Last updated at Fri, 21 Jul 2017 17:07:33 GMT

Mozilla's Firefox and Thunderbird recently removed 1024-bit certificate authority (CA) certificates from their trusted store. This change was announced to the various certificate authorities in May of this year and shipped with Firefox 32 on September 2nd. This change was a long time coming, as the National Institute of Standards and Technology (NIST) recommended that 1024-bit RSA keys be deprecated in 2010 and disallowed after 2013. A blog post at provided a list of specific certificates that would no longer be trusted starting with Firefox 32.

There is a little disagreement that 1024-bit RSA keys may be cracked today by adversaries with the resources of nation states. As technology marches on, the security of 1024-bit keys will continue to deteriorate and become accessible by operators of relatively small clusters of commodity hardware. In the case of a CA key, the successful factoring of the RSA primes would allow an adversary to sign any certificate just as the CA in question would. This would allow impersonation of any "secure" web site, so long as the software you use still trusts these keys.

This is certainly a welcome change, but how many sites are going to be affected by the removal of these CA certificates, and, how many of these sites have certificates that aren't due to expire anytime soon? Fortunately there is a means to answer these questions.

In June of 2012, the University of Michigan began scanning the Internet and collecting SSL certificates from all sites that responded on port 443. At Rapid7, we started our own collection of certificates starting in September of 2013 as part of Project Sonar, and have been conducting weekly scans since.

Both sets of scans record the entire certificate chain, including the intermediate CA keys that Mozilla recently removed from the trusted store. We loaded approximately 150 scans into a Postgres database, resulting in over 65 million unique certificates, and started crunching the data.

The first question we wanted to answer, which is how many sites are affected, was relatively easy to determine. We searched the certificate chain for each of the roughly 20 million web sites we index to check if the SHA1 hashes listed in the blog post are present in the signing chain. After several minutes Postgres listed 107,535 sites that are using a certificate signed by the soon-to-be untrusted CA certificates. That is a relatively large number of sites and represents roughly half a percent of all of the web sites in our database.

The next question we wanted to explore was how long the 1024-bit CA key signed certificates would continue to be used. This proved to be informative and presents a clearer picture of the impact. We modified the first query and grouped the sites by the certificate expiration date, rounded to the start of the month. The monthly counts of affected sites, grouped by expiration date, demonstrated the full extent of the problem.

The resultant data, shown in part in the graph below, makes it clear that the problem isn't nearly as bad as the initial numbers indicated, since a great many of the certificates have already expired and the rest will do so over the next year. Surprisingly, over 13,000 web sites presented a certificate that expired in July of this year. Digging into these, we found that almost all of these had been issued to Vodafone and expired on July 1st. These expired certificates still appear to be in use today.

The graph below demonstrates that the majority of affected certificates have already expired and those that haven't expired are due to expire in the next year. We have excluded certificates from the graph that expired prior to 2013 for legibility.

While Mozilla's decision will affect a few sites, most of those that are active and affected have already expired, and shouldn't be trusted on that basis alone.

In summary, the repeal of trust for these certificates is a sound decision based upon NIST recommendations, and while it initially appeared that a great many sites would be affected, the majority of these sites either have expired certificates or a certificate that expires within the next year. We hope that Chrome and other browsers will also remove these certificates to remove the potential risk involved with these 1024-bit CA keys.

Going forward, we are now tracking the certificates presented by SMTP, IMAP, and POP services, and will keep an eye on those as the data rolls in. If you still use a 1024-bit RSA key for any other purpose, such as a Secure Shell (SSH) or PGP, it is past time to consider those obsolete and start rolling out stronger keys, of at least 2048 bits, and using ECC-based keys where available.

- Labs