CSRF and CORS address distinct web security risks and therefore both are necessary. CSRF (Cross-Site Request Forgery) protects against malicious sites tricking a user's browser into making unintended requests to a trusted site where the user is already authenticated. This is achieved through tokens that verify the request originated from the trusted site itself. CORS (Cross-Origin Resource Sharing), on the other hand, dictates which external sites are permitted to access resources from a particular server, focusing on protecting the server itself from unauthorized access by scripts running on other origins. While they both deal with cross-site interactions, CSRF prevents malicious exploitation of a user's existing session, while CORS restricts access to the server's resources in the first place.
The blog post "Why do we have both CSRF protection and CORS?" explores the distinct roles of Cross-Site Request Forgery (CSRF) protection and Cross-Origin Resource Sharing (CORS) in web security, explaining why both mechanisms are necessary despite appearing to address similar issues. The author emphasizes that while both deal with cross-site requests, they protect against different types of attacks and operate under different assumptions.
CSRF protection is primarily concerned with preventing malicious websites from leveraging a user's existing authenticated session on a target website. It assumes that the user is already logged in and that the attacker's site can trick the user's browser into sending requests to the target site without the user's explicit intent. The core vulnerability lies in the fact that browsers automatically include cookies (including session cookies) with requests to the same origin, even if those requests are initiated from a different origin. CSRF tokens, generated by the server and embedded in forms or request headers, act as a proof of intent, verifying that the request originated from the genuine website and not a malicious actor.
Conversely, CORS focuses on restricting which origins are allowed to make cross-origin requests to a specific server. It's a browser-enforced mechanism that intercepts requests originating from a different origin than the target server and checks whether the server explicitly permits such requests via specific HTTP headers (like Access-Control-Allow-Origin). CORS's primary goal is to prevent malicious websites from directly reading the responses of cross-origin requests, safeguarding sensitive data that might be returned by the server. Without CORS, a malicious script could make requests to a different domain while operating within the context of the user's browser, potentially gaining access to private information if the browser's same-origin policy wasn't enforced.
The author highlights a critical distinction: CORS doesn't prevent the request itself from reaching the server; it merely prevents the malicious script from accessing the response. This is where CSRF protection becomes crucial. A malicious site can still send a POST request to a vulnerable server, even with CORS in place, potentially modifying data or performing unwanted actions if the server lacks proper CSRF protection. Therefore, both mechanisms are essential. CORS protects against unauthorized reading of responses, while CSRF protection prevents unauthorized modification of state through forged requests. They work in tandem to create a more secure web environment. The author concludes by noting that while they may seem redundant at first glance, understanding the distinct threats they address reveals the necessity of both mechanisms.
Summary of Comments ( 64 )
https://news.ycombinator.com/item?id=43231411
Hacker News users discussed the nuances of CSRF and CORS, pointing out that while they both address security concerns related to cross-origin requests, they protect against different threats. Several commenters emphasized that CORS primarily protects the server from unauthorized access by other origins, controlled by the server itself. CSRF, on the other hand, protects users from malicious sites exploiting their existing authenticated sessions on another site, controlled by the user's browser. One commenter offered a clear analogy: CORS is like a bouncer at a club deciding who can enter, while CSRF protection is like checking someone's ID to make sure they're not using a stolen membership card. The discussion also touched upon the practical differences in implementation, like preflight requests in CORS and the use of tokens in CSRF prevention. Some comments questioned the clarity of the original blog post's title, suggesting it might confuse the two distinct mechanisms.
The Hacker News post titled "Why do we have both CSRF protection and CORS?" generated a robust discussion with numerous comments exploring the nuances and interplay between these two security mechanisms. The central theme revolves around the distinct, yet complementary, roles CSRF protection and CORS play in securing web applications.
Several commenters emphasize that while both mechanisms address security concerns related to cross-origin requests, they target different attack vectors. CORS, they explain, is primarily browser-enforced and focuses on protecting the server from unauthorized access by scripts running in a different origin. It acts as a gatekeeper, allowing the server to specify which origins are permitted to make requests and what types of requests are allowed. This helps prevent malicious websites from directly accessing resources on a different domain without explicit server authorization.
CSRF protection, on the other hand, is focused on protecting the user from unknowingly making requests to a trusted site while authenticated. Commenters highlight how CSRF attacks exploit the browser's automatic inclusion of cookies and other authentication details in cross-origin requests. By tricking a user into interacting with a malicious website, an attacker can leverage the user's existing session to perform unwanted actions on the trusted site. CSRF tokens, as explained in the comments, act as a secret, unpredictable value included with each request that the server can verify to ensure the request originated from a legitimate source, not a malicious third-party site.
A key point of discussion revolves around how CORS alone does not prevent CSRF attacks. Commenters explain scenarios where a malicious website, even if blocked by CORS from reading the response from a cross-origin request, can still send the request. This means a CSRF attack can still be executed, potentially modifying data or performing actions on the targeted website without the user's knowledge, even if the attacker cannot directly see the results.
Some comments delve into the specific mechanics of how CSRF tokens work, explaining how they are typically generated on the server, embedded in forms or included as custom headers, and validated upon submission. They also touch upon different methods of implementing CSRF protection, such as double submit cookies and the Synchronizer Token Pattern.
Several commenters provide practical examples to illustrate the differences between CSRF and CORS, using scenarios involving banking transactions, social media interactions, and other web applications. These examples help clarify the distinct vulnerabilities each mechanism addresses and why both are necessary for comprehensive security.
Finally, some commenters discuss the limitations of both CSRF protection and CORS and highlight the importance of employing multiple layers of security. They mention other security best practices, such as proper input validation and output encoding, as essential components of a robust security strategy. The general consensus is that while CSRF and CORS are vital tools, they are not silver bullets, and a comprehensive approach to web security is crucial.