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Pre-signed URLs are a powerful feature of Amazon S3 that allow you to grant time-limited access to private objects without changing your bucket’s ACL or policy. This walkthrough explains how to:
  • Create an S3 bucket with secure default settings
  • Upload and verify objects
  • Generate pre-signed URLs via the AWS Management Console
  • Demonstrate permission boundaries with a restricted IAM user

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1. Create an S3 Bucket

Start by creating a new S3 bucket using default settings:
The image shows the AWS S3 console interface for creating a new bucket, with fields for bucket name, region selection, and object ownership settings.
Ensure all options remain at their defaults and block public access for enhanced security:
The image shows an AWS S3 console screen with settings for blocking public access to a bucket, including options for access control lists and policies. There is also a section for bucket versioning settings.
By blocking public access and relying on IAM, you safeguard your data against unintended exposure.
At this point, only your AWS account (root user or explicitly granted IAM identities) can access the bucket.

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2. Upload an Object

Next, upload a test file (e.g., boat.jpg) to your bucket:
The image shows an Amazon S3 management console with a list of two buckets, their regions, access status, and creation dates. The console also displays options for managing buckets and accessing various AWS services.
After initiating the upload, watch for the completion notification:
The image shows an AWS S3 Management Console screen indicating a successful upload of a file named "boat.jpg" with a size of 3.6 MB.
You can view the object details as an authenticated user:
The image shows an Amazon S3 console interface displaying details of an object named "boat.jpg" in a bucket. It includes information such as the object's size, type, last modified date, and URLs.
If you open the S3 URL in a private browser session, you’ll see an Access Denied error since the object is private.

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3. Generate a Pre-Signed URL via AWS Console

To share your object temporarily without altering bucket policies, generate a pre-signed URL:
  1. Open the object’s detail page and click Share with a pre-signed URL:
The image shows an Amazon S3 permissions overview page, indicating that the bucket and objects are not public, with block public access settings enabled.
  1. In the sharing dialog, set an expiration (e.g., 30 minutes) and click Create pre-signed URL:
The image shows an Amazon S3 console screen with a pop-up for sharing "boat.jpg" using a presigned URL, allowing temporary access to the object. The pop-up includes options to set the expiration time for the URL.
  1. The generated URL is automatically copied to your clipboard. Any user with this link can retrieve boat.jpg until it expires.
While the Console is great for one-off sharing, production workflows typically use the AWS SDK or AWS CLI to programmatically issue pre-signed URLs.

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4. Permission Demo with a Restricted IAM User

Let’s illustrate how IAM policies affect pre-signed URLs. We have a user named user2 who only has permissions to list buckets:
The image shows the AWS Identity and Access Management (IAM) console, displaying a list of users with details such as last activity and MFA status.
Below is the policy attached to user2: 
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor",
            "Effect": "Allow",
            "Action": [
                "s3:ListAllMyBuckets",
                "s3:ListBucket"
            ],
            "Resource": "*"
        }
    ]
}
Action AllowedDescription
s3:ListAllMyBucketsView all buckets in the account
s3:ListBucketList objects within a bucket
Since user2 lacks s3:GetObject, they receive an Access Denied error when directly accessing boat.jpg: 
<Error>
  <Code>AccessDenied</Code>
  <Message>Access Denied</Message>
  <RequestId>AHK7JSNDOI93/Y4/RequestId</RequestId>
  <HostId>ABN4lmOqPyYoV2StMdc9DlyhwFJxl.XoDAp1LhYgoKC8W0</HostId>
</Error>
If user2 generates a pre-signed URL (30-minute expiration) and an anonymous user tries to use it, they encounter the same Access Denied response: 
<Error>
  <Code>AccessDenied</Code>
  <Message>Access Denied</Message>
  <RequestId>5648S6RRMZ5F1S5T</RequestId>
  <HostId>1gXiuD2vm6W7cG7iWcRjdmxisz7sj6FtH8D9ArtnX3i2hdyg42IRzBTrFe0Zmg=</HostId>
</Error>
A pre-signed URL inherits the creator’s IAM permissions. To allow download, the issuer must have s3:GetObject.

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5. Conclusion

Amazon S3 pre-signed URLs offer a secure way to grant temporary, explicit access to private objects without exposing your entire bucket. They respect existing IAM and bucket policies, ensuring your data remains protected.

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