AWS CLI S3 A client error (403) occurred when calling the HeadObject operation: Forbidden

I'm trying to setup a Amazon Linux AMI(ami-f0091d91) and have a script that runs a copy command to copy from a S3 bucket.

 aws --debug s3 cp s3://aws-codedeploy-us-west-2/latest/codedeploy-agent.noarch.rpm .

This script works perfectly on my local machine but fails with the following error on the Amazon Image:

2016-03-22 01:07:47,110 - MainThread - botocore.auth - DEBUG - StringToSign:
HEAD


Tue, 22 Mar 2016 01:07:47 GMT
x-amz-security-token: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
/aws-codedeploy-us-west-2/latest/codedeploy-agent.noarch.rpm
2016-03-22 01:07:47,111 - MainThread - botocore.endpoint - DEBUG - Sending http request: <PreparedRequest [HEAD]>
2016-03-22 01:07:47,111 - MainThread - botocore.vendored.requests.packages.urllib3.connectionpool - INFO - Starting new HTTPS connection (1): aws-codedeploy-us-west-2.s3.amazonaws.com
2016-03-22 01:07:47,151 - MainThread - botocore.vendored.requests.packages.urllib3.connectionpool - DEBUG - "HEAD /latest/codedeploy-agent.noarch.rpm HTTP/1.1" 403 0
2016-03-22 01:07:47,151 - MainThread - botocore.parsers - DEBUG - Response headers: {'x-amz-id-2': '0mRvGge9ugu+KKyDmROm4jcTa1hAnA5Ax8vUlkKZXoJ//HVJAKxbpFHvOGaqiECa4sgon2F1kXw=', 'server': 'AmazonS3', 'transfer-encoding': 'chunked', 'x-amz-request-id': '6204CD88E880E5DD', 'date': 'Tue, 22 Mar 2016 01:07:46 GMT', 'content-type': 'application/xml'}
2016-03-22 01:07:47,152 - MainThread - botocore.parsers - DEBUG - Response body:

2016-03-22 01:07:47,152 - MainThread - botocore.hooks - DEBUG - Event needs-retry.s3.HeadObject: calling handler <botocore.retryhandler.RetryHandler object at 0x7f421075bcd0>
2016-03-22 01:07:47,152 - MainThread - botocore.retryhandler - DEBUG - No retry needed.
2016-03-22 01:07:47,152 - MainThread - botocore.hooks - DEBUG - Event after-call.s3.HeadObject: calling handler <function enhance_error_msg at 0x7f4211085758>
2016-03-22 01:07:47,152 - MainThread - botocore.hooks - DEBUG - Event after-call.s3.HeadObject: calling handler <awscli.errorhandler.ErrorHandler object at 0x7f421100cc90>
2016-03-22 01:07:47,152 - MainThread - awscli.errorhandler - DEBUG - HTTP Response Code: 403
2016-03-22 01:07:47,152 - MainThread - awscli.customizations.s3.s3handler - DEBUG - Exception caught during task execution: A client error (403) occurred when calling the HeadObject operation: Forbidden
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/awscli/customizations/s3/s3handler.py", line 100, in call
    total_files, total_parts = self._enqueue_tasks(files)
  File "/usr/local/lib/python2.7/site-packages/awscli/customizations/s3/s3handler.py", line 178, in _enqueue_tasks
    for filename in files:
  File "/usr/local/lib/python2.7/site-packages/awscli/customizations/s3/fileinfobuilder.py", line 31, in call
    for file_base in files:
  File "/usr/local/lib/python2.7/site-packages/awscli/customizations/s3/filegenerator.py", line 142, in call
    for src_path, extra_information in file_iterator:
  File "/usr/local/lib/python2.7/site-packages/awscli/customizations/s3/filegenerator.py", line 314, in list_objects
    yield self._list_single_object(s3_path)
  File "/usr/local/lib/python2.7/site-packages/awscli/customizations/s3/filegenerator.py", line 343, in _list_single_object
    response = self._client.head_object(**params)
  File "/usr/local/lib/python2.7/site-packages/botocore/client.py", line 228, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/usr/local/lib/python2.7/site-packages/botocore/client.py", line 488, in _make_api_call
    model=operation_model, context=request_context
  File "/usr/local/lib/python2.7/site-packages/botocore/hooks.py", line 226, in emit
    return self._emit(event_name, kwargs)
  File "/usr/local/lib/python2.7/site-packages/botocore/hooks.py", line 209, in _emit
    response = handler(**kwargs)
  File "/usr/local/lib/python2.7/site-packages/awscli/errorhandler.py", line 70, in __call__
    http_status_code=http_response.status_code)
ClientError: A client error (403) occurred when calling the HeadObject operation: Forbidden
2016-03-22 01:07:47,153 - Thread-1 - awscli.customizations.s3.executor - DEBUG - Received print task: PrintTask(message='A client error (403) occurred when calling the HeadObject operation: Forbidden', error=True, total_parts=None, warning=None)
A client error (403) occurred when calling the HeadObject operation: Forbidden

However, when I run it with the --no-sign-request option, it works perfectly:

 aws --debug --no-sign-request s3 cp s3://aws-codedeploy-us-west-2/latest/codedeploy-agent.noarch.rpm .

Can someone please explain what is going on?

in my case the problem was the Resource statement in the user access policy.

First we had "Resource": "arn:aws:s3:::BUCKET_NAME", but in order to have access to objects within a bucket you need a /* at the end: "Resource": "arn:aws:s3:::BUCKET_NAME/*"

From the AWS documentation:

Bucket access permissions specify which users are allowed access to the objects in a bucket and which types of access they have. Object access permissions specify which users are allowed access to the object and which types of access they have. For example, one user might have only read permission, while another might have read and write permissions.

I figured it out. I had an error in my cloud formation template that was creating the EC2 instances. As a result, the EC2 instances that were trying to access the above code deploy buckets, were in different regions (not us-west-2). It seems like the access policies on the buckets (owned by Amazon) only allow access from the region they belong in. When I fixed the error in my template (it was wrong parameter map), the error disappeared

Trying to solve this problem myself, I discovered that there is no HeadBucket permission. It looks like there is, because that's what the error message tells you, but actually the HEAD operation requires the ListBucket permission. I also discovered that my IAM policy and my bucket policy were conflicting. Make sure you check both.