guardianagent

Guardian Agent: secure ssh-agent forwarding for Mosh and SSH

Traditional ssh-agent forwarding can be dangerous: the local ssh-agent has to sign opaque challenges with the user's private key, without knowing (a) what intermediary host is asking for the signature, (b) what remote server that intermediary host wants to authenticate to, or (c) what command the intermediary host wants to execute on the remote server.

A compromised intermediary can send rogue challenges and use the user's identity to authenticate to other servers or to run unauthorized commands. So you might enable ssh-agent forwarding and be asked yes or no on signing "something," and you think it's allowing an EC2 machine to run "git push" to GitHub. But actually it's allowing a different EC2 machine (that you also are logged in to) to connect to some other sensitive server that you have permissions on and add an evil key to your authorized_keys file.)

Guardian Agent provides secure ssh-agent forwarding. A user first runs sga-guard on her local machine (on which she stores her private SSH keys) to securely forward her ssh-agent to an intermediary machine (e.g., on AWS). She can then use sga-ssh on the intermediary machine as a drop-in replacement to ssh. The local sga-guard verifies the identity of (a) the intermediary host, (b) the remote server, and (c) the command^*, either by prompting the user or based on a stored security policy. After all the details are verified, the connection is handed off to the intermediary (so the bulk of the data is not proxied through the local host).

• Installation
• Basic Usage
• Advanced Usage
  • Command verification
  • Prompt types
  • Customizing the SSH command
  • Stub location
• Building from Source
• Troubleshooting
• Development
• FAQ

WARNING!

Installation

Using Guardian Agent requires installation both on your local machine (the one with your SSH private keys) and on each of the intermediary machines you want to securely forward ssh-agent to (the machines on which you want to run an SSH client without having the keys on them). No installation is required on the server side.

sudo apt-get install openssh-client autossh ssh-askpass
curl -L https://api.github.com/repos/StanfordSNR/guardian-agent/releases/latest | grep browser_download_url | grep 'linux' | cut -d'"' -f 4 | xargs curl -Ls | tar xzv
sudo cp sga_linux_amd64/* /usr/local/bin

brew install autossh ssh-askpass
curl -L https://api.github.com/repos/StanfordSNR/guardian-agent/releases/latest | grep browser_download_url | grep 'darwin' | cut -d'"' -f 4 | xargs curl -L | tar xzv
sudo cp sga_darwin_amd64/* /usr/local/bin

Basic Usage

Make sure SSH guardian agent is installed on both your local and intermediary machine.

On your local machine

Start guarded SSH agent forwarding to the intermediary machine:

[local]$ sga-guard <intermediary>

You should then expect to see the following message:

[local]$ sga-guard aws-ubu
Connecting to aws-ubu to set up forwarding...
Forwarding to aws-ubu setup successfully. Waiting for incoming requests...

Guarded agent forwarding is now enabled on the intermediary.

On the intermediary

Connect to the intermediary (e.g., using standard ssh or mosh). Install guardian-agent. To enable several common tools (scp, git, rsync, mosh) to use the guardian agent instead of the default ssh program:

[intermediary]$ source sga-env.sh

You can also add this line to your ~/.bashrc/~.zshrc/... file on the intermediary hosts.

You can then use git, scp, rsync, mosh as you would normally do.

[intermediary]$ git clone git@github.com:user/repo
...
[intermediary]$ scp foo.txt remote-host:Documents/foo.txt
...

You can also use sga-ssh as a drop-in replacement to an ssh client:

[intermediary]$ sga-ssh <server> [command]

Advanced Usage

Command verification

Command verification requires the server to support the no-more-sessions extension. This is extension is present on OpenSSH servers, but unfortunately not implemented on other SSH servers (including GitHub, which uses libssh). When executing a command on a server that does not support this extension, only the identity of the intermediary and the identity of the server can be constrained and verified by the agent (but not the contents of the command).

Prompt types

Guardian Agent supports two types of interactive prompts: graphical and terminal-based. The graphical prompt requires the DISPLAY environment variable to be set to the appropriate X11 server.
If running in a terminal-only session (in which the DISPLAY environment variable is not set), a textual prompt will be used instead.

Customizing the SSH command

When using sga-guard, the default SSH client on the local machine is used to set up the connection. This requires ssh to be found in the user's PATH. To specify an alternative SSH client or specifying additional argument to the client, use the --ssh command-line flag.

Stub location

If the sga-stub is not installed in the user's PATH on the intermediary machine, its location must be specified when setting up secure agent forwarding from the local machine:

[local]$ sga-guard --stub=<PATH-TO-STUB> <intermediary>

Building from Source

1. Install go 1.8+
2. Get and build the sources:

go get github.com/StanfordSNR/guardian-agent/...

1. Copy the built binaries (sga-guard-bin, sga-ssh, and sga-stub) from $GOPATH/bin to a directory in the user's PATH.
2. Copy the scripts $GOPATH/src/github.com/StanfordSNR/guardian-agent/scripts/sga-guard and $GOPATH/src/github.com/StanfordSNR/guardian-agent/scripts/sga-env.sh to a directory in the user's PATH.

Troubleshooting

In case of unexpected behavior, please consider opening an issue in our issue tracker. We'd also greatly appreciate if you could run the tool in debug mode by setting the --debug and --logfile=<LOG-FILE> flags and attach the log file to the issue.

FAQ

Q: Is ssh-agent forwarding really insecure? What is the point of Guardian Agent?

A: The ssh(1) man page warns that "Agent forwarding should be enabled with caution," because the ssh-agent cannot verify (a) which intermediary machine is making the request, (b) which remote server the intermediary wants to authenticate to, or (c) what command the intermediary plans to run on the remote server. The agent simply signs a blank check--- an opaque challenge from an unknown server that will allow the intermediary to execute any sequence of commands on the user's behalf. Several commentators have noted that this creates risks that may not be widely appreciated.

Guardian Agent is a prototype of a system for secure agent forwarding that could be enabled on every outgoing connection, because the local agent can verify and enforce security policies regarding who wants to do what to whom.

Q: What if I only use ssh-agent forwarding when I SSH to intermediaries that I trust?

A: If the user trusts the software (and system administrator) on the intermediary host, it is essentially fine to use ssh-agent forwarding as it exists today. However, with this level of trust, it may also be fine to simply place a private key on the intermediary's hard drive and use that to authenticate to remote servers, rather than forwarding agent requests back to the local agent.

Q: Can I use this to constrain an intermediary to only pull from (or only push to) a limited set of remote Git repositories?

A: Yes, if the remote Git server is running an SSH server (such as OpenSSH) that supports no-more-sessions and allows Guardian Agent to limit the command. (The name of the repository, and the difference between pulling and pushing, are both represented in the command.) Among popular Git-hosting services that we are aware of, currently only GitLab appears to support this currently. GitHub and Bitbucket use other SSH implementations and do not allow Guardian Agent to constrain the intermediary to only push or pull from certain repositories.

Q: Is Guardian Agent secure?

A: Guardian Agent is a technology preview that was first released for beta testing in October 2017. It has not accumulated enough testing and scrutiny to make claims that the implementation is bulletproof.

Q: Why did you write Guardian Agent in Go?

A: Guardian Agent is a technology preview intended to solicit feedback from the community, especially with regards to the basic design of a secure ssh-agent forwarding mechanism that works with unmodified remote SSH servers. We found Go and the Go SSH library to be helpful in rapidly prototyping this tool.

Q: What is the connection to Mosh (mobile shell)?

A: Many Mosh users have asked for ssh-agent forwarding support. Guardian Agent was developed by some of the Mosh developers and can be used with Mosh today. Based on feedback to this prototype, we may integrate Guardian Agent more fully into Mosh as a system for secure ssh-agent forwarding that is safe enough to leave on by default.

Q: Who wrote Guardian Agent?

A: Guardian Agent was developed by students and faculty in the Stanford University Department of Computer Science (Dima Kogan and Henri Stern, advised by Keith Winstein and David Mazières).

Q: Where should I send feedback?

A: Please file an issue on GitHub.

Development

Detailed Design