I've written in the past about the EFF SSL Observatory. It's a great project that has scanned the whole IP space for SSL-certificates used in HTTPS. They provide a database with meta information and their project found a couple of issues where CAs have issued certificates with weak security settings and violation of their own policies. CAcert is a project which tries to be the "better SSL authority" - it issues certificates for free, based on a web-of-trust community. The CAcert root certificate is not part of any major web browser. The EFF has mainly analyzed the browser-accepted CAs - but they provide the data, so I could do it myself.
I did some checks on the all_certs table selecting the certificates from cacert. I found out that there were 143 valid certificates with 512 bit. That is completely insecure and breakable by a home computer today. I also found that the majority of certificates still has 1024 bit, which by today's standards should be considered harmful - there have been no public breaks yet, but it's expected that it's possible to build an RSA-1024 cracker for an attacker with enough money.
I did the following query on the database: SELECT RSA_Modulus_Bits, count(*) FROM all_certs WHERE `Validity:Not After datetime` > '2010-03-08' AND ( `Issuer` like '%CAcert.org%' OR `Issuer` like '%cacert.org') GROUP BY `RSA_Modulus_Bits` ORDER BY count(*);
Now, what further checks can we do? I checked for the RSA exponent. I found two certificates in the database with exponent 3. RSA with low exponent is also considered insecure, although one has to state that this is not a serious issue. RSA with low exponents is not insecure by itself, but it can create vulnerabilities in combination with other issues (if you're interested in details, read my diploma thesis).
I have not checked the CAcert database for the Debian SSL vulnerability, as that would've been non-trivial. There were scripts shipped with the SSL Observatory data, but I found them not easy to use, so I skipped that part.
My suggestions to cacert were to revoke all certificates with serious issues (like the 512 bit certificates). Also, I suggested that new certificates with insecure settings like RSA below 2048 bits or a low exponent should not be allowed. CAcert did most of this. By now, all 512 bit certificates should be revoked and it is impossible to create new ones below 1024 bit or with low exponents. It is however still possible to create 1024 bit certificates, which is due to a limitation in the client certificate creation script for the Internet Explorer. They say they're working on this and plan to prevent 1024 bit certificates in the future. They also told me that they've checked for the Debian SSL bug.
I've reported the issue on the 11th March and got a reply on the same day - that's pretty okay, one slight thing still: There was no security contact with a PGP key listed on the webpage (but I got a PGP-encrypted contact once I asked for it). That's not good, I expect especially from a security project that I can contact them for security issues with encrypted mail. One can also argue if four months is a bit long to fix such an issue, but as it was far away from being trivial, this can be apologized.
I'd say that I'm quite satisfied with the reactions of CAcert. I always got fast replies to questions I had and the issues were resolved in a proper way. I have other points of criticism on the security of CAcert, the issue that bothers me most is that they still use SHA-1 and refuse to switch to a more secure hashing algorithm like SHA-512, although all major browsers have support for this since a long time.
I want to encourage others to do further tests on CAcert. I'd like to see CAcert being an authority that does better than the commercial ones. The database from the observatory is a treasure and should be used by projects like CAcert to improve their security.