empty string, the authorizer function returns an 401 UNAUTHORIZED response.

Node.js
// A simple token-based authorizer example to demonstrate how to use an authorization token 
// to allow or deny a request. In this example, the caller named 'user' is allowed to invoke 
// a request if the client-supplied token value is 'allow'. The caller is not allowed to invoke 
// the request if the token value is 'deny'. If the token value is 'unauthorized' or an empty
// string, the authorizer function returns an HTTP 401 status code. For any other token value, 
// the authorizer returns an HTTP 500 status code. 
// Note that token values are case-sensitive.

export const handler =  function(event, context, callback) {
    var token = event.authorizationToken;
    switch (token) {
        case 'allow':
            callback(null, generatePolicy('user', 'Allow', event.methodArn));
            break;
        case 'deny':
            callback(null, generatePolicy('user', 'Deny', event.methodArn));
            break;
        case 'unauthorized':
            callback("Unauthorized");   // Return a 401 Unauthorized response
            break;
        default:
            callback("Error: Invalid token"); // Return a 500 Invalid token response
    }
};

// Help function to generate an IAM policy
var generatePolicy = function(principalId, effect, resource) {
    var authResponse = {};
    
    authResponse.principalId = principalId;
    if (effect && resource) {
        var policyDocument = {};
        policyDocument.Version = '2012-10-17'; 
        policyDocument.Statement = [];
        var statementOne = {};
        statementOne.Action = 'execute-api:Invoke'; 
        statementOne.Effect = effect;
        statementOne.Resource = resource;
        policyDocument.Statement[0] = statementOne;
        authResponse.policyDocument = policyDocument;
    }
    
    // Optional output with custom properties of the String, Number or Boolean type.
    authResponse.context = {
        "stringKey": "stringval",
        "numberKey": 123,
        "booleanKey": true
    };
    return authResponse;
}
Python
# A simple token-based authorizer example to demonstrate how to use an authorization token
# to allow or deny a request. In this example, the caller named 'user' is allowed to invoke
# a request if the client-supplied token value is 'allow'. The caller is not allowed to invoke
# the request if the token value is 'deny'. If the token value is 'unauthorized' or an empty
# string, the authorizer function returns an HTTP 401 status code. For any other token value,
# the authorizer returns an HTTP 500 status code.
# Note that token values are case-sensitive.

import json


def lambda_handler(event, context):
    token = event['authorizationToken']
    if token == 'allow':
        print('authorized')
        response = generatePolicy('user', 'Allow', event['methodArn'])
    elif token == 'deny':
        print('unauthorized')
        response = generatePolicy('user', 'Deny', event['methodArn'])
    elif token == 'unauthorized':
        print('unauthorized')
        raise Exception('Unauthorized')  # Return a 401 Unauthorized response
        return 'unauthorized'
    try:
        return json.loads(response)
    except BaseException:
        print('unauthorized')
        return 'unauthorized'  # Return a 500 error


def generatePolicy(principalId, effect, resource):
    authResponse = {}
    authResponse['principalId'] = principalId
    if (effect and resource):
        policyDocument = {}
        policyDocument['Version'] = '2012-10-17'
        policyDocument['Statement'] = []
        statementOne = {}
        statementOne['Action'] = 'execute-api:Invoke'
        statementOne['Effect'] = effect
        statementOne['Resource'] = resource
        policyDocument['Statement'] = [statementOne]
        authResponse['policyDocument'] = policyDocument
    authResponse['context'] = {
        "stringKey": "stringval",
        "numberKey": 123,
        "booleanKey": True
    }
    authResponse_JSON = json.dumps(authResponse)
    return authResponse_JSON

In this example, when the API receives a method request, API Gateway passes the source token to this Lambda authorizer function in the event.authorizationToken attribute. The Lambda authorizer function reads the token and acts as follows:

  • If the token value is allow, the authorizer function returns a 200 OK HTTP response and an IAM policy that looks like the following, and the method request succeeds:

    {
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "execute-api:Invoke",
      "Effect": "Allow",
      "Resource": "arn:aws:execute-api:us-east-1:123456789012:ivdtdhp7b5/ESTestInvoke-stage/GET/"
    }
  ]
}

  • If the token value is deny, the authorizer function returns a 200 OK HTTP response and a Deny IAM policy that looks like the following, and the method request fails:

    {
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "execute-api:Invoke",
      "Effect": "Deny",
      "Resource": "arn:aws:execute-api:us-east-1:123456789012:ivdtdhp7b5/ESTestInvoke-stage/GET/"
    }
  ]
}
    Note

    Outside of the test environment, API Gateway returns a 403 Forbidden HTTP response and the method request fails.

  • If the token value is unauthorized or an empty string, the authorizer function returns a 401 Unauthorized HTTP response, and the method call fails.

  • If the token is anything else, the client receives a 500 Invalid token response, and the method call fails.

In addition to returning an IAM policy, the Lambda authorizer function must also return the caller's principal identifier. Optionally, it can return a context object containing additional information that can be passed into the integration backend. For more information, see Output from an API Gateway Lambda authorizer.

In production code, you might need to authenticate the user before granting authorization. You can add authentication logic in the Lambda function by calling an authentication provider as directed in the documentation for that provider.

Additional examples of Lambda authorizer functions

The following list shows additional examples of Lambda authorizer functions. You can create a Lambda function in the same account, or a different account, from where you created your API.

For the previous example Lambda functions, you can use the built-in AWSLambdaBasicExecutionRole, as these functions don't call other AWS services. If your Lambda function calls other AWS services, you'll need to assign an IAM execution role to the Lambda function. To create the role, follow the instructions in AWS Lambda Execution Role.

Additional examples of Lambda authorizer functions