TLS unjammed: scanning for weak Diffie-Hellman groups

On May 20th, 2015, a paper was released that got some media attention. The newly disclosed research focused on a new vulnerability, coined Logjam. eSentire issued an advisory to our clients soon after the vulnerability's disclosure and immediately, we saw the usual round of everyone running initial scans, patching their systems, and cleaning up.

But there was a lot more than just Logjam discussed in the initial paper, and the disclosures discussed are worth a closer look. As was acknowledged in the CVE assignation, Logjam only addressed one part of the multi-faceted cryptographic disclosure. Many different services rely on diffie-hellman and are potentially vulnerable to the weaknesses that were discussed in the paper, and yet all of the scanning tools that have been released were narrowly focused on testing just the specific case in TLS that got the CVE designation.

As time moves on, major vendors continue to issue patches to address areas of the vulnerability affecting their products. With every new patch, the initial concerns around Logjam have become less pressing. But what can we do about all the other potentially vulnerable services? We haven't seen a CVE released to demonstrate a specific attack that takes advantage of some of those services, but it makes sense to consider the rest of the original research paper and identify critical next steps. Rather than wait for the next inevitable breach to land in the news, we should be spending time updating configurations and pro-actively closing cryptographic holes in the services that we rely on.

Jacob Gajek, Principal Security Researcher at eSentire, has released a paper for system administrators which is designed to drive industry momentum toward an improved, proactive security posture. It provides an explanation of security concerns stemming from the initial cryptographic paper, making them more accessible to the layman. Released along with the paper is a useful scanning tool to help identify extended Logjam Man-In-The-Middle vulnerabilities.

We recognize that in the case of Logjam and similar vulnerabilities, it's critical to be vigilant and recommend regular vulnerability scanning to harden infrastructures against aftershock-style attacks.

The discussed scanning tool can be found here.