Guardian Agent: secure agent forwarding for Mosh or SSH

Guardian Agent (now in beta) allows users to securely empower remote hosts to take actions on their behalf, using their SSH credentials. It allows Mosh and SSH users to enable agent forwarding for every connection, even to hosts they may not fully trust.

Guardian Agent is an alternative to traditional ssh-agent forwarding, which can only safely be enabled when connecting to trusted hosts. The traditional ssh-agent protocol doesn't give the agent information about which host is asking to perform a command on the user's behalf, which server that hosts wants to connect to, or which command the host wants to perform:

By contrast, Guardian Agent provides more-constrained agent forwarding that can safely be enabled on any connection. It can be used alongside Mosh or SSH.

To use Guardian Agent, a user runs sga-guard on her local machine to establish a secure forwarding channel to an intermediary host (e.g. in EC2). On the intermediary machine, she can then use sga-ssh as a drop-in replacement for 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 policy. No changes to the server are required, but full functionality is only available when connecting to servers running OpenSSH.

Based on feedback to this beta/prototype, we may incorporate the techniques behind Guardian Agent more deeply into Mosh.

How does Guardian Agent work?

Guardian Agent allows an SSH client (sga-ssh), running on a partially trusted machine, to request the SSH agent (sga-guard), running on a trusted machine, to execute commands on an SSH server (e.g., GitLab), such that the identity of the server as well as the SSH session command can be verified by the SSH agent, with the server's own code unaltered.

The scheme works by first having sga-ssh (on the intermediary host) request that the user's agent allow it to run a particular command on a particular server. The user approves or denies the request, or the request is auto-approved according to a pre-existing policy. (These policies are stored in the ~/.ssh/sga_policy file.)

If approved, sga-ssh then establishes a TCP connection to the server, and securely tunnels it back to sga-guard. sga-guard then establishes an end-to-end authenticated SSH connection directly with the server, over the forwarded TCP connection. The local machine allows the delegate to issue the approved command, and then to ``lock'' the session by using the no-more-sessions request, so that no more commands may be issued on this connection.

After the session is locked, sga-guard allows sga-ssh to rekey the session, taking over in the middle of the connection, and reusing the existing TCP connection to the server. From this point, sga-ssh can complete the operation, with data flowing directly between the intermediary and the server (as in traditional ssh-agent forwarding).

For more, please read our research paper or the detailed design document.

Research paper (to appear at ACM HotNets 2017)

Installation

Warning: this tool is in beta and is intended as a technology prototype. It was first released in October 2017. Feedback is greatly appreciated, but please use at your own risk.

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.

Ubuntu installation

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

CentOS / Fedora installation

sudo yum install -y mosh openssh-clients autossh openssh-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

macOS installation

brew tap theseal/ssh-askpass
brew tap theseal/guardian-agent
brew install guardian-agent

Other

1. Install the following dependencies: OpenSSH client, autossh, ssh-askpass.
2. Obtain the latest release for your platform. Alternatively, you may opt to build from source.
3. Extract the executables (sga-guard, sga-guard-bin, sga-ssh, and sga-stub) from the tarball to a directory in the user's PATH.

Basic Usage

Make sure Guardian Agent is installed on both your local and intermediary machine.

On your local machine

Start guarded 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 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/...

3. Copy the built binaries (sga-guard-bin, sga-ssh, and sga-stub) from $GOPATH/bin to a directory in the user's PATH.
4. 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 protocol does not allow the agent to 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 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.

As a result, ssh-agent forwarding can be used safely only when the user trusts the remote host. 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 safely 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's the exact attack scenario where I could get burned with traditional ssh-agent forwarding?

A: A compromised or malevolent intermediary can send rogue challenges and use the user's identity to authenticate to other servers or to run unauthorized commands. A user that enables ssh-agent forwarding will be asked yes or no on signing "something," and may think she is allowing an EC2 machine to run "git push" to GitHub. Unbeknownst to the user, she is allowing a different machine (that she is also logged in to) to connect to some other sensitive server that she has permissions on and do something evil.

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 administrators on the intermediary host, it is probably 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. Guardian Agent is a prototype of an approach that could safely be enabled by default.

Q: Can I use Guardian Agent 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 the no-more-sessions extension. (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 beta 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: What language is Guardian Agent written in?

A: Go! Guardian Agent is a beta intended to solicit feedback from the community, especially with regards to the basic design of a secure protocol that works with unmodified remote SSH servers. We found Go and the Go SSH library to be helpful in 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.

Q: Where can I read more about how Guardian Agent works?

A: Please read our research paper or the detailed design document.