Another critical flaw affecting Transport Layer Security (TLS) was discovered recently that could put some organizations at risk. The “Logjam” attack exploits a weakness in how the Diffie-Hellman key exchange is used. An attacker, acting as a man-in-the-middle, can potentially force a downgrade of the TLS connection, resulting in the use of weak cryptography. The Qualys SSL Labs SSL Server Test has been updated to identify this vulnerability. When testing a DirectAccess server you will receive the following warning message.
“This server supports weak Diffie-Hellman (DH) key exchange parameters. Grade capped to B.”
DirectAccess leverages SSL and TLS as part of the IP-HTTPS IPv6 transition protocol, which is used to tunnel IPv6 packets over the IPv4 Internet. These IPv6 packets are encrypted using IPsec. If an attacker were to break the SSL/TLS connection they would gain nothing. Because of this, a dedicated DirectAccess server is unaffected by the Logjam attack. Mitigating it would provide no additional protection, so you can safely ignore the warning about weak DH key exchange parameters being supported.
However, if DirectAccess has been configured to use one-time password (OTP) authentication, the client-based VPN role has been enabled and configured, or the Web Application Proxy (WAP) role has been installed on the DirectAccess server, then the Logjam attack represents a serious risk and should be mitigated. Also, in some cases it may be desirable to make this change on a dedicated DirectAccess server just to prevent an audit finding and avoid having to explain why the DirectAccess workload would be unaffected by this attack.
To mitigate this vulnerability it will be necessary to remove support for cipher suites that use the Diffie-Hellman key exchange protocol on the DirectAccess server. This is accomplished by opening the Local Group Policy Editor (gpedit.msc) on the DirectAccess server and expanding Computer Configuration, Administrative Templates, and Network. Select SSL Configuration Settings and then double-click SSL Cipher Suite Order. Select Enabled and then replace the default list of cipher suites with the following list.
Once complete, restart the DirectAccess server. The Qualys SSL Labs server test should no longer give a warning about the use of weak Diffie-Hellman keys. In addition, this reordering and optimization of cipher suites will also improve the protocol support and key exchange scores, as shown here.
As a reminder, and overall rating of “F” is expected when testing a dedicated DirectAccess server. By design, DirectAccess provides support for null cipher suites to improve scalability and performance for Windows 8.x and later DirectAccess clients. More details here.