ssh-mitm VS openssh-portable

I wanted to share some insights into a security concern that echoes the Terrapin-Attack scenario, highlighting a similar vulnerability that has been observed in other tools.

Recently, I came across a Pull Request on GitHub for the SSH-MITM tool, which sheds light on a critical aspect of SSH protocol security, specifically regarding RFC 4253 during the KEXINIT process. The Pull Request, available at GitHub https://github.com/ssh-mitm/ssh-mitm/pull/163, describes the necessity of discarding certain packages during the KEXINIT phase to prevent issues with intercepted clients.

Moreover, a look into the GitHub Blame for SSH-MITM reveals that these crucial changes in the KEXINIT step were integrated into SSH-MITM about 1-2 years ago. You can see the specific changes at this link: https://github.com/ssh-mitm/ssh-mitm/blame/4fc3ef418847c35d17d0c427e2701b33a03c323c/sshmitm/workarounds/transport.py#L178-L188

An important note to add is that this information suggests that a similar form of attack, akin to the Terrapin-Attack, could potentially have been exploited for the last two years. This raises significant concerns about the historical vulnerability of systems to such attack techniques and emphasizes the importance of retroactive security analysis in addition to ongoing vigilance.

There is now an issue ticket in ssh-mitm to discuss the similarities between ssh-mitm and terrapin attack: https://github.com/ssh-mitm/ssh-mitm/issues/165

During some audits, it's likely that you find some ssh public keys laying around on some servers.

If you want to verify if this key is known by other services, but you don't have access to those services, this task might be hard.

SSH-MITM has an additional command, which allows to check if a public ssh key is known by GitHub, GitLab, and other code hosters. It's not limited to GitHub and other major platforms and even works with each service, which is accessible over SSH.

First you must install SSH-MITM. It's recommended to use the AppImage, because this works out of the box on most Linux machines.

$ wget https://github.com/ssh-mitm/ssh-mitm/releases/latest/download/ssh-mitm-x86_64.AppImage

SSH-MITM is a tool to audit ssh sessions and protocols, which uses SSH as the transport protocol: https://github.com/ssh-mitm/ssh-mitm

you can find the project on github: https://github.com/ssh-mitm/ssh-mitm

You should check the Revisionhistory of the Readme file first.. https://github.com/ssh-mitm/ssh-mitm/commit/564028af25c395528446fbb679c7392469d59bfd

The “customized wording” you’re seeing is “The LGPL”. It’s a different license from the GPL.

The history on the repo shows that the original license was GPL (June 2020), the author changed the license to LGPL (December 2022), and now they’re changing it to GPL again. https://github.com/ssh-mitm/ssh-mitm/commits/master/LICENSE

Unless I'm misunderstanding what this is about RFC5647 merely points out that the sequence number is included as AAD due to RFC4253 requirements. The [email protected] specification is not exactly the most rigorous thing I've ever seen (https://github.com/openssh/openssh-portable/blob/master/PROT...) but reading it, the sequence number is only included in the IV, and not as AAD, which directly runs afoul of the RFC4253 section 6.4 requirement for it to be included in the MAC.

The key layout is described in https://github.com/openssh/openssh-portable/blob/master/PROT... and you can view it pretty easily via

cat private_key_here | head -n -1 | tail -n +2 | base64 -d | xxd

One I created in 2016 is using aes256-cbc with bcrypt for the kdf, which isn't awful at all.

Interestingly, it looks like ssh-agent disables core dumps[1], but I don't see similar usage for sshd

1: https://github.com/openssh/openssh-portable/blob/694150ad927...

There's a current pull request for adding AF_UNIX support, which should make all kinds of exciting forwarding possible, since it will make it easy to proxy ssh connections through an arbitrary local process which can do anything to forward the data to the remote end.

https://github.com/openssh/openssh-portable/pull/431

OpenSSH server (allows connecting to containers)

It doesn't, at least not for generic/unmodified cryptographic applications.

WebAuthN signatures are of a very specific challenge/response format that applications need to explicitly support. For example, SSH had to add new key and signature formats [1] to support it.

Theoretically, a blockchain/cryptocurrency application could adopt the WebAuthN signature format as its canonical or an alternative signature format, but I'm not aware of any popular one having done so.

[1] https://github.com/openssh/openssh-portable/blob/master/PROT...

I just tested it and looked at the code briefly; the client fortunately does seem to remove all keys not provided by the server: https://github.com/openssh/openssh-portable/blob/36c6c3eff5e...

It seems like at least a `known_hosts` compromise would be "self-healing" after connecting to the legitimate github.com server once.

And OpenVAS and OpenSSH and OpenBSD and OpenNN and OpenAFS and on and on and on