What are Pre-Signed URLs in S3?

A Pre-Signed URL is a mechanism provided by Amazon S3 (Simple Storage Service) that allows users to access objects in private buckets without requiring AWS credentials or permissions. A pre-signed URL is generated with a specific expiration time and grants temporary access to an object for actions such as downloading or uploading.

Key Features of Pre-Signed URLs :

Time-Limited Access:
- A pre-signed URL expires after a specified time (e.g., 15 minutes, 1 hour).
- Once expired, the URL cannot be used to access the object.
Temporary Permissions:
- Permissions for accessing the object are tied to the user or application that generated the URL.
- Actions allowed (e.g., GET, PUT) are defined when creating the URL.
No Need for AWS Credentials:
- Users accessing the object with a pre-signed URL do not need direct AWS credentials or S3 permissions.
Supports GET, PUT, DELETE Operations:
- You can generate pre-signed URLs for downloading (GET), uploading (PUT), or deleting objects.

Common Use Cases :

Secure File Sharing:
- Temporarily share files stored in private S3 buckets with users without making the bucket public.
Direct Uploads from Clients:
- Allow users to upload files directly to S3 without routing through your backend server.
Controlled Access in Applications:
- Provide temporary access to resources for specific operations in web or mobile applications.

How It Works

Generate the URL:
- A user with valid AWS credentials generates the pre-signed URL using AWS SDKs (e.g., Python's boto3, Node.js, or AWS CLI).
- The URL contains:
  - The bucket and object key.
  - The allowed operation (e.g., GET or PUT).
  - A signature that validates the request.
  - An expiration timestamp.
Share the URL:
- The generated URL is shared with the intended recipient.
Recipient Accesses the Object:
- The recipient uses the URL to perform the specified operation within the allowed time frame.

Example Code (Python with Boto3)

Generate a Pre-Signed URL for Downloading:

import boto3
from botocore.exceptions import NoCredentialsError

# Initialize S3 client
s3_client = boto3.client('s3')

# Parameters
bucket_name = 'my-private-bucket'
object_key = 'example-file.txt'
expiration = 3600  # URL valid for 1 hour

try:
    # Generate pre-signed URL
    pre_signed_url = s3_client.generate_presigned_url(
        'get_object',
        Params={'Bucket': bucket_name, 'Key': object_key},
        ExpiresIn=expiration
    )
    print("Pre-Signed URL:", pre_signed_url)
except NoCredentialsError:
    print("AWS credentials not available.")

Generate a Pre-Signed URL for Uploading :

pre_signed_url = s3_client.generate_presigned_url(
    'put_object',
    Params={'Bucket': bucket_name, 'Key': object_key},
    ExpiresIn=expiration
)
print("Upload Pre-Signed URL:", pre_signed_url)

Advantages of Pre-Signed URLs :

Enhanced Security: Ensures private buckets remain private while allowing temporary access.
Granular Access Control: Specifies operation and expiration time.
Server Offloading: Enables direct uploads/downloads without passing through your server, reducing load.

Limitations :

Expiration Time: Once expired, a new URL must be generated.
Scope: URL permissions are limited to the action defined during its creation.
User Responsibility: The URL should be shared securely, as anyone with the URL can access the object.