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ObPlug for Guardian Agent, which is basically "safe" ssh-agent forwarding (and works with Mosh and SSH): https://github.com/StanfordSNR/guardian-agent https://g

ObPlug for Guardian Agent, which is basically "safe" ssh-agent forwarding (and works with Mosh and SSH): https://github.com/StanfordSNR/guardian-agent https://github.com/StanfordSNR/guardian-agent The basic story is that ssh-agent really just exposes a primitive of "please sign this challenge," which is useful locally, but the protocol wasn't designed to be forwarded. If requests are coming from a semi-trusted intermediary host, the protocol doesn't tell the agent (a) "what remote server is being authenticated to [i.e., who generated the challenge?]", or (b) "what command is going to be executed?" It doesn't even really know (c) "what (semi-trusted) host forwarded the challenge and is asking to authenticate to the remote server?" (although this one you can approximate today by running lots of agents and giving each local ssh command a different agent to forward requests to). Guardian Agent is a sort of hack that allows the agent to know (a), (b), and (c) before deciding whether to grant or deny the request, so the user can set up policies like, "I'd like to allow `semi-trusted host x` to use <SSH identity i> to run "git pull from <repository name y>" when talking to `git server z`, but that's it." The basic ssh-agent protocol just doesn't have enough info to be able to do something like that.

pritambaral 5y ago
But this doesn't solve the trust problem, though; only papers over it with a false sense of security. The problem with traditional Agent Forwarding is that an intermediary may be compromised. A compromised intermediary can still fool Guardian Agent with false values for (a), (b), and (c). The only way Guardian Agent is more secure than unmodified ssh is that it's relatively unknown. Obscurity, however, isn't a defensible security perimeter.
- keithwinstein 5y ago
  No, a compromised intermediary can't fool Guardian Agent with false values for (a), (b), or (c). The client checks/enforces these. The details of how it works are in the linked paper.

pritambaral 5y ago

But this doesn't solve the trust problem, though; only papers over it with a false sense of security. The problem with traditional Agent Forwarding is that an intermediary may be compromised. A compromised intermediary can still fool Guardian Agent with false values for (a), (b), and (c). The only way Guardian Agent is more secure than unmodified ssh is that it's relatively unknown. Obscurity, however, isn't a defensible security perimeter.

keithwinstein 5y ago
No, a compromised intermediary can't fool Guardian Agent with false values for (a), (b), or (c). The client checks/enforces these. The details of how it works are in the linked paper.