CloudFront with Lambda#Edge results in CORS problem - amazon-web-services

Question

I've configured S3 with access only through CloudFront and protected with lambda executed on Viewer request. The problem is that I'm not able to access the files from SPA because of a failing preflight call.
When I removed the lambda function everything is starting to work. This is surprising to me because lambda is not modifying the request at all.
Here is my configuration:
S3:
CloudFront:
Lambda#Edge (executed at Viewer request)
exports.handler = async (event, context, callback) => {
let request;
let token;
try {
request = event.Records[0].cf.request;
const headers = request.headers;
const authorization = headers['authorization'][0];
const authorizationValue = authorization.value;
token = authorizationValue.substring(7);
} catch (error) {
console.error("Missing authorization header", error);
callback(null, missingAuthorizationHeaderResponse);
}
if (token) {
try {
if (!secret) {
secret = await getSecret();
}
jwt.verify(token, secret);
console.log("Token valid");
callback(null, request);
} catch (error) {
console.error("Token not valid", error);
callback(null, invalidTokenResponse);
}
} else {
console.error("Token not found");
callback(null, missingAuthorizationHeaderResponse);
}
};
I will be very grateful for help since I've spent a lot of time on this case, thanks!

Answer 1

The problem is that the preflight call are executed without any additional headers and in my case "authorization" header was missing and was generating 403. I found that by looking into logs of the lambda. I've added handling of options call to the lambda. Also I had to change s3 config to have the response with visible CORS headers.
Lambda Code:
const preflightCall = {
status: "204",
headers: {
'access-control-allow-origin': [{
key: 'Access-Control-Allow-Origin',
value: "*",
}],
'access-control-request-method': [{
key: 'Access-Control-Request-Method',
value: "PUT, GET, OPTIONS, DELETE",
}],
'access-control-allow-headers': [{
key: 'Access-Control-Allow-Headers',
value: "*",
}]
},
};
exports.handler = async (event, context, callback) => {
let request;
let token;
try {
request = event.Records[0].cf.request;
if(request.method === 'OPTIONS') {
console.log('preflight call');
callback(null, preflightCall);
return;
}
const headers = request.headers;
const authorization = headers['authorization'][0];
const authorizationValue = authorization.value;
token = authorizationValue.substring(7);
} catch (error) {
console.error("Missing authorization header", error);
callback(null, missingAuthorizationHeaderResponse);
}
if (token) {
try {
if (!secret) {
secret = await getSecret();
}
jwt.verify(token, secret);
console.log("Token valid");
callback(null, request);
} catch (error) {
console.error("Token not valid", error);
callback(null, invalidTokenResponse);
}
} else {
console.error("Token not found");
callback(null, missingAuthorizationHeaderResponse);
}
};
S3:
[
{
"AllowedHeaders": [
"Authorization"
],
"AllowedMethods": [
"GET",
"HEAD",
"DELETE",
"POST",
"PUT"
],
"AllowedOrigins": [
"*"
],
"ExposeHeaders": [
"Access-Control-Allow-Origin"
],
"MaxAgeSeconds": 3000
}
]