When Certificate Authorities are Hacked
By Sigbjørn VikSigbjorn. Tuesday, August 30, 2011 12:45:34 PM
Certificate Authorities (CAs) have a lot of responsibility, they are in charge of ensuring who can present their site as secure and verified, which they do by issuing certificates to trusted site owners. There are hundreds of CAs around the world, and it will inevitably happen that some of them issue a certificate incorrectly, either by mistake or by hacking. The certificate system is built to tackle this, by having a built-in revocation system. CAs add a revocation URL1 to all certificates, and when a browser encounters such a certificate, the browser will check with that URL if the certificate is still valid. This allows CAs to immediately cancel any misissued or fraudulent certificates.
If a browser gets a negative response when checking the revocation URL, the browser will warn the user, and refuse to load the page. However, in most cases where an attacker is trying to spoof another server, the attacker is in full control of the network, so as to direct users to his own server. It is then easy for the attacker to additionally block the revocation URL. Some browsers will present a site as secure if the revocation URL is blocked,
Opera's address barwhich allows for abusing even a revoked certificate. Opera will downgrade the security level of the site to the same as any other regular web page in such unverified cases, which means that once a certificate is revoked by the issuer, it cannot be abused in Opera, even if the revocation URL is blocked. The most an attacker can do, is the same as he could without a certificate.
Browsers that do not have protection against blocked revocation lists will need to rapidly issue an update to fix any new certificate abuse. In Opera, users are protected automatically when the certificate is revoked. If the CA has a general problem, or a CA is no longer being used, we can remove it from our list of trusted CAs behind the scenes, and the user will also be secure, without needing to change anything in her browser.
You may well encounter reports in the media about fraudulent certificates. But rest assured that Opera takes care of these for you. Our advice is that as long as you are using Opera, pay attention to the address bar badge, and you will still be secure.
1 Edit: Technically there are two such URLs, but this post treats them both as one.
Edit 2: Regarding specific incidents with DigiNotar, please see this blogpost on rootstore.
Edit 3: Further actions regarding DigiNotar have been deployed, please see the announcement on the rootstore home page.