Friday, July 2, 2010

Intercepting SSL traffic using WebScarab

The last time I wrote about intercepting web requests using WebScarab, I was successful in intercepting SSL traffic generated through a custom Java client. Even though the process to do that was quite tedious – involving exporting the WebScarab server certificates into .cer format, importing the certificate into a Java keystore and then running WebScarab as a reverse proxy – I was able to intercept and view the SSL traffic that was being generated. But there was an inherent issue with that process that I overlooked.

When a proxy is setup to intercept SSL traffic, the security issue is that the SSL certificate that is presented by the proxy is not signed by a trusted authority. Web browsers detect this and give the user an option to accept or not accept this risk. So there is no problem in using the proxy to intercept web traffic to secure sites and we can just point the browser to the proxy and accept when warned about certificate error. But in case of Java clients using JSSE, there is no assumption of an interactive user session and so by default it throws an exception if there are any certificate related issues – be it an unknown certificate in which case it throws the exception:

javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

or a hostname mismatch:

javax.net.ssl.SSLHandshakeException: java.security.cert.CertificateException: No name matching … found

The latter is thrown because when a new HTTPS connection is created using HttpsURLConnection class, it implements a default HostnameVerifier interface which checks if the host we’re trying to connect to matches the name in the certificate in its certificate store (specifically the cn within the certificate). If it doesn’t, it throws the above exception. The client I was using earlier overrode the default HostnameVerifier with a custom one, which ignored the hostname mismatch. But this time with a new client for a different application, it didn’t and I had to go one extra step to intercept the requests, which is detailed below. So first:

  1. Start WebScarab and run it as a reverse proxy on port 443. This is so that WebScarab behaves as a secure server to the client, even if with a self-signed certificate instead of one signed by a trusted authority. (If running WebScarab from the same machine that is generating the requests, we should also select “Intercept requests” check box. This is important because in that case, the proxy is an infinite loop to its own interface and so we want to be able to break the flow and Abort after the first intercept)
  2. Modify the hosts file to point WebScarab hostname to the IP of the machine where it is running. In case of local, it should be:
    127.0.0.1    WebScarab
    This is specifically so we can get around the issue of hostname mismatch because we’ll try to connect to host “WebScarab” instead of actual target server. If the client overrides the default HostnameVerifier to ignore those errors, it can be setup so that the client points to the actual host:
    <ip where WebScarab is running>    <target hostname>
  3. Use the java program available here to create a keystore with the WebScarab certificate
    >>java InstallCert WebScarab
    Since WebScarab hostname is pointing to the WebScarab proxy, this program will connect to it and retrieve its certificate. It will create a keystore file called jssecacerts with WebScarab’s certificate (keystore password is blank by default).
  4. Configure the client to use WebScarab as the host within the URL. So instead of https://<hostname>/<path>, it should be https://WebScarab/<path>.
  5. Run the java client with the truststore and password properties: -Djavax.net.ssl.trustStore=<location to jssecacerts file> -Djavax.net.ssl.trustStorePassword=<password, default blank>

At this point, WebScarab proxy should intercept the request. I can review it, and abort it so it doesn’t repeat. Obviously, the request can’t be sent to the actual destination server. As I’ve noted above and as far as I know, there’s no way to get around the hostname mismatch error unless the default HostnameVerifier is overridden. But in my case, I was fine with just intercepting the request and creating my LoadRunner scripts using the raw HTTP request.

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