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> This seems like dubious statement. SSH authentication does not need to be key based. Let's say you just use an ID Token as a bearer token to authenticate to

> This seems like dubious statement. SSH authentication does not need to be key based. Let's say you just use an ID Token as a bearer token to authenticate to SSH. The SSH server now has the secret you used to authenticate with. Doesn't this introduce replay attacks where the SSH server can replay your ID Token to log into other SSH servers? Whereas if your ID Token functions like a "certificate" issued by your IDP binding your identity to a public key, it is no longer a secret. You can just use your public key to prove you are you. No secrets leave your computer. My motto: always use public key rather than a bearer secret if possible. > I understand the practicality of their approach, but I would have preferred this to be proper first-class authentication method instead of smuggling it through publickey auth method Me too. I have a PR open to SSH3 (not connected with OpenSSH) so it can be support OpenPubkey as a built-in authentication mechanism. https://github.com/francoismichel/ssh3/pull/146 https://github.com/francoismichel/ssh3/pull/146

slt2021 2y ago
> The SSH server now has the secret you used to authenticate with. secrets can be made unique per connection and single use
- confiq 2y ago
  this ^ GSSAPI can be more secured than public/private key if configured right.
  - dcow 2y ago
    Doesn’t this require the server to consult the IDP on every log in, though, to make sure the id token is valid? One of the staples of ssh from a UX standpoint is that it’s peer to peer.
    - lxgr 2y ago
      I suppose you could do something based on IDP-signed tokens, e.g. "valid for authentication to service x until <timestamp>"?
      - megous 2y ago
        This is basically a ssh certificate then.
        pluto_modadic 2y ago
        the difference is in /key management/. Key management is the hard part. Especially keyless SSH management. (things like sigstore's rekor/fulcio remove some complexity here). It is not "just a (manually generated) ssh certificate"
      - XorNot 2y ago
        Kerberos tickets have timeouts on them already, it's a matter of configuration how long you wait. The thing is most enterprises want "user disabled" to be instant. Which of course leads to SSH keys all over the place anyway.
  - EthanHeilman 2y ago
    Can you explain more? I want to be a fan of GSSAPI
    - thyristan 2y ago
      Don't know what the grandparent meant by GSSAPI, that is just an API for various underlying auth methods. But what people usually use together with GSSAPI is Kerberos. Kerberos can be very secure, much more so than CA-based or generally asymmetric crypto based approaches. Kerberos (if you ignore some extensions) uses symmetric cryptography, so it is less vulnerable to quantum computers. Use AES256 and you are fine, a quantum attacker can at the most degrade this to a 128bit level (according to current theories). Also, no weak RSA exponents, elliptic curve points at zero, variable-time implementations or other common pitfalls of asymmetric crypto. The trusted third party ("KDC" in Kerberos) distributes keys ("tickets") for pairs of user and service-on-server, so mutual authentication is always assured, not like in ssh or https where you can just ignore that pesky certificate or hostkey error. Keys are short-lived (hours to weeks usually), but there are builtin mechanisms for autorenew if desired. Each side of a key can also be bound to a host identity, so stealing tickets (like cookies in HTTP) can be made harder (but not impossible). The KDC can (theoretically, rarely implemented) also enforce authorisation by preventing users from obtaining tickets for services they are not authorized to use (the original idea of Kerberos was to just use it for authentication, the authorisation step is done by the service after authentication has been established). Single-Sign-On with all the usual protocols is included automatically, you log in to your workstation and get a ticket-granting-ticket that can then be used to transparently get all the subsequent tickets for services you are using. The hardest one to implement is actually HTTP, because browsers suck and just implement the bare minimum. However, the whole of Kerberos implementations is ancient, 1990s era software, packed with extensions upon extensions. You don't really want to expose a KDC to the open internet nowadays. The whole thing needs a redesign and rewrite in something safer than 1990s era C. Oh, and there are mechanisms for federation like trust relationsships between realms and KDCs, but nobody uses those beyond merging internal corporate networks.
      - slt2021 2y ago
        the weak point of Kerberos is not the Kerberos protocol itself, but the most popular implementation of it being Microsoft Active Directory. Due to an incredible bloat of AD and entire Windows/Azure ecosystem, it has an enormous attack surface (multiply the universe of all windows ecosystem by the decades of old versions being supported for compatibility), and any vulnerability in the ecosystem (past and present) can lead to escalation and compromise of the Active Directory itself. so is Kerberos secure? as a protocol it is fine, cause it was developed at MIT by smart people. is MSFT AD/Windows ecosystem secure? HELL NO, stay away
_cenw 2y ago
This is the purpose of the not so well known audience claim. Though I'd still prefer to authenticate to something like Vault's SSH engine and get a very short lived SSH certificate instead. No new software to install on your servers, just the CA key.
- c45y 2y ago
  CA key also allows those servers to avoid reaching out to some central location to validate which I've found to be a nice side bonus for disaster recovery type scenarios.
  - deleted 2y ago
    [deleted]
pcthrowaway 2y ago
I'd assume the auth handshake would prevent this. - client connects to SSH server at IP X.X.X.X or hostname SomeHost - redirected to oAuth server - Client signs in and receives token scoped to X.X.X.X or hostname SomeHost - Client provides token to SSH server
jrozner 2y ago
I think it's interesting they're choosing to use certificates this way. If they're already using certs, why not just leverage sshca auth? Also, at the end of the day, it's still effectively a bearer token. I founded a company called Based Security last year in this space. We're looking for design partners currently. We host a CA for you (or you can host yourself if you want) and use ssh certificates and bind the user identity (oidc to the IdP) to a physical device (yubikey, secure enclave, tpm, etc.) This ensures that the user is both in possession of the physical device and that the credential can't be stolen without stealing the device, unlike the bearer token examples here. Currently we're offering support for GitHub and GitLab authentication but it works out of the box with standard ssh tooling as well. It just currently requires manually handling user provisioning for standard ssh access.
- EthanHeilman 2y ago
  > Why not just leverage sshca auth? Because that has two trusted parties: the IDP and the SSH CA. OPKSSH has just one trusted party: the IDP. > This ensures that the user is both in possession of the physical device and that the credential can't be stolen without stealing the device, unlike the bearer token examples here. Currently we're offering support for GitHub and GitLab authentication but it works out of the box with standard ssh tooling as well. It just currently requires manually handling user provisioning for standard ssh access. That sounds valuable. Have you looked in OpenPubkey, the cosigner protocol supports binding hardware tokens to ID Tokens? Although not as fancy as having the SSH key pair live in the hardware token but maybe we could figure out a way to get the best of both worlds.
  - jrozner 2y ago
    I can understand the concern about having a second trusted party but think that the value of utilizing the standard ssh ca auth flow is worth the potential risk. If you require keys in attested hardware and verify that before issuing certs, the actual attack becomes very difficult. You need to compromise the actual hardware or compromise the CA in a pretty substantial way to issue certs to untrusted private keys. The certificate alone doesn't actually do anything without the key. In addition to just being supported out of the box, we can also issue hardware bound host keys, which allow us to offer bi-directional verification. We gain the benefit of all the standard PKI tooling (eg. revocation lists, ACME, etc.) and can use the same PKI for other scenarios (eg. mTLS, piv, etc.) by issuing x509 certificates instead. That's our long term plan is moving past ssh auth and having it be an attestable, immovable, hardware backed identity that can be usable for continuous authentication in other areas. I have looked into OpenPubKey briefly in the past but haven't spent a ton of time with it. We were going in a very different direction and it didn't seem particularly useful based on our goals or what we wanted to achieve. edit: Looking at the documentation https://docs.bastionzero.com/openpubkey-ssh/openpubkey-ssh/installation-and-deployment https://docs.bastionzero.com/openpubkey-ssh/openpubkey-ssh/i... It seems like to use OpenPubKey you also need a fairly modern version of OpenSSH. It also requires that the user authenticating have sudo access on the machine, which doesn't sound great. It's not clear to me whether it's possible for the existing authorized_keys file to co-exist or whether that's just to stop access using existing keys but using the standard ssh certs will co-exist allowing for a non-binary rollout if there are use cases that need to be worked around.

slt2021 2y ago

> The SSH server now has the secret you used to authenticate with. secrets can be made unique per connection and single use

confiq 2y ago
this ^ GSSAPI can be more secured than public/private key if configured right.
- dcow 2y ago
  Doesn’t this require the server to consult the IDP on every log in, though, to make sure the id token is valid? One of the staples of ssh from a UX standpoint is that it’s peer to peer.
  - lxgr 2y ago
    I suppose you could do something based on IDP-signed tokens, e.g. "valid for authentication to service x until <timestamp>"?
    - megous 2y ago
      This is basically a ssh certificate then.
      - pluto_modadic 2y ago
        the difference is in /key management/. Key management is the hard part. Especially keyless SSH management. (things like sigstore's rekor/fulcio remove some complexity here). It is not "just a (manually generated) ssh certificate"
    - XorNot 2y ago
      Kerberos tickets have timeouts on them already, it's a matter of configuration how long you wait. The thing is most enterprises want "user disabled" to be instant. Which of course leads to SSH keys all over the place anyway.
- EthanHeilman 2y ago
  Can you explain more? I want to be a fan of GSSAPI
  - thyristan 2y ago
    Don't know what the grandparent meant by GSSAPI, that is just an API for various underlying auth methods. But what people usually use together with GSSAPI is Kerberos. Kerberos can be very secure, much more so than CA-based or generally asymmetric crypto based approaches. Kerberos (if you ignore some extensions) uses symmetric cryptography, so it is less vulnerable to quantum computers. Use AES256 and you are fine, a quantum attacker can at the most degrade this to a 128bit level (according to current theories). Also, no weak RSA exponents, elliptic curve points at zero, variable-time implementations or other common pitfalls of asymmetric crypto. The trusted third party ("KDC" in Kerberos) distributes keys ("tickets") for pairs of user and service-on-server, so mutual authentication is always assured, not like in ssh or https where you can just ignore that pesky certificate or hostkey error. Keys are short-lived (hours to weeks usually), but there are builtin mechanisms for autorenew if desired. Each side of a key can also be bound to a host identity, so stealing tickets (like cookies in HTTP) can be made harder (but not impossible). The KDC can (theoretically, rarely implemented) also enforce authorisation by preventing users from obtaining tickets for services they are not authorized to use (the original idea of Kerberos was to just use it for authentication, the authorisation step is done by the service after authentication has been established). Single-Sign-On with all the usual protocols is included automatically, you log in to your workstation and get a ticket-granting-ticket that can then be used to transparently get all the subsequent tickets for services you are using. The hardest one to implement is actually HTTP, because browsers suck and just implement the bare minimum. However, the whole of Kerberos implementations is ancient, 1990s era software, packed with extensions upon extensions. You don't really want to expose a KDC to the open internet nowadays. The whole thing needs a redesign and rewrite in something safer than 1990s era C. Oh, and there are mechanisms for federation like trust relationsships between realms and KDCs, but nobody uses those beyond merging internal corporate networks.
    - slt2021 2y ago
      the weak point of Kerberos is not the Kerberos protocol itself, but the most popular implementation of it being Microsoft Active Directory. Due to an incredible bloat of AD and entire Windows/Azure ecosystem, it has an enormous attack surface (multiply the universe of all windows ecosystem by the decades of old versions being supported for compatibility), and any vulnerability in the ecosystem (past and present) can lead to escalation and compromise of the Active Directory itself. so is Kerberos secure? as a protocol it is fine, cause it was developed at MIT by smart people. is MSFT AD/Windows ecosystem secure? HELL NO, stay away

_cenw 2y ago

This is the purpose of the not so well known audience claim. Though I'd still prefer to authenticate to something like Vault's SSH engine and get a very short lived SSH certificate instead. No new software to install on your servers, just the CA key.

c45y 2y ago
CA key also allows those servers to avoid reaching out to some central location to validate which I've found to be a nice side bonus for disaster recovery type scenarios.
- deleted 2y ago
  [deleted]

pcthrowaway 2y ago

I'd assume the auth handshake would prevent this. - client connects to SSH server at IP X.X.X.X or hostname SomeHost - redirected to oAuth server - Client signs in and receives token scoped to X.X.X.X or hostname SomeHost - Client provides token to SSH server

jrozner 2y ago

I think it's interesting they're choosing to use certificates this way. If they're already using certs, why not just leverage sshca auth? Also, at the end of the day, it's still effectively a bearer token. I founded a company called Based Security last year in this space. We're looking for design partners currently. We host a CA for you (or you can host yourself if you want) and use ssh certificates and bind the user identity (oidc to the IdP) to a physical device (yubikey, secure enclave, tpm, etc.) This ensures that the user is both in possession of the physical device and that the credential can't be stolen without stealing the device, unlike the bearer token examples here. Currently we're offering support for GitHub and GitLab authentication but it works out of the box with standard ssh tooling as well. It just currently requires manually handling user provisioning for standard ssh access.

EthanHeilman 2y ago
> Why not just leverage sshca auth? Because that has two trusted parties: the IDP and the SSH CA. OPKSSH has just one trusted party: the IDP. > This ensures that the user is both in possession of the physical device and that the credential can't be stolen without stealing the device, unlike the bearer token examples here. Currently we're offering support for GitHub and GitLab authentication but it works out of the box with standard ssh tooling as well. It just currently requires manually handling user provisioning for standard ssh access. That sounds valuable. Have you looked in OpenPubkey, the cosigner protocol supports binding hardware tokens to ID Tokens? Although not as fancy as having the SSH key pair live in the hardware token but maybe we could figure out a way to get the best of both worlds.
- jrozner 2y ago
  I can understand the concern about having a second trusted party but think that the value of utilizing the standard ssh ca auth flow is worth the potential risk. If you require keys in attested hardware and verify that before issuing certs, the actual attack becomes very difficult. You need to compromise the actual hardware or compromise the CA in a pretty substantial way to issue certs to untrusted private keys. The certificate alone doesn't actually do anything without the key. In addition to just being supported out of the box, we can also issue hardware bound host keys, which allow us to offer bi-directional verification. We gain the benefit of all the standard PKI tooling (eg. revocation lists, ACME, etc.) and can use the same PKI for other scenarios (eg. mTLS, piv, etc.) by issuing x509 certificates instead. That's our long term plan is moving past ssh auth and having it be an attestable, immovable, hardware backed identity that can be usable for continuous authentication in other areas. I have looked into OpenPubKey briefly in the past but haven't spent a ton of time with it. We were going in a very different direction and it didn't seem particularly useful based on our goals or what we wanted to achieve. edit: Looking at the documentation https://docs.bastionzero.com/openpubkey-ssh/openpubkey-ssh/installation-and-deployment https://docs.bastionzero.com/openpubkey-ssh/openpubkey-ssh/i... It seems like to use OpenPubKey you also need a fairly modern version of OpenSSH. It also requires that the user authenticating have sudo access on the machine, which doesn't sound great. It's not clear to me whether it's possible for the existing authorized_keys file to co-exist or whether that's just to stop access using existing keys but using the standard ssh certs will co-exist allowing for a non-binary rollout if there are use cases that need to be worked around.