AWS: Restrict Cognito Authorized User to specific Lambda Functions - amazon-web-services

Question

I'm working with AWS and I've the following setup: UserPool; API Gateway, Lambda Functions
The api gateway is using a UserPool authorizer to protect the lambda functions. This is working so far. Now I want to restrict every lambda function to a specific group of users. Therefore I've created two user groups in the CognitoPool (user and admin) and I've assigned a specific role to each group with a policy. Afterwards I've created a user in the UserPool and added him to the user group. That user is still able to submit requests to each route/lambda function.
How do I submit a request?
Postman
set IdToken (of the authenticated user) in the Authorization header
without Authorization header the response is a 401 (as expected)
with Authorization header every lambda function can be triggered (not expected)
Configuration of the UserPool Groups:
Group User:
Arn: Role ARN: arn:aws:iam::xxxxxx:role/User
UserRole is specified as
{
"Version": "2012-10-17",
"Statement": [
"Action": [
"lambda:InvokeFunction",
"lambda:InvokeAsync"
],
"Resource": [
"arn:aws:lambda:region:xxxxxx:function:api-dev-getItems
],
"Effect": "Allow"
]
}
Group Admin:
Arn: Role ARN: arn:aws:iam::xxxxxx:role/Admin
AdminRole is specified as
{
"Version": "2012-10-17",
"Statement": [
"Action": [
"lambda:InvokeFunction",
"lambda:InvokeAsync"
],
"Resource": [
"arn:aws:lambda:region:xxxxxx:function:api-dev-getItems
"arn:aws:lambda:region:xxxxxx:function:api-dev-getUsers
],
"Effect": "Allow"
]
}
The payload of the id token also contains:
'cognito:roles': [ 'arn:aws:iam::xxxxxx:role/User' ]

Answer 1

So I've found a solution to my problem. Here is the summary of my experiences:
Cognito Authorizer is more like a yes/no authorizer (authenticated or not; user groups are not evaluated)
Therefore I went with AWS IAM Authorizer in the API Gateway, which will evaluate the user group roles
Instead of a JWT the AWS signature v4 authorization has to be passed (there is a plugin for postman and several packages on npm)
Since I am using an API Gateway I had to change the role policy resources to execute-api:Invoke
In detail:
UserRole:
{
"Version": "2012-10-17",
"Statement": [
"Action": [
"lambda:InvokeFunction",
"lambda:InvokeAsync"
],
"Resource": [
"arn:aws:execute-api:region:accountid:api-id/stage/GET/items
],
"Effect": "Allow"
]
}
AdminRole:
{
"Version": "2012-10-17",
"Statement": [
"Action": [
"lambda:InvokeFunction",
"lambda:InvokeAsync"
],
"Resource": [
"arn:aws:execute-api:region:accountid:api-id/stage/GET/items
"arn:aws:execute-api:region:accountid:api-id/stage/*/users
],
"Effect": "Allow"
]
}
Instead of passing the ID Token into the Authorization header, I had to use Postman AWS Signature, which requires at least an AccessKey and a SecretKey. Those two can be retrieved when I sign in my user using the aws-sdk. aws-sdk-js with TypeScript as example:
import { CognitoUserPool, CognitoUser, AuthenticationDetails } from 'amazon-cognito-identity-js';
const userPool = new CognitoUserPool({
UserPoolId: 'my pool id',
ClientId: 'my client id'
});
function signIn(username: string, password: string) {
const authData = {
Username: username,
Password: password,
};
const authDetails = new AuthenticationDetails(authData);
const userData = {
Username: username,
Pool: userPool,
};
const cognitoUser = new CognitoUser(userData);
cognitoUser.authenticateUser(authDetails, {
onSuccess: (result) => {
const cognitoIdpKey = `cognito-idp.${region}.amazonaws.com/${userPool.getUserPoolId()}`;
const credentials = new AWS.CognitoIdentityCredentials({
IdentityPoolId: 'identity pool id,
Logins: {
[cognitoIdpKey]: result.getIdToken().getJwtToken(),
}
});
AWS.config.update({
credentials,
});
credentials.refreshPromise()
.then(() => {
console.log('Success refresh. Required data:', (credentials as any).data.Credentials);
})
.catch(err => console.error('credentials refresh', err));
}
});
}