Securing AWS S3 Pre-Signed URLs

Travis Felder
3 min readAug 17, 2023
Photo by Scott Rodgerson on Unsplash

AWS pre-signed URLs are a powerful feature that allows temporary access to a specific S3 object without requiring AWS credentials. Here we will outline the secure implementation of pre-signed URLs, the associated risks, common use cases, and alternative solutions to consider.

Use Cases

1. Temporary Access to Files: Pre-signed URLs can be used to provide temporary access to files, such as images, videos, or documents, without exposing them to the public.

2. Third-Party Access: They enable third-party users to upload or download files without needing to manage AWS IAM (Identity and Access Management) credentials.

3. Streamlined User Experience: Pre-signed URLs can be used to create a seamless user experience by allowing direct uploads and downloads without intermediate servers.

Implementation

Step 1: Configure IAM Permissions

Ensure that the IAM user or role generating the pre-signed URL has the necessary permissions for the S3 object.

Step 2: Generate the Pre-Signed URL

Utilize the AWS SDK to create a pre-signed URL with a specific expiration time.

Step 3: Secure the URL

Implement security measures such as HTTPS and secure tokens to protect the URL from unauthorized access.

Step 4: Monitor and Log

Monitor and log the usage of pre-signed URLs to detect any suspicious activity.

Risks

1. URL Exposure: If a pre-signed URL is leaked, unauthorized users can access the S3 object until the URL expires.

2. Long Expiration Times: Setting a long expiration time can increase the risk of unauthorized access if the URL is compromised.

3. Lack of IP Restriction: Without IP restrictions, anyone with the URL can access the object from any location.

4. Replay Attacks: Attackers may reuse a pre-signed URL if proper security measures are not in place.

Mitigation Strategies

1. Set Short Expiration Times: Limit the validity of the URL to minimize exposure.

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