SSL Hardening for Web Application Proxy Servers

The Web Application Proxy (WAP) Servers act as an SSL termination instance towards the Internet. External connections that try to access the Active Directory Federation Services (ADFS) farm or internal applications that are published via the Web Application Proxy will terminate their SSL connections at the Web Application Proxy. Unfortunately, the Windows 2012R2 server default settings allow a lot of SSL Cipher Suites that are publically known as weak or “outdated” like SSLv3, DES encryption and key length below 128bit. The preferred Server Ciphers of a freshly installed and updated Windows 2012R2 server are SSLv3    168 bits        DES-CBC3-SHA TLSv1    256 bits        AES256-SHA Therefore from a network security standpoint it is mandatory to harden the SSL settings on the Web Application Servers BEFORE opening the WAP server in the DMZ for incoming Internet connections. The best option to harden the SSL settings on a standalone Windows Server 2012R2 is to modify the Local Group Policy: From a commandline run: “gpedit.msc”. In the computer section navigate to “Administrative Templates – Network – SSL Configuration Settings”  Edit the “SSL Cipher Suite Order”:

The listed Cipher suites can be exported and adjusted to the actual security requirements by deleting the unwanted Ciphers from the list. As a minimum all combinations that contain SSL2, SSL3, DES, 3DES, MD5 elements are deleted as well as all combination with a cipher length below 128bit.

Information

The list needs to be sorted in a way that the preferred SSL ciphers are on top.

Afterwards create a string of the values from list and separate each cipher by “,” without any blank. Don’t leave a “,” at the end of the string. The input box in the GPO menu has a limited size. Make sure that your string fits into this limit. If not, delete further ciphers which are not widely used.

Warning!

It looks like simply activating the new local GPO by running “gpupdate /force” is not sufficient. Please reboot the WAP servers one-by-one after setting the SSL cipher policy

However, before we open the firewall, an internal test should be executed to validate the SSL hardening. You can run the sslscan tool (you can download from here sslscan) from another computer in the DMZ or the WAP server itself. DNS resolving of the federation or application name must resolve to the external Load Balancer or interface of the WAP server. Example for SSL Server Ciphers before SSL Hardening (left side) and after SSL Hardening (right side):

When the Web Application Proxy server has been connected to the Internet finally, a second check can be achieved by using one of the proven Internet based SSL scan tools, e.g. https://www.ssllabs.com/ssltest/ .

You can find a string of the recommended SSL ciphers for importing into local GPO here.

4 thoughts on “SSL Hardening for Web Application Proxy Servers

    1. Hello Thomas,
      I have added a link to a text file at the end of the blog post. The file contains a string of ciphers that can be inserted into a local GPO on the WAP to define the allowed cipher Suite.
      Hope that helps
      Robert

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