Cross-Origin Resource Sharing (CORS)

Definition

Cross-Origin Resource Sharing (CORS) is a security feature implemented in web browsers that allows restricted resources on a web page to be requested from another domain outside the domain from which the first resource was served. This mechanism is crucial for web application architecture as it helps define how browsers handle cross-origin requests and protect users from malicious attacks, while still allowing legitimate cross-origin requests for resources like APIs or fonts.

5 Must Know Facts For Your Next Test

1. CORS is essential for modern web applications that rely on APIs hosted on different domains, facilitating secure data exchange.
2. When a browser makes a cross-origin request, it checks the server's CORS policy through specific HTTP headers, determining if the request should be allowed or blocked.
3. If a preflight request is required, it checks if the actual request type (like PUT or DELETE) is permitted before sending it to the server.
4. CORS can be configured using various HTTP response headers, allowing developers to fine-tune which domains can access their resources and under what conditions.
5. Improper CORS configuration can lead to security vulnerabilities, such as exposing sensitive data to unauthorized domains.

Review Questions

• How does CORS enhance security in web applications that make cross-origin requests?
  • CORS enhances security by implementing a set of rules that browsers follow when handling cross-origin requests. By using HTTP headers like 'Access-Control-Allow-Origin', servers can specify which domains are allowed to access their resources. This helps prevent malicious sites from making unauthorized requests to sensitive data, ensuring that only trusted origins can interact with the resources.
• Discuss the role of preflight requests in CORS and their impact on web application performance.
  • Preflight requests play a critical role in CORS by allowing browsers to check if cross-origin requests are safe before actually sending them. When a request method other than GET or POST is used, or when custom headers are included, the browser sends an OPTIONS request first. This adds an extra round trip to the server, potentially impacting performance, especially if many cross-origin requests are made in rapid succession.
• Evaluate the implications of incorrect CORS configurations on web application security and user privacy.
  • Incorrect CORS configurations can expose web applications to significant security risks, such as Cross-Site Scripting (XSS) or data theft. If a server improperly allows all origins with a wildcard ('*'), it may unintentionally permit malicious sites to access sensitive information. This not only jeopardizes user privacy but also undermines trust in the application. Consequently, developers must carefully implement and test their CORS settings to mitigate these risks effectively.

Related terms

Same-Origin Policy: A security measure that restricts how a document or script loaded from one origin can interact with resources from another origin, preventing potential malicious activities.

Preflight Request: A preliminary request sent by the browser using the OPTIONS method to determine whether the actual request is safe to send, often involved in CORS scenarios.

Access-Control-Allow-Origin: An HTTP header used in CORS responses that specifies which origins are permitted to access the resource, crucial for enabling or restricting cross-origin requests.