> By issuing a single certificate with Subject Alternate Names to cover multiple domains, LetsEncrypt can leak the IP of an origin server that is behind a service such as Cloudflare. This increases the risk of DDOS attack.
I echo Hugo and Rich's position that ACME is the wrong place to solve this particular problem. It's also worth noting that certificate attributes are just one way[0] that your origin might be discovered by an adversary attempting to bypass your CDN/WAF. [0] https://cloudpiercer.org/paper/CloudPiercer.pdf On Sat, Sep 24, 2016 at 8:51 PM, Bryan Jones <[email protected]> wrote: > Summary: > > By issuing a single certificate with Subject Alternate Names to cover > multiple domains, LetsEncrypt can leak the IP of an origin server that is > behind a service such as Cloudflare. This increases the risk of DDOS attack. > > > Scenario: > > 1. I run a VPS that, through Apache Virtual Hosts, serves content for 5 > websites. All five sites are secured with LetsEncrypt. > > 2. Suppose that two of those websites are protected by Cloudflare. The IP > address of my origin server is hidden and public traffic can only resolve > my DNS to the IP address of Cloudflare’s server for these two websites. > > 3. By pulling up the LetsEncrypt SSL certificate in my browser, I can see > all five of my websites listed in the Subject Alternate Name field. An > attacker could reasonably assume that these sites might be related (i.e. > running on the same server). Using that information, the attacker could > resolve the DNS for each of the five sites and note that three of them > resolve to a non-Cloudflare IP. The attacker now has the IP address of the > origin server that is supposed to be protected by CloudFlare. > > > Suggestion: > > Is it possible for LetsEncrypt to (optionally?) issue one SSL certificate > per domain instead of using SAN? > > > Reasoning: > > It’s very common for a single server to host several different websites > through Virtual Hosts. It would be ideal if LetsEncrypt did not “leak” the > fact that those websites are all related. > > > Alternate Ways To Mitigate: > > 1. Ensure that ALL websites using the same LetsEncrypt certificate are > protected by Cloudflare and that none of them have any CNAME aliases that > can resolve to the origin server’s IP. > > 2. Run a separate server for each website. > > > -Bryan > > _______________________________________________ > Acme mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/acme >
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