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98 votes
Accepted

How do hackers trick frontend validation?

I think you are very confused about what both CORS and SOP do... neither is relevant to these attacks at all. There are lots of ways to bypass client-side validation. HTTP is just a stream of bytes, ...
CBHacking's user avatar
  • 47.5k
63 votes
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How does CORS prevent XSS?

TL;DR: How does CORS prevent XSS? It does not. It is not meant to do so. CORS is intended to allow resource hosts (any service that makes its data available via HTTP) to restrict which websites may ...
marstato's user avatar
  • 2,315
29 votes

How does CORS prevent XSS?

Cross-Site-Scripting (XSS) is the execution of attacker defined script code in the context of another site. CORS does not prevent XSS, in fact it is unrelated to XSS. Instead CORS offers a way to ...
Steffen Ullrich's user avatar
23 votes

How do hackers trick frontend validation?

Maybe a very short answer will help as well. I never thought about it much, I just thought this meant someone could bypass the validations by making a request on something like Postman. But then I ...
Vincent's user avatar
  • 329
19 votes
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HTTP access control (CORS) purpose

For your own good. If you disable CORS the following can happen: I send you link to a page called hackfacebook.com for example. When you visit my page, I then request the facebook.com page using an ...
Bubble Hacker's user avatar
16 votes
Accepted

Access-control-allow-origin: * with a bearer token

There are a few things that will mean exploitation is unlikely. To start with access-control-allow-credentials: true access-control-allow-origin: * is an invalid combination: Important note: ...
SilverlightFox's user avatar
16 votes

What are the security risk of enabling cors on localhost?

The answer really depends on what API you created and how it works. This site gives a very good explanation on the goods and bads of CORS. In short, the author creates a fictional API that is used to ...
Ben's user avatar
  • 2,054
15 votes
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Vary: origin response header and CORS exploitation

It's important to include the Vary: Origin header to prevent caching. The header indicates that the response is in some way dependent on the origin and should therefore not be served from cache for ...
Arminius's user avatar
  • 44.7k
15 votes
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Can we use CORS policy to restrict API calls from native mobile app?

CORS doesn't prevent anything, and it doesn't protect the server. It simply tells a conforming client (a browser) what is permitted for the protection of the browser's user; CORS is a way to carefully ...
CBHacking's user avatar
  • 47.5k
11 votes
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How did the Facebook Originull vulnerablity of Access-Control-Allow-Origin: null allow cross-origin access?

I'm Ysrael and I'm the researcher that found this vulnerability. Let's divide your question into 2 parts: A. How did the Origin of the browser become null? B. How the null Origin affect Facebook ...
Ysrael Gurt's user avatar
11 votes

How does CORS prevent XSS?

CORS and XSS are related, but not directly. The best way to explain it is by example: we shall consider 3 servers (your_bank.com, api.your_bank.com, badguy.com*) and 1 client (your browser). You are ...
Daniel's user avatar
  • 119
11 votes
Accepted

What attacks are mitigated by requiring CORS for subresource integrity verification?

The attack I think the attack they are trying to protect against is the following. Imagine santaclause.com serves an image at santaclause.com/naughty_or_nice.png to logged in users. The image is a ...
Anders's user avatar
  • 65.6k
11 votes
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How does setting Origin to null in a redirected CORS request protect against a confused deputy attack?

That paragraph is tersely written, and the use of "redirects to another resource at a new origin" in the first sentence isn't quite right. Here's a simple contrived example. Let's say you are ...
Jonah Benton's user avatar
  • 3,497
11 votes
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CORS error but only from AJAX and not from HTML form

This is the expected behavior. Before CORS it was already possible to make cross-origin requests using HTML forms, cross-origin inclusion of images ... . But it was not possible to do cross-origin XHR ...
Steffen Ullrich's user avatar
10 votes
Accepted

How do CORS proxy websites work?

The CORS policy is enforced by the browser, not by the server. The server just sets a couple of HTTP headers telling the browser how it wants it to behave. There is nothing that forces a proxyserver ...
Anders's user avatar
  • 65.6k
10 votes

How do hackers trick frontend validation?

Postman and same origin policy aren't obstacles. To understand this, I need to explain why, as a developer, you virtually never trust the client/front end. Front and back end trust If someone controls ...
Stilez's user avatar
  • 1,684
9 votes
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Why is a "tainted canvas" a risk?

What exactly would that image request URL look like? It need not be anything complicated or abnormal. There are two main ways this could work (were it not for the restrictions in the browser): In ...
Anders's user avatar
  • 65.6k
9 votes

How do hackers trick frontend validation?

Your backend is accessible via the network. That means I don't need to use your frontend. I can find out what endpoints it uses, and and what format the request looks like, and use my own tools to ...
hobbs's user avatar
  • 647
8 votes

Why CORS preflight is not available for POST requests when Content-Type is application/x-www-form-urlencoded

So my understanding of why this is allowed is so that the implementation of CORS wouldn't break existing and well understood functionality which was already allowed by browsers. This also means that ...
GreatSeaSpider's user avatar
8 votes

Security Headers: Access-Control-Allow-Origin vs. Cross-Origin-Resource-Policy (CORP)

CORS (Cross-Origin Resource Sharing) in general, and the Access-Control-Allow-... headers in particular, are a way to relax same-origin policy. They are used when you need to allow cross-origin ...
CBHacking's user avatar
  • 47.5k
7 votes

Is CORS ever needed during any aspect of OAuth / OpenIDConnect Authentication?

I'm not sure about OpenID, but for OAuth there is no CORS involved for the actual authentication part, though it may be required on the resource server depending on the type of client that is ...
JackW's user avatar
  • 713
7 votes

Ways to bypass browsers CORS Policy

You can use the following content types without CORS access: application/x-www-form-urlencoded multipart/form-data text/plain You can try a couple of things: Try sending valid JSON with another ...
Sjoerd's user avatar
  • 30.4k
7 votes
Accepted

Why are web font resource requests not no-cors?

This mechanism exists mainly as a way for font creators to control which websites can access their possibly copyrighted and licensed products. As MDN puts it: ... Web Fonts (for cross-domain ...
AlphaD's user avatar
  • 893
6 votes

vuln Cross-origin resource sharing: arbitrary origin trusted

https://xrmtfgxgjkzw.com is just an example generated by Burp. What it is in fact saying is that any website on the internet that the user is visiting can grab content from your site (possibly private ...
SilverlightFox's user avatar
6 votes

How does CORS prevent XSS?

CORS does not protect anything, SOP (Same Origin Policy) protects something instead. SOP protects the target domain and the browser user. In fact, CORS weaken existing restrictions of SOP to help ...
Keaser's user avatar
  • 161
6 votes

If a CORS request doesn't need authentication, why can't I send it without the cookies?

History The original XMLHttpRequest implementations did not have the concept of "allow credentials" - all requests had credentials attached. Of course, this would be extremely dangerous to allow ...
paj28's user avatar
  • 33.4k
6 votes

How does setting Origin to null in a redirected CORS request protect against a confused deputy attack?

Look at the Origin header in terms of CSRF defense. Let's say a.com hosts... a service that uses the Origin header to block CSRF attacks, and a web page that makes CORS requests to b.com. If the ...
Andre D's user avatar
  • 171
6 votes

is every site without a CSRF token vulnerable to CSRF attack?

The simple answer is “no”. Cross-Site Request Forgery (CSRF) tokens are a generalized defense against CSRF, but other conditions must be at play to be vulnerable. For example, the attacker needs to be ...
nbering's user avatar
  • 4,028
6 votes
Accepted

Exploitability of allowed wildcard (*) CORS Origins with Bearer Token Authorization

ACAO: * will prevent the browser from automatically sending any form of credentials, but that's only relevant for the kinds that the browser can actually send automatically (cookies and HTTP BASIC, ...
CBHacking's user avatar
  • 47.5k
6 votes

Concrete example of how can Access-Control-Allow-Origin:* cause security risks?

[S]uppose website B set the header Access-Control-Allow-Credentials to false, and Access-Control-Allow-Origin: *, can this cause any concrete security risk to the user who is browsing website A (...
jub0bs's user avatar
  • 363

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