Thursday, November 26, 2009

How fast are Debian-flawed certificates being re-issued?

In 2008 it was discovered that the OpenSSL package in Debian had been producing low entropy public keys for about a year and a half on its Etch distribution. While it was relatively easy to patch the offending code (only a few lines), it was going to be more difficult to track down and re-issue all the weak public keys that had found their way into SSL server certificates. From my post on the topic

An article in the Register, called Debian's Epic SSL Blunder, states that the number of SSL certificates that may need replacing could be in the hundreds of thousands or even millions. So while the OpenSSL PRNG code can be easily patched, identifying and replacing all the weak keys generated by the flawed code is a big operational headache. It may be months or years before all the weak keys and their corresponding certificates are tracked down and upgraded.

At the Internet Measurement Conference (IMC) held in early November, researchers Scott Yilek, Eric Rescorla, Hovav Shacham, Brandon Enright, and Stefan Savage presented a study on the rate at which Debian-flawed SSL server certificates were being replaced. In short, the news could have been better.

The researchers tracked a collection of approximately 50,000 public web servers over a period of 6 months. Initially around 1.5% of the servers (751 to be exact) were using Debian-flawed keys in their certificates, and the observed rate at which these certificate were being re-issued  is shown in the graph below


The researchers stress that as compared to typical patching rates for general vulnerabilities, re-issuing certificates for the sample of weak servers was very slow. A long term study by Qualys reported this year that the patching half-life for vulnerabilities is 30 days, and so over a 6 month period we should see an exponential decrease in unpatched endpoints. However the graph above is approximately linear, and 30% of the Debian-flawed certificates were still not re-issued after almost 180 days. The authors conclude that

… unlike other vulnerabilities which have been studied and typically show a short, fast, fixing phase followed by levelling off, certificates were replaced on a slower cycle with substantial fixing extending well past five months after the announcement. We also found that in some cases certificate authorities continued to issue certificates to weak keys long after the vulnerability was announced.

Incidentally the researchers also found that approximately 2% of the sampled servers (1000 or so) were still using 512-bit RSA keys. While such keys are not as weak as those produced by the Debian flaw, recovering the associated private keys was recently shown to require nothing more than a  3-day desktop calculation. Nonetheless, this fraction of 512-but keys is a dramatic improvement over the results of a  survey conducted in 2000 which found that almost a quarter of the 8,000 servers sampled were using 512-bit keys.

No comments: