A client recently ran Qualys SSL Server Test against their web applications published through the Azure Application Gateway. The test graded the SSL security on the site as a “B” mainly because the server supported weak Diffie-Hellman (DH) key exchange parameters.
Diffie-Hellman key exchange is a popular cryptographic algorithm that allows Internet protocols to agree on a shared key and negotiate a secure connection. SSL sites that support export cipher suites and don’t use 2048-bit or stronger Diffie-Hellman groups with “safe” primes are susceptible to attacks like LogJam. Luckily, a feature known as SSL Policy in the Azure Application Gateway allows you to reduce the potential for these types of attacks.
The SSL handling in Azure Application Gateway (used for things such as SSL offloading and centralized SSL handling) allows you to specify a central SSL policy that’s suited to your organizational security requirements. The SSL policy includes control of the SSL protocol version as well as the cipher suites and the order in which ciphers are used during an SSL handshake. Application Gateway offers two mechanisms for controlling SSL policy: either a predefined policy or a custom policy. Here’s a link to the documentation for SSL policy with Azure Application Gateway. Changing the SSL policy for a new Application Gateway deployment can be accomplished using PowerShell and changing an existing deployment’s SSL policy is also easily done via the cmdlets. Below is an example of how to do this with a few lines of PowerShell.
One “gotcha” is that the predefined SSL policy which disables the weaker cipher suites also sets a minimum TLS version of v1.2 and breaks most older browsers. If that’s not a concern, use the latest predefined SSL policy – otherwise you’ll have to use a custom policy and specify a lower minimum TLS version to support older IE browsers running on Windows 7, for example.