CORS configuration in AWS - Commands & Configuration

Introduction

Sometimes, web browsers block requests from one website to another for security. CORS configuration lets you tell browsers which websites can talk to your AWS resources safely.

When you want a web app hosted on one domain to access an AWS API on another domain.

When your frontend JavaScript needs to read data from an AWS S3 bucket in a different domain.

When you want to allow specific websites to upload files directly to your AWS S3 bucket.

When you want to prevent unauthorized websites from accessing your AWS resources.

When you want to enable safe sharing of resources between different web applications.

Config File - cors.json

cors.json

{
  "CORSRules": [
    {
      "AllowedHeaders": ["*"],
      "AllowedMethods": ["GET", "POST", "PUT"],
      "AllowedOrigins": ["https://example.com"],
      "ExposeHeaders": ["ETag"],
      "MaxAgeSeconds": 3000
    }
  ]
}

This JSON file sets CORS rules for an AWS S3 bucket.

• AllowedOrigins: Which websites can access the bucket.
• AllowedMethods: What actions are allowed (like GET to read, POST to upload).
• AllowedHeaders: Which headers the browser can send.
• ExposeHeaders: Which headers the browser can see in the response.
• MaxAgeSeconds: How long browsers can remember these rules.

Commands

This command applies the CORS rules from the cors.json file to the S3 bucket named my-example-bucket.

Terminal

aws s3api put-bucket-cors --bucket my-example-bucket --cors-configuration file://cors.json

Expected Output

No output (command runs silently)

→

--bucket - Specifies the name of the S3 bucket to configure.

→

--cors-configuration - Points to the JSON file with the CORS rules.

This command retrieves and shows the current CORS configuration of the S3 bucket to verify the settings.

Terminal

aws s3api get-bucket-cors --bucket my-example-bucket

Expected Output

{

    "CORSRules": [

        {

            "AllowedHeaders": [

                "*"

            ],

            "AllowedMethods": [

                "GET",

                "POST",

                "PUT"

            ],

            "AllowedOrigins": [

                "https://example.com"

            ],

            "ExposeHeaders": [

                "ETag"

            ],

            "MaxAgeSeconds": 3000

        }

    ]

}

→

--bucket - Specifies the name of the S3 bucket to check.

Key Concept

If you remember nothing else from this pattern, remember: CORS rules tell browsers which websites can safely access your AWS resources.

Common Mistakes

Using '*' as AllowedOrigins to allow all websites.

This can expose your resources to any website, causing security risks.

Specify only the trusted website URLs in AllowedOrigins.

Not including the correct HTTP methods in AllowedMethods.

Requests using methods not listed will be blocked by the browser.

List all HTTP methods your application needs, like GET, POST, PUT.

Forgetting to apply the CORS configuration after editing the JSON file.

Changes won't take effect until you run the command to update the bucket.

Always run the put-bucket-cors command after changing the config file.

Summary

Create a JSON file with CORS rules specifying allowed origins, methods, and headers.

Use the AWS CLI put-bucket-cors command to apply these rules to your S3 bucket.

Verify the configuration with get-bucket-cors to ensure your settings are active.

Practice

1. What does CORS stand for and why is it important in AWS S3 bucket configuration?

easy

A. Cross-Origin Resource Sharing; it controls access to resources from different websites.

B. Cloud-Origin Resource Security; it encrypts data in S3 buckets.

C. Cross-Object Resource Sync; it syncs files between buckets.

D. Cloud-Origin Resource Sharing; it manages AWS user permissions.

Solution

1. Step 1: Understand the acronym CORS

   CORS stands for Cross-Origin Resource Sharing, a web security feature.

2. Step 2: Identify its role in AWS S3

   CORS controls which websites can access resources in an S3 bucket, preventing unauthorized cross-site requests.

3. Final Answer:

   Cross-Origin Resource Sharing; it controls access to resources from different websites. -> Option A

4. Quick Check:

   CORS = Cross-Origin Resource Sharing [OK]

Hint: Remember CORS controls cross-site resource access [OK]

Common Mistakes:

• Confusing CORS with encryption or syncing
• Thinking CORS manages AWS user permissions
• Assuming CORS is only for AWS internal use

2. Which of the following is the correct JSON snippet to allow GET and POST methods from any origin in an S3 bucket CORS configuration?

easy

A. { "CORSRules": [{ "AllowedOrigins": ["http://example.com"], "AllowedMethods": ["PUT"] }] }

B. { "CORSRules": [{ "AllowedOrigins": ["*"], "AllowedMethods": ["DELETE"] }] }

C. { "CORSRules": [{ "AllowedOrigins": ["*"], "AllowedMethods": ["GET", "POST"] }] }

D. { "CORSRules": [{ "AllowedOrigins": ["*"], "AllowedMethods": ["PATCH"] }] }

Solution

1. Step 1: Identify AllowedOrigins and AllowedMethods

   The question asks for allowing any origin (*) and methods GET and POST.

2. Step 2: Match JSON snippet to requirements

   { "CORSRules": [{ "AllowedOrigins": ["*"], "AllowedMethods": ["GET", "POST"] }] } correctly sets AllowedOrigins to ["*"] and AllowedMethods to ["GET", "POST"].

3. Final Answer:

   { "CORSRules": [{ "AllowedOrigins": ["*"], "AllowedMethods": ["GET", "POST"] }] } -> Option C

4. Quick Check:

   AllowedOrigins=* and AllowedMethods=GET,POST [OK]

Hint: Look for AllowedOrigins '*' and methods GET, POST [OK]

Common Mistakes:

• Using wrong HTTP methods like DELETE or PATCH
• Setting AllowedOrigins to a specific domain instead of '*'
• Incorrect JSON structure or missing brackets

3. Given this CORS configuration snippet for an S3 bucket:

{
  "CORSRules": [
    {
      "AllowedOrigins": ["https://example.com"],
      "AllowedMethods": ["GET"],
      "AllowedHeaders": ["*"],
      "MaxAgeSeconds": 3000
    }
  ]
}

What will happen if a browser from https://example.com sends a POST request to this bucket?

medium

A. The POST request will succeed because MaxAgeSeconds is set.

B. The POST request will succeed because AllowedHeaders is '*'.

C. The POST request will be redirected to GET automatically.

D. The POST request will be blocked due to method not allowed.

Solution

1. Step 1: Check AllowedMethods in CORS configuration

   Only GET method is allowed from https://example.com.

2. Step 2: Understand effect on POST request

   POST is not in AllowedMethods, so the browser will block the request due to CORS policy.

3. Final Answer:

   The POST request will be blocked due to method not allowed. -> Option D

4. Quick Check:

   AllowedMethods=GET only, POST blocked [OK]

Hint: AllowedMethods must include the request method [OK]

Common Mistakes:

• Confusing AllowedHeaders with AllowedMethods
• Thinking MaxAgeSeconds affects allowed methods
• Assuming POST auto-converts to GET

4. You configured this CORS rule on your S3 bucket but your web app still gets CORS errors when making PUT requests from https://myapp.com:

{
  "CORSRules": [
    {
      "AllowedOrigins": ["https://myapp.com"],
      "AllowedMethods": ["GET", "POST"],
      "AllowedHeaders": ["*"]
    }
  ]
}

What is the most likely cause of the error?

medium

A. PUT method is not included in AllowedMethods.

B. AllowedOrigins should be '*', not a specific domain.

C. AllowedHeaders cannot be '*'.

D. CORSRules must include MaxAgeSeconds.

Solution

1. Step 1: Review AllowedMethods for PUT requests

   PUT is not listed in AllowedMethods, only GET and POST are allowed.

2. Step 2: Understand CORS behavior for unsupported methods

   Requests with methods not allowed by CORS are blocked by browsers.

3. Final Answer:

   PUT method is not included in AllowedMethods. -> Option A

4. Quick Check:

   AllowedMethods missing PUT causes CORS error [OK]

Hint: Check AllowedMethods includes your HTTP method [OK]

Common Mistakes:

• Assuming AllowedOrigins must be '*' for CORS to work
• Thinking AllowedHeaders '*' is invalid
• Believing MaxAgeSeconds is required

5. You want to configure an S3 bucket to allow only GET and OPTIONS requests from two specific domains: https://site1.com and https://site2.com. You also want browsers to cache the CORS response for 1 hour. Which is the correct CORS configuration JSON?

hard

A. {
"CORSRules": [
{
"AllowedOrigins": ["*"],
"AllowedMethods": ["GET", "OPTIONS"],
"AllowedHeaders": ["*"],
"MaxAgeSeconds": 3600
}
]
}

B. {
"CORSRules": [
{
"AllowedOrigins": ["https://site1.com", "https://site2.com"],
"AllowedMethods": ["GET", "OPTIONS"],
"AllowedHeaders": ["*"],
"MaxAgeSeconds": 3600
}
]
}

C. {
"CORSRules": [
{
"AllowedOrigins": ["https://site1.com"],
"AllowedMethods": ["GET"],
"AllowedHeaders": ["*"],
"MaxAgeSeconds": 3600
},
{
"AllowedOrigins": ["https://site2.com"],
"AllowedMethods": ["GET", "OPTIONS"],
"AllowedHeaders": ["*"],
"MaxAgeSeconds": 3600
}
]
}

D. {
"CORSRules": [
{
"AllowedOrigins": ["https://site1.com", "https://site2.com"],
"AllowedMethods": ["GET"],
"AllowedHeaders": ["*"],
"MaxAgeSeconds": 3600
}
]
}

Solution

1. Step 1: Check AllowedOrigins for both domains

   { "CORSRules": [ { "AllowedOrigins": ["https://site1.com", "https://site2.com"], "AllowedMethods": ["GET", "OPTIONS"], "AllowedHeaders": ["*"], "MaxAgeSeconds": 3600 } ] } correctly lists both https://site1.com and https://site2.com in AllowedOrigins.

2. Step 2: Verify AllowedMethods and MaxAgeSeconds

   { "CORSRules": [ { "AllowedOrigins": ["https://site1.com", "https://site2.com"], "AllowedMethods": ["GET", "OPTIONS"], "AllowedHeaders": ["*"], "MaxAgeSeconds": 3600 } ] } allows GET and OPTIONS methods and sets MaxAgeSeconds to 3600 (1 hour), matching requirements.

3. Final Answer:

   { "CORSRules": [ { "AllowedOrigins": ["https://site1.com", "https://site2.com"], "AllowedMethods": ["GET", "OPTIONS"], "AllowedHeaders": ["*"], "MaxAgeSeconds": 3600 } ] } -> Option B

4. Quick Check:

   AllowedOrigins two domains, methods GET & OPTIONS, cache 3600s [OK]

Hint: List all domains in AllowedOrigins and set MaxAgeSeconds for caching [OK]

Common Mistakes:

• Using '*' for AllowedOrigins when only specific domains are needed
• Missing OPTIONS method which browsers send for CORS preflight
• Splitting domains into multiple rules unnecessarily