AWS Assume Role via .Net SDK gives Access Denied but works with CLI

Question

I am trying to upload a file in S3 by AWS Assume Role. When I am trying to access it from CLI it works fine but from .Net SDK it gives me Access Denied error.

Here are the steps I followed in CLI -

1. Setup the access key/secret key for user using aws configure
2. Assume the Role - “aws sts assume-role --role-arn "arn:aws:iam::1010101010:role/Test-Account-Role" --role-session-name AWSCLI-Session”
3. Take the access key / secret key / session token from the assumed role and setup an AWS profile. The credentials are printed out/returned from the assumed role.
4. Switch to the assume role profile: “set AWS_PROFILE=”
5. Verify that the user has the role: “aws sts get-caller-identity”
6. Access the bucket using ls or cp or rm command - Works Successfully.

Now I am trying to access it from .Net core App -

Here is the code snippet- Note that I am using same Access and Secret key as CLI from my local.

        try
        {
            var region = RegionEndpoint.GetBySystemName(awsRegion);

            SessionAWSCredentials tempCredentials = await GetTemporaryCredentialsAsync(awsAccessKey, awsSecretKey, region, roleARN);

            //Use the temp credentials received to create the new client
            IAmazonS3 client = new AmazonS3Client(tempCredentials, region);
            
            TransferUtility utility = new TransferUtility(client);
            // making a TransferUtilityUploadRequest instance
            TransferUtilityUploadRequest request = new TransferUtilityUploadRequest
            {
                BucketName = bucketName, 
                Key = $"{subFolder}/{fileName}", 
                FilePath = localFilePath 
            utility.Upload(request); //transfer
            fileUploadedSuccessfully = true;

        }
        catch (AmazonS3Exception ex)
        {
            // HandleException
        }
        catch (Exception ex)
        {
             // HandleException
        }

The method to get temp credentials is as follow - GetTemporaryCredentialsAsync

  private static async Task<SessionAWSCredentials> GetTemporaryCredentialsAsync(string awsAccessKey, string awsSecretKey, RegionEndpoint region, string roleARN)
        {
            using (var stsClient = new AmazonSecurityTokenServiceClient(awsAccessKey, awsSecretKey, region))
            {

                var getSessionTokenRequest = new GetSessionTokenRequest
                {
                    DurationSeconds = 7200
                };

                await stsClient.AssumeRoleAsync(
                    new AssumeRoleRequest()
                    {
                        RoleArn = roleARN,
                        RoleSessionName = "mySession"
                    });

                GetSessionTokenResponse sessionTokenResponse =
                              await stsClient.GetSessionTokenAsync(getSessionTokenRequest);

                Credentials credentials = sessionTokenResponse.Credentials;

                var sessionCredentials =
                    new SessionAWSCredentials(credentials.AccessKeyId,
                                              credentials.SecretAccessKey,
                                              credentials.SessionToken);
                return sessionCredentials;
            }
        }

I am getting back the temp credentials but it gives me Access Denied while uploading the file. Not sure if I am missing anything here.

Also noted that the token generated via SDK is shorter than that from CLI. I tried pasting these temp credentials to local profile and then tried to access the bucket and getting the Access Denied error then too.

Answer 1

There is an AWS .NET V3 example that shows this exact use case. To assume a role, you use an AmazonSecurityTokenServiceClient. In this example, the user assumes the role that allows the role to be used to list all S3 buckets. See this .NET scenario here.

https://github.com/awsdocs/aws-doc-sdk-examples/blob/main/dotnetv3/IAM/Scenarios/IAMBasics/IAMBasics.cs