Category Archives: Security

SSL Termination and Secure Cookies/requireSSL with ASP.NET Forms Authentication

If you’re running a HTTPS-only web application, then you probably have requireSSL set to true in your web.config like so:

<httpCookies requireSSL="true" httpOnlyCookies="true"

With requireSSL set, any cookies ASP.NET sends with the HTTP response – in particular, the forms authentication cookies – will have the “secure” flag set. This ensures that they will only be sent to your website when being accessed over HTTPS.

What happens if you put your web application behind a load balancer with SSL termination? In this case, ASP.NET will see the request coming in as non-HTTPS (Request.IsSecureConnection always returns false) and refuse to set your cookies:

“The application is configured to issue secure cookies. These cookies require the browser to issue the request over SSL (https protocol). However, the current request is not over SSL.”

Fortunately, we have a few tricks up our sleeve:

  1. If the HTTPS server variable is set to ‘on’, ASP.NET will think we are over HTTPS
  2. The HTTP_X_FORWARDED_PROTO header will contain the original protocol running at the load balancer (so we can check that the end connection is in fact HTTPS)

With this knowledge, and the rewrite module available in IIS 7 upwards, we can set up the following:

    <rewrite>
        <rules>
            <rule name="HTTPS_AlwaysOn" patternSyntax="Wildcard">
                <match url="*" />
                <serverVariables>
                    <set name="HTTPS" value="on" />
                </serverVariables>
                <action type="None" />
                <conditions>
                    <add input="{HTTP_X_FORWARDED_PROTO}" pattern="https" />
                </conditions>
            </rule>
        </rules>
    </rewrite>

You’ll also need to add HTTPS to the list of allowedServerVariables in the applicationHost.config (or through the URL Rewrite config)

        <rewrite>
            <allowedServerVariables>
                <add name="HTTPS" />
            </allowedServerVariables>
        </rewrite>

With thanks to Levi Broderick on the ASP.NET team who sent me in the right direction to this solution!

Get ASP.NET auth cookie using PowerShell (when using AntiForgeryToken)

At FundApps we run a regular SkipFish scan against our application as one of our tools for monitoring for security vulnerabilities. In order for it to test beyond our login page, we need to provide a valid .ASPXAUTH cookie (you’ve renamed it, right?) to the tool.

Because we want to prevent Cross-site request forgeries to our login pages, we’re using the AntiForgeryToken support in MVC. This means we can’t just post our credentials to the login url and fetch the cookie that is returned. So here’s the script we use to fetch a valid authentication cookie before we call SkipFish with its command line arguments:

Forms Authentication loginUrl ignored

I hit this issue a while back, and someone else just tripped up on it so thought it was worth posting here. If you’ve got loginUrl in your Forms Authentication configuration in web.config set, but your ASP.NET Forms or MVC app has suddenly started redirecting to ~/Account/Login for no apparent reason, then the new simpleMembership(ish) provider is getting in the way. This seems to happen after updating the MVC version, or installing .NET 4.5.1 at the moment.

Try adding the following to your appSettings in the web.config file:

<add key="enableSimpleMembership" value="false"/>

which resolved the issue for me. Still trying to figure out with Microsoft why this is an issue.

Achieving an A+ grading at Qualys SSL Labs (Forward Secrecy in IIS)

At FundApps we love the SSL Labs tool from Qualys for checking best practice on our SSL implementations. They recently announced a bunch of changes introducing stricter security requirements for 2014, and a new A+ grade – so I was curious what it would take to achieve the new A+ grading. There are a few things required to now achieve A grading and then beyond:

  • TLS 1.2 required
  • Keys must be 2048 bits and above
  • Secure renegotiation
  • No RC4 on TLS 1.1 and 1.2 (RC4 has stuck around longer than it would be liked in order to mitigate the BEAST attack)
  • Forward secrecy for all browers that support it
  • HTTP Strict Transport Security with a long max age (Qualsys haven’t defined exactly what this is, but we use a 1 year value).

We’re using IIS so the focus of this entry is how to achieve an A+ grading in IIS 7/8.

Forward Secrecy & Best Practice Ciphers

Attention to Forward Secrecy has been increasing in recent time – the key benefit being if, say, the NSA obtain your keys in the future, this will not compromise previous communications that were encrypted using session keys derived from your long term key.

To set up support for Forward Secrecy, the easiest approach (in a Windows/IIS world) is to download the latest version of the IIS Crypto tool. This makes it really easy to get your SSL Ciphers in the right order and the correct ones enabled rather than messing directly with the registry.

Once downloaded, if you click the ‘Best Practice’ option, this will enable ECHDE as the preferred cipher (required for forward secrecy). The tool does also keep SSL 3.0, RC4 and 3DES enabled in order to support IE 6 on Windows XP. If you don’t require this, you can safely disable SSL 3.0, TLS_RSA_WITH_RC4_128_SHA and TLS_RSA_WITH_3DES_EDE_CBC_SHA in the cipher list. We also disable MD5.

HTTP Strict Transport Security

The other part of the equation is enabling a HTTP Strict Transport header. The idea with this is to stop man-in-the-middle attacks whereby they transparently convert a secure HTTPS connection to a plain HTTP connection. Visitors can see the connection is insecure, but there is no way of knowing that the connection *should* have been secure. By adding a HTTP Strict Transport header (which is remembered by the browser and stored for a specified period), then provided first communication with the server is not tampered with (by stripping out the header), the browser will prevent non-secure communication from then on.

Doing this is simple – but you need to ensure that you only return a Strict-Transport-Security header on a HTTPS connection. Any requests on HTTP should *not* have this header, and should be 301 redirect-ing to the HTTPS version. especially if your website only responds to HTTPS requests in the first place (we use a seperate website to redirect from non-HTTPS requests).

In our case, we have a seperate website already responsible for the non-HTTP redirection, so it was simply a case of adding the following in our system.webServer section of the web.config

<system.webServer>
  <httpProtocol>
    <customHeaders>
       <add name="Strict-Transport-Security" value="max-age=31536000" />
    </customHeaders>
  </httpProtocol>
</system.webServer>

If you have to deal with both HTTPS and non-HTTPS, then implementation section on WikiPedia gives an example of how.

The end result? An A+ grading from the SSL Labs tool.