CORS - Misconfigurations & Bypass
What is CORS
The CORS (Cross-origin resource sharing) standard is needed because it allows servers to specify who can access its assets and which HTTP request methods are allowed from external resources.
In a same-origin policy, is needed that both the server requesting a resource and the server where the resource is located uses the same protocol (http://),domain name (internal-web.com) and the same port (80). Then, if the server forces the same-origin policy, only web pages from the same domain and port will be able to access the resources.
The following table shows how the same-origin policy will be applied in http://normal-website.com/example/example.html
:
*Internet Explorer will allow this access because IE does not take account of the port number when applying the same-origin policy.
Access-Control-Allow-Origin
Header
Access-Control-Allow-Origin
HeaderThe specification of Access-Control-Allow-Origin
allows for multiple origins, or the value null
, or the wildcard *
. However, no browser supports multiple origins and there are restrictions on the use of the wildcard *
.(The wildcard can only be used alone, this will fail Access-Control-Allow-Origin: https://*.normal-website.com
and it cannot be used with Access-Control-Allow-Credentials: true)
This header is returned by a server when a website requests a cross-domain resource, with an Origin
header added by the browser.
Access-Control-Allow-Credentials
Header
Access-Control-Allow-Credentials
HeaderThe default behaviour of cross-origin resource requests is for requests to be passed without credentials like cookies and the Authorization header. However, the cross-domain server can permit reading of the response when credentials are passed to it by setting the CORS Access-Control-Allow-Credentials
header to true
.
If the value is set to true
then the browser will send credentials (cookies, authorization headers or TLS client certificates).
Pre-flight request
Under certain circumstances, when a cross-domain request includes a non-standard HTTP method or headers, the cross-origin request is preceded by a request using the OPTIONS
method, and the CORS protocol necessitates an initial check on what methods and headers are permitted prior to allowing the cross-origin request. This is called the pre-flight check. The server returns a list of allowed methods in addition to the trusted origin and the browser checks to see if the requesting website's method is allowed.
For example, this is a pre-flight request that is seeking to use the PUT
method together with a custom request header called Special-Request-Header
:
The server might return a response like the following:
Access-Control-Allow-Headers
Allowed headersAccess-Control-Expose-Headers
Access-Control-Max-Age
Defines a maximum timeframe for caching the pre-flight response for reuseAccess-Control-Request-Headers
The header the cross-origin request wants to sendAccess-Control-Request-Method
The method the cross-origin request wants to useOrigin
Origin of the cross-origin request (Set automatically by the browser)
Note that usually (depending on the content-type and headers set) in a GET/POST request no pre-flight request is sent (the request is sent directly), but if you want to access the headers/body of the response, it must contains an Access-Control-Allow-Origin header allowing it. Therefore, CORS doesn't protect against CSRF (but it can be helpful).
{% hint style="info" %} Check in this link the conditions of a request to avoid sending of a pre-flight request {% endhint %}
Exploitable misconfigurations
Notice that most of the real attacks require Access-Control-Allow-Credentials
to be set to true
because this will allow the browser to send the credentials and read the response. Without credentials, many attacks become irrelevant; it means you can't ride on a user's cookies, so there is often nothing to be gained by making their browser issue the request rather than issuing it yourself.
One notable exception is when the victim's network location functions as a kind of authentication. You can use a victim’s browser as a proxy to bypass IP-based authentication and access intranet applications. In terms of impact this is similar to DNS rebinding, but much less fiddly to exploit.
Reflected Origin
in Access-Control-Allow-Origin
Origin
in Access-Control-Allow-Origin
In the real world this cannot happen as this 2 values of the headers are forbidden together.
It is also true that a lot of developers want to allow several URLs in the CORS, but subdomain wildcards or lists of URLs aren't allowed. Then, several developers generates the **Access-Control-Allow-Origin
**header dynamically, and in more than one occasion they just copy the value of the Origin header.
In that case, the same vulnerability might be exploited.
In other cases, the developer could check that the domain (victimdomain.com) appears in the Origin header, then, an attacker can use a domain called attackervictimdomain.com
to steal the confidential information.
The null
Origin
null
Originnull
is a special value for the Origin header. The specification mentions it being triggered by redirects, and local HTML files. Some applications might whitelist the null
origin to support local development of the application.
This is nice because several application will allow this value inside the CORS and any website can easily obtain the null origin using a sandboxed iframe:
Regexp bypasses
If you found the domain victim.com to be whitelisted you should check if victim.com.attacker.com is whitelisted also, or, in case you can takeover some subdomain, check if somesubdomain.victim.com is whitelisted.
Advance Regexp bypasses
Most of the regex used to identify the domain inside the string will focus on alphanumeric ASCII characters and .-
. Then, something like victimdomain.com{.attacker.com
inside the Origin header will be interpreted by the regexp as if the domain was victimdomain.com
but the browser (in this case Safari supports this character in the domain) will access the domainattacker.com
.
The _
character (in subdomains) is not only supported in Safari, but also in Chrome and Firefox!
Then, using one of those subdomains you could bypass some "common" regexps to find the main domain of a URL.
For more information and settings of this bypass check: https://www.corben.io/advanced-cors-techniques/ and https://medium.com/bugbountywriteup/think-outside-the-scope-advanced-cors-exploitation-techniques-dad019c68397****
From XSS inside a subdomain
One defensive mechanism developers use against CORS exploitation is to white-list domains that frequently requests access for information. However, this isn’t entirely secure, because if even one of the subdomains of the whitelisted domain is vulnerable to other exploits such as XSS, it can enable CORS exploitation.
Let us consider an example, the following code shows the configuration that allows subdomains of requester.com to access resources of provider.com.
Assuming that a user has access to sub.requester.com but not requester.com, and assuming that sub.requester.com
is vulnerable to XSS. The user can exploit provider.com
by using cross-site scripting attack method.
Server-side cache poisoning
If the stars are aligned we may be able to use server-side cache poisoning via HTTP header injection to create a stored XSS vulnerability.
If an application reflects the Origin header without even checking it for illegal characters like \r, we effectively have a HTTP header injection vulnerability against IE/Edge users as Internet Explorer and Edge view \r (0x0d) as a valid HTTP header terminator:GET / HTTP/1.1 Origin: z[0x0d]Content-Type: text/html; charset=UTF-7
Internet Explorer sees the response as:
HTTP/1.1 200 OK Access-Control-Allow-Origin: z Content-Type: text/html; charset=UTF-7
This isn't directly exploitable because there's no way for an attacker to make someone's web browser send such a malformed header, but I can manually craft this request in Burp Suite and a server-side cache may save the response and serve it to other people. The payload I've used will change the page's character set to UTF-7, which is notoriously useful for creating XSS vulnerabilities.
Client-Side cache poisoning
You may have occasionally encountered a page with reflected XSS in a custom HTTP header. Say a web page reflects the contents of a custom header without encoding:`GET / HTTP/1.1 Host: example.com X-User-id: <svg/onload=alert(1)>
HTTP/1.1 200 OK Access-Control-Allow-Origin: * Access-Control-Allow-Headers: X-User-id Content-Type: text/html ... Invalid user: <svg/onload=alert(1)>`
With CORS, we can send any value in the Header. By itself, that's useless since the response containing our injected JavaScript won't be rendered. However, if Vary: Origin hasn't been specified the response may be stored in the browser's cache and displayed directly when the browser navigates to the associated URL. I've made a fiddle to attempt this attack on a URL of your choice. Since this attack uses client-side caching, it's actually quite reliable.
Bypass
XSSI (Cross-Site Script Inclusion) / JSONP
XSSI designates a kind of vulnerability which exploits the fact that, when a resource is included using the script
tag, the SOP doesn’t apply, because scripts have to be able to be included cross-domain. An attacker can thus read everything that was included using the script
tag.
This is especially interesting when it comes to dynamic JavaScript or JSONP when so-called ambient-authority information like cookies are used for authentication. The cookies are included when requesting a resource from a different host. BurpSuite plugin: https://github.com/kapytein/jsonp
****Read more about the difefrent types of XSSI and how to exploit them here.****
Try to add a callback
parameter in the request. Maybe the page was prepared to send the data as JSONP. In that case the page will send back the data with Content-Type: application/javascript
which will bypass the CORS policy.
Easy (useless?) bypass
You can ask a web-application to make a request for you and send back the response. This will bypass the the Access-Control-Allow-Origin
but notice that the credentials to the final victim won't be sent as you will be contacting a different domain (the one that will make the request for you).
CORS-escape
CORS-escape provides a proxy that passes on our request along with its headers, and it also spoof the Origin header (Origin = requested domain). So the CORS policy is bypassed. The source code is on Github, so you can host your own.
simple-cors-escape
Proxying is kinda like “passing on" your request, exactly as you sent it. We could solve this in an alternative way that still involves someone else making the request for you, but this time, instead of using passing on your request, the server makes its own request, but with whatever parameters you specified.
DNS Rebinding
Basically you make the victim access your page, then you change the DNS of your domain (the IP) and make it points to your victims web page. You make your victim execute (JS) something when the TLS is over so a new DNS request will be made and then you will be able to gather the information (as you will always mantains the user in your domain, he won't send any cookie to the victim server, so this options abuses the speciall privileges of the IP of the victim).
Also, I don't know why this attack plays with the TLS of the DNS instead of just having a subdomain always pointing to the victims IP.
Tools
Fuzz possible misconfigurations in CORS policies
References
{% embed url="https://portswigger.net/web-security/cors" caption="" %}
{% embed url="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers\#CORS" caption="" %}
{% embed url="https://portswigger.net/research/exploiting-cors-misconfigurations-for-bitcoins-and-bounties" caption="" %}
{% embed url="https://www.codecademy.com/articles/what-is-cors" caption="" %}
{% embed url="https://www.we45.com/blog/3-ways-to-exploit-misconfigured-cross-origin-resource-sharing-cors" caption="" %}
{% embed url="https://medium.com/netscape/hacking-it-out-when-cors-wont-let-you-be-great-35f6206cc646" caption="" %}
{% embed url="https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/CORS%20Misconfiguration" caption="" %}
{% embed url="https://medium.com/entersoftsecurity/every-bug-bounty-hunter-should-know-the-evil-smile-of-the-jsonp-over-the-browsers-same-origin-438af3a0ac3b" caption="" %}
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