When developing web applications that interact with AWS S3, developers might occasionally encounter the dreaded CORS (Cross-Origin Resource Sharing) issues. These issues can appear in various scenarios, one notable case being while using presigned URLs. In this blog post, we’ll dive deep into what causes these CORS issues and how to resolve them.
What is CORS?
Cross-Origin Resource Sharing (CORS) is a security feature implemented by web browsers. It controls how web pages from one origin can request and interact with resources on another origin.
Imagine a scenario where a web application hosted on https://example.com tries to fetch a resource from an S3 bucket with a different origin (like https://mybucket.s3.amazonaws.com). Without the right CORS headers from the S3 bucket, the browser will block this request, leading to a CORS error.
When Do CORS Issues Arise with S3?
CORS issues typically arise when:
- An S3 bucket’s CORS policy doesn’t permit requests from the domain in question.
- The client-side request doesn’t match what the S3 bucket’s CORS policy allows in terms of methods or headers.
- There’s a region or header mismatch during the request.
One notable use case where CORS errors might surprise you is when using presigned URLs. These are temporary URLs generated to grant temporary access to a private object in S3. Even if the URL is valid and the object is accessible, if the CORS configuration isn’t set up correctly, accessing the object via the browser can lead to a CORS error.
How to Resolve CORS Issues with S3
Configure the S3 Bucket’s CORS Policy:
Navigate to AWS Management Console -> S3 -> Select your bucket -> Permissions -> CORS configuration.
Here’s a basic CORS policy example:
[
{
"AllowedHeaders": ["*"],
"AllowedMethods": ["GET", "PUT", "POST", "DELETE"],
"AllowedOrigins": ["*"],
"MaxAgeSeconds": 3000
}
]For security, avoid using “*” for AllowedOrigins in production. Specify the exact domains that should have access.
