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Yes, the SSH flow is completely broken and, therefore, in reality only really protects against passive snooping. I have never met a real human, in real life, wh

Yes, the SSH flow is completely broken and, therefore, in reality only really protects against passive snooping. I have never met a real human, in real life, who ever checked the host key on first connect. For what it’s worth: The way to solve this is require the host key fingerprint as an argument to the ssh command, along with host and user. This would force people to look it up. This is not a popular opinion, but I think it’s inescapable: SSH was not made for humans. (This is purely a client issue, by the way. But at the end of the day: does that matter for anyone?) (Edit: when this comes up people often mention key based auth. Unfortunately, that relies on nobody else having your public key. You can find mine anywhere, it used to be available through the GitHub api , even. It won’t let the attacker connect to the original server but they can definitely impersonate it. Perhaps get your key agent forwarded, and continue. Not great.)

aaronmdjones 8y ago
No, the way to solve this is to use client public key authentication. Then a successful MITM will require the client's private key too.
- nothrabannosir 8y ago
  No, it will only require the public key (which is public) to impersonate the server. It won’t be a MITM but it’s still bad; see e.g. the bug in this very article, or if (god forbid) you’ve set up agent forwarding. Or if you do e.g. a git pull from github on a fresh comp, it can serve you bad code. Asymmetric key is not a solution to the problem of users blindly trusting hosts without checking the fingerprint. It’s a UI problem. Every time I bring this up it gets shot down (e.g. now -3 votes), and every time I wait for someone to put forward an actual counter argument. Unfortunately, no luck. SSH is broken and it needs fixing.
  - fwip 8y ago
    I'm not sure you know how public/private keys work.
    - nothrabannosir 8y ago
      This reply is tragicomically ironic :( See it like this: The bug is in the verification of the host, not the user. Eve can intercept a connection from Alice to Bob. It can "pretend" to be Bob, but it can't MITM (aka "proxy") because it won't be able to auth with Bob. However, auth depends on the server encrypting a message with Alice's public key, which everyone has ( https://github.com/torvalds.keys https://github.com/torvalds.keys !), including Eve, so Alice will connect to Eve. Really this is nothing different from just connecting to a fresh host: you upload your public key to the remote ~/.ssh/authorized_keys somehow, and voila, right? How does your ssh client know that was the same server? The fingerprint. Did you check it? No. Check out this article: > The more well-discussed use of asymmetrical encryption with SSH comes from SSH key-based authentication. SSH key pairs can be used to authenticate a client to a server. The client creates a key pair and then uploads the public key to any remote server it wishes to access. This is placed in a file called authorized_keys within the ~/.ssh directory in the user account's home directory on the remote server. > After the symmetrical encryption is established to secure communications between the server and client, the client must authenticate to be allowed access. The server can use the public key in this file to encrypt a challenge message to the client. If the client can prove that it was able to decrypt this message, it has demonstrated that it owns the associated private key. The server then can set up the environment for the client. - https://www.digitalocean.com/community/tutorials/understanding-the-ssh-encryption-and-connection-process https://www.digitalocean.com/community/tutorials/understandi... As you can see, ssh public key auth authenticates the user, not the host. anyone could be that host. Now: you as a human will quickly notice "where is my homedir? wait a second... this is not Bob—This is Eve!" the question is: quickly enough? If you were using scp: no, look at TFA. If you were connecting to github for the first time using git, perhaps using homebrew on a fresh machine: now you'll definitely never know. If you had set up your ssh key agent to forward keys to your "trusted" Bob host: serious trouble. There are loads of reasons why intercepting an SSH connection is a dramatic bug, even without being able to MITM it. Or just try and turn it around: why do you think SSH prompts you to confirm the host key fingerprint, if it doesn't matter? For fun? They didn't add that as a joke. Alas, few people understand this, as you so elegantly demonstrated :/
      - 8y ago

aaronmdjones 8y ago

No, the way to solve this is to use client public key authentication. Then a successful MITM will require the client's private key too.

nothrabannosir 8y ago
No, it will only require the public key (which is public) to impersonate the server. It won’t be a MITM but it’s still bad; see e.g. the bug in this very article, or if (god forbid) you’ve set up agent forwarding. Or if you do e.g. a git pull from github on a fresh comp, it can serve you bad code. Asymmetric key is not a solution to the problem of users blindly trusting hosts without checking the fingerprint. It’s a UI problem. Every time I bring this up it gets shot down (e.g. now -3 votes), and every time I wait for someone to put forward an actual counter argument. Unfortunately, no luck. SSH is broken and it needs fixing.
- fwip 8y ago
  I'm not sure you know how public/private keys work.
  - nothrabannosir 8y ago
    This reply is tragicomically ironic :( See it like this: The bug is in the verification of the host, not the user. Eve can intercept a connection from Alice to Bob. It can "pretend" to be Bob, but it can't MITM (aka "proxy") because it won't be able to auth with Bob. However, auth depends on the server encrypting a message with Alice's public key, which everyone has ( https://github.com/torvalds.keys https://github.com/torvalds.keys !), including Eve, so Alice will connect to Eve. Really this is nothing different from just connecting to a fresh host: you upload your public key to the remote ~/.ssh/authorized_keys somehow, and voila, right? How does your ssh client know that was the same server? The fingerprint. Did you check it? No. Check out this article: > The more well-discussed use of asymmetrical encryption with SSH comes from SSH key-based authentication. SSH key pairs can be used to authenticate a client to a server. The client creates a key pair and then uploads the public key to any remote server it wishes to access. This is placed in a file called authorized_keys within the ~/.ssh directory in the user account's home directory on the remote server. > After the symmetrical encryption is established to secure communications between the server and client, the client must authenticate to be allowed access. The server can use the public key in this file to encrypt a challenge message to the client. If the client can prove that it was able to decrypt this message, it has demonstrated that it owns the associated private key. The server then can set up the environment for the client. - https://www.digitalocean.com/community/tutorials/understanding-the-ssh-encryption-and-connection-process https://www.digitalocean.com/community/tutorials/understandi... As you can see, ssh public key auth authenticates the user, not the host. anyone could be that host. Now: you as a human will quickly notice "where is my homedir? wait a second... this is not Bob—This is Eve!" the question is: quickly enough? If you were using scp: no, look at TFA. If you were connecting to github for the first time using git, perhaps using homebrew on a fresh machine: now you'll definitely never know. If you had set up your ssh key agent to forward keys to your "trusted" Bob host: serious trouble. There are loads of reasons why intercepting an SSH connection is a dramatic bug, even without being able to MITM it. Or just try and turn it around: why do you think SSH prompts you to confirm the host key fingerprint, if it doesn't matter? For fun? They didn't add that as a joke. Alas, few people understand this, as you so elegantly demonstrated :/
    - 8y ago