For anyone else who didn't have "remote perspectives" in their personal technical lexicon: this refers to performing the ACME challenge/response check when validating the domain of a certificate signing request from multiple physical locations, and therefore via multiple physical network routes, in order to make network-level attacks more difficult.
For example, if you were validating example.com, you'd need to set up some response to the ACME challenge somewhere, either through direct DNS-based validation (DNS-01, DNS-PERSIST-01) or through a server running at the resolved A/AAAA IP address for example.com (HTTP-01, TLS-ALPN-01). In all cases, when a CA goes to validate your challenge response, it relies both upon DNS and upon the network integrity between the CA's datacenters and your DNS and/or app servers. Both of these can be subverted, eg via BGP attack, to allow an attacker to replace a DNS response, re-route the destination IP address to a different server, etc, at which point they can inject their own challenge response, and therefore issue their own (unauthorized) certificates.
Multi-perspective validation means that the CA validates your challenge response from multiple datacenters on different networks instead of just one, such that a hypothetical network attack would have to target multiple independent networks in order to successfully issue their unauthorized certificate.
In August an attacker was able to get a certificate issued even with multi-perspective validation:
> For 33 hours in late August, a rival network hijacked a block of Hetzner-hosted infrastructure serving Softaculous and Virtualizor, using it to issue a valid TLS certificate and push a malicious software update. The stolen route was RPKI-invalid the entire time, on file, checkable by anyone. It still won on most of the internet.
> At 20:57 UTC on 28 August, a network nobody at Hetzner or Softaculous had dealt with before started announcing 162.55.80.0/24 to the internet. It was a small, unremarkable-looking slice of Hetzner’s own address space — the block hosting Softaculous’s client area, its billing pages, and the update endpoint that every Virtualizor-licensed control panel on the internet periodically calls home to. For the next 33 hours, with an 11-hour lull in the middle, a meaningful share of global traffic bound for that block went to the hijacker instead. Some of it was Let’s Encrypt’s own domain-control validation, which is how the attacker walked away with a certificate for virtualizor.com that no browser would ever flag.
So it's all very well to have multiple network locations to prevent hijacking on the path (i.e., you have to attack multiple paths), but if the network as a whole thinks the destination is someplace else then all those paths will lead to the same (false) location.
Perhaps the CAB should consider RPKI validation as a "MUST" rather than piling on multiple sources of validation who still have the same single point of failure (the spoofing of the destination).
70% adoption doesn't seem high enough or it to be a mandatory component of validation[1], that seems like it would be very disruptive to nearly a third of the Internet.
I think having multiple authoritative DNS servers in different networks might help with this.
I'm not sure how CAs deal with situations when a domain has several authoritative DNS servers, but not all of them respond with expected reply. Would it would be a good idea for CAs to ask *all* authoritative DNS servers and require a coherent answer from them?
For example, if you were validating example.com, you'd need to set up some response to the ACME challenge somewhere, either through direct DNS-based validation (DNS-01, DNS-PERSIST-01) or through a server running at the resolved A/AAAA IP address for example.com (HTTP-01, TLS-ALPN-01). In all cases, when a CA goes to validate your challenge response, it relies both upon DNS and upon the network integrity between the CA's datacenters and your DNS and/or app servers. Both of these can be subverted, eg via BGP attack, to allow an attacker to replace a DNS response, re-route the destination IP address to a different server, etc, at which point they can inject their own challenge response, and therefore issue their own (unauthorized) certificates.
Multi-perspective validation means that the CA validates your challenge response from multiple datacenters on different networks instead of just one, such that a hypothetical network attack would have to target multiple independent networks in order to successfully issue their unauthorized certificate.
More info:
https://letsencrypt.org/2020/02/19/multi-perspective-validat...
https://community.letsencrypt.org/t/multi-perspective-valida...
> For 33 hours in late August, a rival network hijacked a block of Hetzner-hosted infrastructure serving Softaculous and Virtualizor, using it to issue a valid TLS certificate and push a malicious software update. The stolen route was RPKI-invalid the entire time, on file, checkable by anyone. It still won on most of the internet.
> At 20:57 UTC on 28 August, a network nobody at Hetzner or Softaculous had dealt with before started announcing 162.55.80.0/24 to the internet. It was a small, unremarkable-looking slice of Hetzner’s own address space — the block hosting Softaculous’s client area, its billing pages, and the update endpoint that every Virtualizor-licensed control panel on the internet periodically calls home to. For the next 33 hours, with an 11-hour lull in the middle, a meaningful share of global traffic bound for that block went to the hijacker instead. Some of it was Let’s Encrypt’s own domain-control validation, which is how the attacker walked away with a certificate for virtualizor.com that no browser would ever flag.
* https://cloudpunk.blog/post/bgp-hijack-rpki-gap/
So it's all very well to have multiple network locations to prevent hijacking on the path (i.e., you have to attack multiple paths), but if the network as a whole thinks the destination is someplace else then all those paths will lead to the same (false) location.
Perhaps the CAB should consider RPKI validation as a "MUST" rather than piling on multiple sources of validation who still have the same single point of failure (the spoofing of the destination).
[1]: https://ipinfo.io/blog/rpki-protects-70-internet-routes
I'm not sure how CAs deal with situations when a domain has several authoritative DNS servers, but not all of them respond with expected reply. Would it would be a good idea for CAs to ask *all* authoritative DNS servers and require a coherent answer from them?