I'm trying to understand vulnerabilities arising from HTTP(S) header Host. I heard that webservers may use the value of the Host header from incoming requests to do different stuff such as constructing URLs. For example here is a excerpt from Django documentation:
Django uses the Host header provided by the client to construct URLs in certain cases.
I know that any information in HTTP(S) requests may not be trusted. Web servers know what host name they are behind. So why would they take it from Host header that cannot be trusted if they can have their host name configured manually?
I have a general networking question but it's related with security aspect.
Here is my case: I have a host which is infected by a malware. The malware creates an http packet to communicate with it's command and control server. While constructing the packet, the IP layer contains the correct IP address of the command and control server. The tcp layer contains the correct port number 80.
Before sending the packet out, the malware modifies the http header to replace the host header with “google.com" instead of it's server address. It then attaches the stolen data with the packet and sends it out.
My understanding is that the packet will get delivered to the correct server because the routing will happen based on the IP.
But can I host a webserver on this IP that would receive all packets with header host google.com and parse it correctly?
Based on my reading on the internet, it is possible but if it is that easy then why have malware authors not adopted this technique to spoof the http headers and bypass traditional domain whitelisting engines.
When you make a request to let's say Apache2 server, what actually Apache does is match your "Host" header with any VirtualHost within server's configuration. Only if it cannot be found / is invalid, Apache will route the request to default virtualhost if it's defined. Basically nothing stops you from changing these headers.
You can simply test it by editing your hosts file and pointing google.com to any other IP - you will be able to handle the google.com domain on your server, but only you will be to use it this way - no one else.
Anything you send inside HTTP headers shouldn't be trusted - it just a guide for your server on how to actually handle the traffic.
The fake host header is just there to trick some deep-inspection firewalls ("it's for Google? you may pass..."). The server on that IP either doesn't care about the host header (default vhost) or is explicitly configured to accept it.
Passing the loot on by using fake headers or just as plain data behind the headers is another trick to fool data loss prevention.
These methods can mislead shallow application-layer inspection but won't pass a decent firewall.
I've found some interesting reading on the X-Forwarded-* headers, including the Reverse Proxy Request Headers section in the Apache documentation, as well as the Wikipedia article on X-Forwarded-For.
I understand that:
X-Forwarded-For gives the address of the client which connected to the proxy
X-Forwarded-Port gives the port the client connected to on the proxy (e.g. 80 or 443)
X-Forwarded-Proto gives the protocol the client used to connect to the proxy (http or https)
X-Forwarded-Host gives the content of the Host header the client sent to the proxy.
These all make sense.
However, I still can't figure out a real life use case of X-Forwarded-Host. I understand the need to repeat the connection on a different port or using a different scheme, but why would a proxy server ever change the Host header when repeating the request to the target server?
If you use a front-end service like Apigee as the front-end to your APIs, you will need something like X-FORWARDED-HOST to understand what hostname was used to connect to the API, because Apigee gets configured with whatever your backend DNS is, nginx and your app stack only see the Host header as your backend DNS name, not the hostname that was called in the first place.
This is the scenario I worked on today:
Users access certain application server using "https://neaturl.company.com" URL which is pointing to Reverse Proxy. Proxy then terminates SSL and redirects users' requests to the actual application server which has URL of "http://192.168.1.1:5555". The problem is - when application server needed to redirect user to other page on the same server using absolute path, it was using latter URL and users don't have access to this. Using X-Forwarded-Host (+ X-Forwarded-Proto and X-Forwarded-Port) allowed our proxy to tell application server which URL user used originally and thus server started to generate correct absolute path in its responses.
In this case there was no option to stop application server to generate absolute URLs nor configure it for "public url" manually.
I can tell you a real life issue, I had an issue using an IBM portal.
In my case the problem was that the IBM portal has a rest service which retrieves an url for a resource, something like:
{"url":"http://internal.host.name/path"}
What happened?
Simple, when you enter from intranet everything works fine because internalHostName exists but... when the user enter from internet then the proxy is not able to resolve the host name and the portal crashes.
The fix for the IBM portal was to read the X-FORWARDED-HOST header and then change the response to something like:
{"url":"http://internet.host.name/path"}
See that I put internet and not internal in the second response.
For the need for 'x-forwarded-host', I can think of a virtual hosting scenario where there are several internal hosts (internal network) and a reverse proxy sitting in between those hosts and the internet. If the requested host is part of the internal network, the requested host resolves to the reverse proxy IP and the web browser sends the request to the reverse proxy. This reverse proxy finds the appropriate internal host and forwards the request sent by the client to this host. In doing so, the reverse proxy changes the host field to match the internal host and sets the x-forward-host to the actual host requested by the client. More details on reverse proxy can be found in this wikipedia page http://en.wikipedia.org/wiki/Reverse_proxy.
Check this post for details on x-forwarded-for header and a simple demo python script that shows how a web-server can detect the use of a proxy server: x-forwarded-for explained
One example could be a proxy that blocks certain hosts and redirects them to an external block page. In fact, I’m almost certain my school filter does this…
(And the reason they might not just pass on the original Host as Host is because some servers [Nginx?] reject any traffic to the wrong Host.)
X-Forwarded-Host just saved my life. CDNs (or reverse proxy if you'd like to go down to "trees") determine which origin to use by Host header a user comes to them with. Thus, a CDN can't use the same Host header to contact the origin - otherwise, the CDN would go to itself in a loop rather than going to the origin. Thus, the CDN uses either IP address or some dummy FQDN as the Host header fetching content from the origin. Now, the origin may wish to know what was the Host header (aka website name) the content is asked for. In my case, one origin served 2 websites.
Another scenario, you license your app to a host URL then you want to load balance across n > 1 servers.
I've got an internal web application that's designed to work in concert with a server running locally on the client machine. (For the curious: the local server is used to decrypt data retrieved from the server using the client machine's GPG key.)
The internal web app is served over HTTPS while the local app is accessible via localhost. It used to be that I could make unencrypted AJAX requests from the page to localhost without any issues; but it seems that recently Chrome was updated to disallow HTTP requests to any destination from pages served over HTTPS.
I understand that in the vast majority of cases, HTTP requests from a page served via HTTPS constitute a security hole. However, since I have complete control over the endpoint in this case (i.e., localhost), it seems to me that it should still be perfectly safe to make HTTP requests to that one destination even when the host page has been served via HTTPS.
Is this possible? To whitelist localhost somehow?
Since you are in control of both the client and the server, it sounds like a good candidate for Cross-Origin Resource Sharing (CORS). The server will have to set a few response headers to give access to the client. You can learn more here: http://www.html5rocks.com/en/tutorials/cors/
Why does HttpContext.Current.Request.Url.Host return a different URL than the URL used in the Web browser? For example, when entering "www.someurl.com" in the browser, the HttpContext.Current.Request.Url.Host variable is equal to "www.someotherurl.com".
HttpContext.Current.Request.Url.Host is the contents of the Host header that the ASP.net application receives. (see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html for more info about HTTP headers like Host).
Usually the header that ASP.NET sees is identical to the Host header sent by the browser. However, it's possible they won't match if software or hardware is sitting in between the browser and your ASP.net code and is rewriting the Host header.
For example, large-scale budget hosters like GoDaddy do this so they can support multiple top-level domains on a single IIS website, even on their cheaper hosting plans. Instead of creating a separate IIS website (which adds to server load and hence cost), GoDaddy remaps requests for http://secondsite.com/ to a virtual directory on your "main" hosted site, e.g. http://firstsite.com/secondsite). They will change both the Host: header as well as the URL.
BTW, you can easily verify that this is what's happening by dumping the contents of HTTP Request Headers that your app is receiving.
Anyway, if you want to figure out who is changing the Host header, start with the people who host your web app (or the team which is responsible for your load balancer and/or reverse proxy), since they're likely the ones responsible for rewriting your Host header.