CORS configuration in AWS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allow all origins in the CORS configuration for an S3 bucket.

AWS

{
  "CORSRules": [
    {
      "AllowedOrigins": ["[1]"],
      "AllowedMethods": ["GET"]
    }
  ]
}

Drag options to blanks, or click blank then click option'

A"*"

B"https://example.com"

C"http://localhost:3000"

D"https://mydomain.com"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a specific origin when the task asks for all origins.

Forgetting to put quotes around the origin string.

Using "*" in AllowedOrigins allows all origins to access the resource.

2fill in blank

medium

Complete the code to allow the POST method in the CORS configuration.

AWS

{
  "CORSRules": [
    {
      "AllowedOrigins": ["https://example.com"],
      "AllowedMethods": ["GET", "[1]"]
    }
  ]
}

Drag options to blanks, or click blank then click option'

A"PUT"

B"DELETE"

C"POST"

D"PATCH"

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing POST with PUT or DELETE methods.

Not including the method in quotes.

Adding "POST" to AllowedMethods allows POST requests from the specified origin.

3fill in blank

hard

Fix the error in the CORS configuration by completing the missing header name.

AWS

{
  "CORSRules": [
    {
      "AllowedOrigins": ["https://example.com"],
      "AllowedMethods": ["GET"],
      "AllowedHeaders": ["[1]"]
    }
  ]
}

Drag options to blanks, or click blank then click option'

A"Authorization"

B"Content-Type"

C"X-Custom-Header"

D"Accept"

Attempts:

3 left

💡 Hint

Common Mistakes

Using headers not needed for the request.

Forgetting to include quotes around header names.

"Content-Type" header is commonly required to allow browsers to send content type information in requests.

4fill in blank

hard

Fill the blanks to allow GET and PUT methods from any origin.

AWS

{
  "CORSRules": [
    {
      "AllowedOrigins": [[1]],
      "AllowedMethods": [[2], [3]]
    }
  ]
}

Drag options to blanks, or click blank then click option'

A"*"

B"GET"

C"PUT"

D"POST"

Attempts:

3 left

💡 Hint

Common Mistakes

Using specific origins instead of wildcard.

Using wrong methods like POST.

Forgetting quotes around strings.

"*" allows all origins, "GET" and "PUT" allow those methods from any origin.

5fill in blank

hard

Fill all three blanks to allow POST and DELETE methods from https://myapp.com with headers Authorization and Content-Type.

AWS

{
  "CORSRules": [
    {
      "AllowedOrigins": [[1]],
      "AllowedMethods": [[2], [3]],
      "AllowedHeaders": ["Authorization", "Content-Type"]
    }
  ]
}

Drag options to blanks, or click blank then click option'

A"https://myapp.com"

B"POST"

C"DELETE"

D"GET"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wildcard origin instead of specific origin.

Including methods not requested.

Forgetting to put quotes around strings.

AllowedOrigins must be the specific origin "https://myapp.com". AllowedMethods include "POST" and "DELETE" as requested.

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