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Excuse my ignorance but how does this allow someone to forge TLS certs for a given website/domain name that already has active certs? Wouldn't they have to revo

Excuse my ignorance but how does this allow someone to forge TLS certs for a given website/domain name that already has active certs? Wouldn't they have to revoke existing certs and then get new ones during the attack to pull this off?

zerocrates 9y ago
You don't need to revoke existing certificates to generate/forge new ones.
libeclipse 9y ago
You can have multiple certificates for a single host.
evgen 9y ago
The new, fake cert just has to validate for your phish/drove-by/cred-jacking site that is mimicing a real site. You are not planning a long-term take-over but just to make sure the new site passes checks by your victims.
jerf 9y ago
Step one: Hijack the DNS entries so they point at your site. Step two: Obtain a certificate from Let's Encrypt using website validation. Step three: Proxy traffic through a proxy you provide that SSL cert for and capture anything you like. In the event that a website legitimately has a higher-class Extended Validation cert or other such thing, hope users don't notice the downgrade from Extended Validation correct SSL cert to a merely correct SSL cert, which is a pretty solid hope. Certs don't have any sort of overriding or superseding property. A given cert and its chain of signatures is either accepted because you trust the signers, or it isn't; at the time you are making that decision, you don't have access to know whether any other certs have been issued since then or anything. One thing that comes to mind is that browsers really ought to scream if they see a domain that was previously Extendedly Verified drop down to a Let's Encrypt-level cert. Though how much good this would do in practice is hard to tell... "Hmmm, what's this about the SSL cert being less good? It still has one, right? Let's just click through this... hmm... looks like the web site I expected... must just be the computer being weird enters password"
- 24gttghh 9y ago
  For some reason I thought CA's at least talked to each other to see if someone else had issued a valid cert for a given domain along the lines of owning a domain name itself through a registrar. Clearly that only works when DNS isn't being abused like this...
  - xnyanta 9y ago
    Why should websites be limited to having only one certificate? For example, if you have a service running on multiple backed servers, you might want to have once certificate per server for all of your domains.
    - notriddle 9y ago
      That use case still wouldn't explain a domain having certs from two different CAs.
manigandham 9y ago
You can generate as many certificates as you want for a given name. There is no master list of all the live certificates out there and there are no checks for existing certificates when generating new ones (otherwise we'd never be able to renew certs until the old ones expire, resulting in downtime).

zerocrates 9y ago

You don't need to revoke existing certificates to generate/forge new ones.

libeclipse 9y ago

You can have multiple certificates for a single host.

evgen 9y ago

The new, fake cert just has to validate for your phish/drove-by/cred-jacking site that is mimicing a real site. You are not planning a long-term take-over but just to make sure the new site passes checks by your victims.

jerf 9y ago

Step one: Hijack the DNS entries so they point at your site. Step two: Obtain a certificate from Let's Encrypt using website validation. Step three: Proxy traffic through a proxy you provide that SSL cert for and capture anything you like. In the event that a website legitimately has a higher-class Extended Validation cert or other such thing, hope users don't notice the downgrade from Extended Validation correct SSL cert to a merely correct SSL cert, which is a pretty solid hope. Certs don't have any sort of overriding or superseding property. A given cert and its chain of signatures is either accepted because you trust the signers, or it isn't; at the time you are making that decision, you don't have access to know whether any other certs have been issued since then or anything. One thing that comes to mind is that browsers really ought to scream if they see a domain that was previously Extendedly Verified drop down to a Let's Encrypt-level cert. Though how much good this would do in practice is hard to tell... "Hmmm, what's this about the SSL cert being less good? It still has one, right? Let's just click through this... hmm... looks like the web site I expected... must just be the computer being weird enters password"

24gttghh 9y ago
For some reason I thought CA's at least talked to each other to see if someone else had issued a valid cert for a given domain along the lines of owning a domain name itself through a registrar. Clearly that only works when DNS isn't being abused like this...
- xnyanta 9y ago
  Why should websites be limited to having only one certificate? For example, if you have a service running on multiple backed servers, you might want to have once certificate per server for all of your domains.
  - notriddle 9y ago
    That use case still wouldn't explain a domain having certs from two different CAs.

manigandham 9y ago

You can generate as many certificates as you want for a given name. There is no master list of all the live certificates out there and there are no checks for existing certificates when generating new ones (otherwise we'd never be able to renew certs until the old ones expire, resulting in downtime).