You Can Now Use CAA to Auto-Authorize Certificates in Cert Spotter, Without Compromising Transparency
One of the most requested features for the new Cert Spotter has been for Cert Spotter to consult CAA records to decide if a certificate is authorized. This makes a lot of sense - many domain operators have already published CAA records, and it's redundant to duplicate the information in your Cert Spotter settings. Also, CAA records can apply different policies to different domain names, which is more flexible than the list of authorized CAs in your Cert Spotter settings, which applies to every domain you monitor.
However, implementing this feature without degrading the security of Cert Spotter was trickier than it sounds. DNS is, by and large, not authenticated, which means that when Cert Spotter goes to look up CAA records, a network-layer attacker could return bogus CAA records which trick Cert Spotter into thinking a certificate is authorized.
DNSSEC, if we set aside its low adoption rate and deployment challenges, would at least make CAA lookups authenticated, but they still wouldn't be transparent. A misbehaving parent zone, such as a TLD registry, could return validly-signed but malicious CAA records and no one would be the wiser. That's a problem for Cert Spotter, which takes great care not to blindly trust third parties. Cert Spotter verifies the Merkle Trees of all Certificate Transparency logs, and gossips information about them with other auditors, including Google's Certificate Transparency monitor and a network of "honeybees" located all over the Internet. This gives Cert Spotter a high degree of confidence that the logs which it relies upon are not hiding certificates. DNSSEC doesn't provide the same level of confidence that DNS servers are not misbehaving.
For this reason, using CAA was never a consideration during the development of Cert Spotter. But our customers have spoken, so we set out to find a way to mitigate the risks of CAA as much as possible, and settled on the following solution:
You can optionally enable the use of CAA records for certificate authorization in your settings.
At any time you can examine the list of CAA records that Cert Spotter is relying upon to make authorization decisions.
Cert Spotter will email you any time it detects that a CAA record has been added, modified, or removed.
While this solution won't stop Cert Spotter from being fooled by a malicious CAA record, you will at least know about it and be able to take action in response - much like Certificate Transparency doesn't stop malicious certificates, but makes sure you know about them.
Cert Spotter's use of CAA records is unique among Certificate Transparency monitors and is just one example of how SSLMate is innovating to bring you certificate monitoring that reduces alert fatigue by making it easy to automatically authorize (and elide notifications for) legitimate certificates - without reducing security. We have some other features in the pipeline to offer even more flexibility in auto-authorizing certificates. Get in touch if you are interested in being added to our beta tester list.
Do you operate a website? Do you like security and uptime? Cert Spotter monitors your website's security certificates to give you early warning about security and uptime problems like malicious certificates, DNS tampering, and certificate expiration - so you can act before your customers notice a problem, not after.