I developped a client and a server app. In the server's Web.config, I set the property
<add name="Access-Control-Allow-Origin" value="http://domain.tld:4031" />
And, indeed, when I try connecting with a client installed in a different location, I get rejected. But I do not get rejected when I use Chrome's Advanced REST Client from the very same location!
In the extensions, the header of the response indicates
Access-Control-Allow-Origin: http://www.domain.tld:4031
So how comes I still get a answer "200 OK", and the data I requested?
UPDATE:
I do not think this topic is enough of an answer : How does Google Chrome's Advanced REST client make cross domain POST requests?
My main concern is : how comes is it possible to "ask" for those extra permissions. I believe the client shouldn't be allowed to just decide which permission it receives. I thought it was up to the server only. What if I just "ask" for extra permissions to access your data on your computer? It doesn't make sense to me...
The reason that the REST client (or really the browser) is able to bypass the CORS restriction is that it is a client side protection. It isn't the responsibility of the server to provide this protection but it is a feature implemented by most modern browser vendors to protect their users from XSS-hazards.
The following quote from the Wikipedia CORS page sums it up quite good
"Although some validation and authorization can be performed by the server, it is generally the browser's responsibility to support these headers and respect the restrictions they impose." - Wikipedia
You could of course, like the quote imposes, do some server side validation on your own. The "Access-Control-Allow-Origin" header however is more of a indication to the browser that is is okay for the browser to allow the specified origins. It's then up to the browser to decide if it want's to honor this header. In previous versions of for example chrome there actually existed a flag to turn off the same origin policy all together.
Related
Let's say that a page is just printing the value of the HTTP 'referer' header with no escaping. So the page is vulnerable to an XSS attack, i.e. an attacker can craft a GET request with a referer header containing something like <script>alert('xss');</script>.
But how can you actually use this to attack a target? How can the attacker make the target issue that specific request with that specific header?
This sounds like a standard reflected XSS attack.
In reflected XSS attacks, the attacker needs the victim to visit some site which in some way is under the attacker's control. Even if this is just a forum where an attacker can post a link in the hope somebody will follow it.
In the case of a reflected XSS attack with the referer header, then the attacker could redirect the user from the forum to a page on the attacker's domain.
e.g.
http://evil.example.com/?<script>alert(123)>
This page in turn redirects to the following target page in a way that preserves referer.
http://victim.example.org/vulnerable_xss_page.php
Because it is showing the referer header on this page without the proper escaping, http://evil.example.com/?<script>alert(123)> gets output within the HTML source, executing the alert. Note this works in Internet Explorer only.
Other browsers will automatically encode the URL rendering
%3cscript%3ealert%28123%29%3c/script%3e
instead which is safe.
I can think of a few different attacks, maybe there are more which then others will hopefully add. :)
If your XSS is just some header value reflected in the response unencoded, I would say that's less of a risk compared to stored. There may be factors to consider though. For example if it's a header that the browser adds and can be set in the browser (like the user agent), an attacker may get access to a client computer, change the user agent, and then let a normal user use the website, now with the attacker's javascript injected. Another example that comes to mind is a website may display the url that redirected you there (referer) - in this case the attacker only has to link to the vulnerable application from his carefully crafted url. These are kind of edge cases though.
If it's stored, that's more straightforward. Consider an application that logs user access with all request headers, and let's suppose there is an internal application for admins that they use to inspect logs. If this log viewer application is web based and vulnerable, any javascript from any request header could be run in the admin context. Obviously this is just one example, it doesn't need to be blind of course.
Cache poisoning may also help with exploiting a header XSS.
Another thing I can think of is browser plugins. Flash is less prevalent now (thankfully), but with different versions of Flash you could set different request headers on your requests. What exactly you can and cannot set is a mess and very confusing across Flash plugin versions.
So there are several attacks, and it is necessary to treat all headers as user input and encode them accordingly.
Exploitation of xss at referrer header is almost like a traditional reflected xss, Just an additional point to make is "Attacker's website redirects to victim website and hence referrer header with required javascript will be appended to the victim website request".
Here One essential point that needs to be discussed is Why only with IE one can exploit this vulnerability why not with other browsers?
Traditional answer for this question is 'Chrome and firefox automatically encodes URL parameters and hence XSS is not possible..' But interesting thing here is when we have n number of bypasses for traditional xss bypasses. why can't we have bypasses for this scenario.
Yes.. We can bypass with following payload which is same way to bypass HTML validation in traditional payload.
http://evil.example.com/?alert(1)//cctixn1f .
Here the response could be something like this:
The link on the
referring
page seems to be wrong or outdated.
Response End
If victim clicks on referring page, alert will be generated..
Bottomline: Not just only IE, XSS can be possible even in Mozilla and Firefox when referrer is being used as part of href tag..
I'm doing some work with this right now and I have to say, it makes no sense at all to me! Basically, I have some CDN server which provides css, images ect for a site. For whatever reason, in order for my browser to stop blocking those resources with a CORS error, I had to have that server (the CDN) add the Access-Control-Allow-Origin header. But as far as I can tell that does absolutely nothing to increase security. Shouldn't the page I request which references those cross-domain resources be telling the browser it's safe to get stuff from the other domain? If that were a malicious domain wouldn't it just have the Access-Control-Allow-Origin set to * so that sites load their malicious responses (you don't have to answer that because obviously they would)?
So can someone explain how this mechanism/feature provides security? As far as I can tell the implementors fucked up and it actually does nothing. The header should be required from the page which references/requests cross-domain resources rather than from that domain being requested.
To be clear; if I request a page at domain A it would make sense for the response to include the Access-Control-Allow-Origin header white listing resources from domain B (Access-Control-Allow-Origin:.B.com), however it makes no sense at all for domain B to effectively white list itself by providing the header; Access-Control-Allow-Origin: which is how this is currently implemented. Can anyone clarify what the benefit of this feature is?
If I have a protected resource hosted on site A, but also control sites B, C, and D, I may want to use that resource on all of my sites but still prevent anyone else from using that resource on theirs. So I instruct my site A to send Access-Control-Allow-Origin: B, C, D along with all of its responses. It's up to the web browser itself to honor this and not serve the response to the underlying Javascript or whatever initiated the request if it didn't come from an allowed origin. Error handlers will be invoked instead. So it's really not for your security as much as it's an honor-system (all major browsers do this) access control method for servers.
Primarily Access-Control-Allow-Origin is about protecting data from leaking from one server (lets call it privateHomeServer.com) to another server (lets call it evil.com) via an unsuspecting user's web browser.
Consider this scenario:
You are on your home network browsing the web when you accidentally stumble onto evil.com. This web page contains malicious javascript that tries to look for web servers on your local home network and then sends their content back to evil.com. It does this by trying to open XMLHttpRequests on all local IP addresses (eg. 192.168.1.1, 192.168.1.2, .. 192.168.1.255) until it finds a web server.
If you are using an old web browser that isn't Access-Control-Allow-Origin aware or you have set Access-Control-Allow-Origin * on your privateHomeServer then your browser would happily retrieve the data from your privateHomeServer (which presumably you didn't bother passwording as it was safely behind your home firewall) and then handing that data to the malicious javascript which can then send the information on to the evil.com server.
On the other hand using an Access-Control-Allow-Origin aware browser and default web configuration on privateHomeServer (ie. not sending Access-Control-Allow-Origin *) your web browser would block the malicious javascript from seeing any data retrieved from privateHomeServer. So this way you are protected from such attacks unless you go out of your way to change the default configuration on your server.
Regarding the question:
Shouldn't the page I request which references those cross-domain
resources be telling the browser it's safe to get stuff from the other
domain?
The fact that your page contains code that is attempting to get resources from a particular server is implicitly telling the web browser that you believe the resources are safe to fetch. It wouldn't make sense to need to repeat this again elsewhere.
CORS makes only sense for Mashup content provider and nothing more.
Example: You are a provider of a embedded maps mashup service which requires a registration. Now you want to make sure that your ajax mashup map will only work for your registered users on their domains. Other domains should be excluded. Only for this reason CORS makes sense.
Another example: Someone misuse CORS for a REST-Service. The clever developer set up a ajax proxy and et voilà you can access from every domain on that service.
Such a ajax proxy would make no sense for a mashup, on the other way the CORS makes no sense for REST-Services, because you could bypass the restriction with a simple http-client.
I'm trying to understand how an IIS server handles different browsers in the header of an HTTP request.
The situation is that I have some load tests set up that fire off HTTP requests to an IIS server, constructing them and sending them over the wire. My code allows me to specify the browser in the header, but I'm not sure what that would actually change.
So what does IIS do with that particular information in the header?
As far as i am aware IIS doesn't actually do anything with the header.
You can create rules to explicitly handle a type of browser, this is pretty useful if you block traffic from countries but you still want to allow bots for example.
Its useful to also have this information in Log Files too
I have difficulties in seeing the point of the Access-Control-Allow-Origin http header.
I thought that if a client (browser) gets a "no" from a server once, than it will not send any further requests. But chrome and firefox keep sending requests.
Could anyone tell me a real life example where a header like this makes sense?
thanks!
The Access-Control-Allow-Origin header should contain a list of origins which are "allowed" to access the resource.
Thus, determining which domains can make requests to your server for resources.
For example, sending back a header of Access-Control-Allow-Origin: * would allow all sites to access the requested resource.
On the other hand, sending back Access-Control-Allow-Origin: http://foo.example.com will allow access only to http://foo.example.com.
There's some more information on this over at the Mozilla Developer Site
For example
Let's suppose we have a URL on our own domain that returns a JSON collection of Music Albums by Artist. It might look like this:
http://ourdomain.com/GetAlbumsByArtist/MichaelJackson.json
We might use some AJAX on our website to get this JSON data and then display it on our website.
But what if someone from another site wishes to use our JSON object for themselves? Perhaps we have another website http://subdomain.ourdomain.com which we own and would like to use our feed from ourdomain.com.
Traditionally we can't make cross-domain requests for this data.
By specifying other domains that are allowed access to our resource, we now open the doors to cross-domain requests.
CORS implements a two-part security view of cross-origin. The problem it is trying to solve is that there are many servers sitting out there on the public internet written by people who either (a) assumed that no browser would ever allow a cross-origin request, or (b) didn't think about it at all.
So, some people want to permit cross-origin communications, but the browser-builders do not feel that they can just unlock browsers and suddenly leave all these websites exposed. To avoid this, they invented a two-part structure. Before a browser will permit a cross-origin interaction with a server, that server has to specifically indicate that it is willing to allow cross-origin access. In the simple cases, that's Access-Control-Allow-Origin. In more complex cases, it's the full preflight mechanism.
It's still true that servers have to implement appropriate resource access control on their resources. CORS is just there to allow the server to indicate to browsers that it is aware of all the issues.
I've recently enabled Digest Authentication on an intranet website/application I am creating for my company in ASP.NET.
The reason I have done so is because Windows Authentication seemed to only work for some users, and not for others. I could not figure out why nor do I know enough about IIS to try and trace the issue. After some trial and error, I found that digest authentication seemed to give me the behaviour that I wanted. That is: allow only users with a valid account on the domain to log in to the website with their credentials.
The problem now, is that Firefox (3+) seems to ask for the user to authenticate on every HTTP request sent to the server. This does not appear to occur in Internet Explorer (6+) or Chrome.
I've tried searching for solutions but I always arrive at dead-ends. I'll find a discussion about the issue, and every posted solution leads to a dead link...or it's on Experts Exchange and I don't have access to view to solution.
The issue appears to be related (from what I've read) to the way the different browsers send their authentication headers vs how IIS interprets them. I'm not sure what I can do to change this though? One of the solutions I had found mentioned writing an ISAPI filter to fix this, but of course the link to the finished filter was broken and I have no idea how to go about making one myself.
I've tried messing with the NTLM and other auth related strings in about:config to try and force Firefox to trust my server but that doesn't seem to work either.
From a few other sources I've read, it appears that everything should work if I switch back to Windows Authentication, but then I'm back at square one where the authentication would work only for some users and not others.
A solution for either problem would work for me, but I have very little information for the Windows Authentication issue. If someone could guide me through tracing the problem I'd gladly post more information for it as well.
Here are the URLs I've found discussing what seems like the same problem. (Sorry I couldn't make them all links, it wouldn't let me post otherwise)
support.mozilla.com/tiki-view_forum_thread.php?locale=pt-BR&forumId=1&comments_parentId=346851
www.experts-exchange.com/Software/Internet_Email/Web_Browsers/Mozilla/Q_24427378.html
channel9.msdn.com/forums/TechOff/168006-Twin-bugs-in-IIS-IE-unfair-competitive-advantage-EDIT-SOLVED/
www.derkeiler.com/Newsgroups/microsoft.public.inetserver.iis.security/2006-03/msg00141.html
This is a know bug in FF. See Advanced digest authentication works from Internet Explorer however we receive multiple authentication prompts on each GET request from fire fox
IE 6 had the same bug.A potential workaround would be to re-enable "old" Digest in IIS6:
http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/1d6e22ac-0215-4d12-81e9-c9262c91b797.mspx?mfr=true
Currently, if the server send an opaque directive, the IE client will return this directive value as specified in the RFC. Unfortunately, for follow-on requests from the client where the nonce count is incremented (count 2 and beyond) the opaque directive value is not sent. This then fails authentication on the server and a 401 Unauthorized is returned. The IE client now requests the username and password for the new challenge and the file is retrieved.
This requires an additional round trip and the user is prompted for credential each time.
The RFC states that the opaque must always be sent on requests from the client.
The Digest implementation that IE6 is using is not RFC compliant (http://www.ietf.org/rfc/rfc2617.txt).
3.2.2 The Authorization Request Header
The values of the opaque and algorithm fields must be those supplied
in the WWW-Authenticate response header for the entity being
requested.
3.3 Digest Operation
A client should remember the username, password, nonce, nonce count and
opaque values associated with an authentication session to use to
construct the Authorization header in future requests within that
protection space.
Because the client is required to return the value of the opaque
directive given to it by the server for the duration of a session,
the opaque data may be used to transport authentication session state
information.
-------- Edit addition -----
Windows Authentication seemed to only work for some users, and not for others.
How did it fail? Did you enable impersonation?