In Fargate container why can I CRUD S3 but can't create a presigned post - amazon-s3

Question

I'm using node in a docker container and locally I use my IAM keys for both creating, reading and deleting files to an S3 bucket as well as creating pre-signed posts. When up on a Fargate container, I create a taskRole and attach a policy which gives it full access to S3.
taskRole.attachInlinePolicy(
new iam.Policy(this, `${clientPrefix}-task-policy`, {
statements: [
new iam.PolicyStatement({
effect: iam.Effect.ALLOW,
actions: ['S3:*'],
resources: ['*'],
}),
],
})
);
With that role, I can create, read and delete files with no issues from the API. When the API tries to create a pre-signed post however, I get the error:
Error: Unable to create a POST object policy without a bucket, region, and credentials
It seems super strange to me that I can run the other operations, but it fails with the presignedPOST, especially since my S3 actions are all allowed.
const post: aws.S3.PresignedPost = await s3.createPresignedPost({
Bucket: bucket,
Fields: { key },
Expires: 60,
Conditions: [['content-length-range', 0, 5242880]],
});
Here is the code I use. I am logging the bucket and key so I'm positive that they are valid values. One thought I had was when running locally, I will run aws.configure to set my keys but in Fargate I purposefully omit that. I thought that it was getting the right keys since the other s3 operations work without fail. Am I approaching this right?

Answer 1

When using IAM role credentials with AWS sdk, you must either use the asynchronous (callback) version of createPresignedPost or guarantee that your credentials have been resolved before calling the await version of this method.
Something like this will work with IAM based credentials:
const s3 = new AWS.S3()
const _presign = params => {
return new Promise((res, rej) => {
s3.createPresignedPost(params, (err, data) => {
if (err) return rej(err)
return res(data)
})
})
}
// await _presign(...) <- works
// await s3.createPresignedPost(...) <- won't work
Refer: https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html#createPresignedPost-property