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Enhancing Trust in the CA/Browser System

Created on Apr 10 2015, 7:36 PM by Dean Coclin

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Browsers and Certificate Authorities are in the news again over the reported mis-issuance of an SSL server certificate to google.com domains. Discovered by Google most likely via technology known as key pinning and discussed by Google’s Adam Langley in this blog, a Chinese certificate authority, CNNIC (Chinese Internet Network Information Center), apparently issued an intermediate certificate to an Egyptian company called MCS Holdings. Because the CNNIC root certificate is included in the root store of most major browsers, users would not see any warnings on sites that have certificates issued by CNNIC or MCS Holdings. When MCS installed their intermediate into a Man in the Middle (MITM) proxy device, that device could then issue certificates for sites which users connected via that proxy would visit.

There are several violations of the CA/B Forum Baseline Requirements and Mozilla Root Program Requirements here. First, Mozilla specifically prohibits using public roots for MITM applications. (While there may be legitimate corporate use cases for these proxy devices, using public root certificates as part of the implementation is prohibited and is a violation of public trust). Second, any sub CA certificates (issued from the Root) must be publicly disclosed and audited or be technically constrained(using the technology known as “name constraints” which limits the domains which the CA can issue to. Neither appears to be the case here. Third, indications are that the key was not generated and stored in a proper Hardware Security Module (HSM). There are several other mistakes as well but these are the major ones.

CNNIC documents show that the sub CA certificate was only issued for a short duration and was to be used for test purposes only. While this may be the case, it ignores the reality that the misuse of such a certificate can cause great harm to end users. Users can be deceived to go to a fraudulent website and have their credentials stolen. The fact that bogus certificates found their way onto the public Internet due to this “test” makes it clear that improper controls were in place at both CNNIC and MCS Holdings as well as a limited understanding of the rules surrounding public CAs.

The major browsers quickly moved to un-trust the MCS Holdings certificate to protect their users from potential fraud. MCS sent a report to Mozilla with their assessment of the situation. Google has announced that they are taking action to distrust the CNNIC root certificates.  Google will “whitelist” all existing CNNIC certificates and has provided a path for re-inclusion into their browser by insisting all future certificates use Certificate Transparency.  Firefox will be updated to distrust any CNNIC certificate with a notBefore date of April 1, 2015. The current CNNIC root will remain in the Mozilla root store to validate current certificates and CNNIC can reapply for full inclusion but may be subject to additional scrutiny and controls during the process. This is essentially a punishment for violating the Baseline Requirements and the Mozilla root program rules.  Microsoft is still evaluating whether to take further action than just distrusting the MCS Holdings Intermediate certificate. No word from Apple so far.

Three recently introduced technologies and controls namely Certificate Authority Authorization (CAA) and Certificate Key Pinning (HPKP), which are designed to prevent mis-issuance, and Certificate Transparency (CT) which is designed to detect mis-issuance, significantly raise the level of security of the CA/Browser cryptography system. CT and HPKP are being implemented by some browsers and CAA is a function that CAs will have to deploy.

What is the lesson learned here? Not all CAs are created equal. Clearly CNNIC broke the rules and got caught. Whether it was intentional or not is being debated in the public. It doesn’t appear from the evidence that this was intentionally malicious. Symantec and all the SSL issuing CAs are held to high standards with regard to the ecosystem rules including CA/B Forum Baseline Requirements, Network Security controls, and Mozilla, Microsoft, Google, Apple and other root program requirements. We have strict controls in place to insure sub CA certificates are either disclosed or constrained, have strong and knowledgeable vetting and authorization teams, obtain regular audits from accredited WebTrust auditors and work closely with the major browser vendors in the CA/B Forum. While we do issue sub CA certificates to third parties, we are well aware of the strict rules surrounding this practice and the need to remain vigilant. Symantec supports the use of CT, CAA, and HPKP technologies and urges adoption by all participants in the ecosystem.  In the end, it matters which CA you choose so pick one that has a long track record and invests in its infrastructure to insure its customers are protected.

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