vault-ssh-helper

Please note: We take Vault's security and our users' trust very seriously. If you believe you have found a security issue in Vault, please responsibly disclose by contacting us at [email protected].

vault-ssh-helper is a counterpart to HashiCorp Vault's SSH backend. It allows a machine to consume One-Time-Passwords (OTP) created by Vault servers by allowing them to be used as client authentication credentials at SSH connection time.

All of the remote hosts that belong to the SSH backend's OTP-type roles will need this helper installed. In addition, each host must have its SSH configuration changed to enable keyboard-interactive authentication and redirect its client authentication responsibility to vault-ssh-helper.

Vault-authenticated users contact the Vault server and retrieve an OTP issued for a specific username and IP address. While establishing an SSH connection to the host, the vault-ssh-helper binary reads the OTP from the password prompt and sends it to the Vault server for verification. Client authentication is successful (and the SSH connection allowed) only if the Vault server verifies the OTP. True to its name, once the OTP has been used a single time for authentication, it is removed from Vault and cannot be used again.

vault-ssh-helper is not a PAM module, but it does the job of one. vault-ssh-helper's binary is run as an external command using pam_exec.so with access to the entered password (in this case, the issued OTP). Successful execution and exit of this command is a PAM 'requisite' for authentication to be successful. If the OTP is not validated, the binary exits with a non-zero status and authentication fails.

PAM modules are generally shared object files; rather than writing and maintaining a PAM module in C, vault-ssh-helper is written in Go and invoked as an external binary. This allows vault-ssh-helper to be contained within one code base with known, testable behavior. It also allows other authentication systems that are not PAM-based to invoke vault-ssh-helper and take advantage of its capabilities.

Usage

vault-ssh-helper [options]

Options

Download vault-ssh-helper

Download the latest version of vault-ssh-helper at releases.hashicorp.com.

Build and Install

You'll first need Go installed on your machine (version 1.8+ is required).

Install Go on your machine and set GOPATH accordingly. Clone this repository into $GOPATH/src/github.com/hashicorp/vault-ssh-helper. Install all of the dependent binaries like godep, gox, vet, etc. by bootstrapping the environment:

$ make updatedeps

Build and install vault-ssh-helper:

$ make
$ make install

Follow the instructions below to modify your SSH server configuration, PAM configuration and vault-ssh-helper configuration. Check if vault-ssh-helper is installed and configured correctly and also is able to communicate with Vault server properly. Before verifying vault-ssh-helper, make sure that the Vault server is up and running and it has mounted the SSH backend. Also, make sure that the mount path of the SSH backend is properly updated in vault-ssh-helper's config file:

$ vault-ssh-helper -verify-only -config=<path-to-config-file>
Using SSH Mount point: ssh
vault-ssh-helper verification successful!

If you intend to contribute to this project, compile a development version of vault-ssh-helper using make dev. This will put the binary in the bin and $GOPATH/bin folders.

$ make dev

If you're developing a specific package, you can run tests for just that package by specifying the TEST variable. For example below, only helper package tests will be run.

$ make test TEST=./helper
...

If you intend to cross compile the binary, run make bin.

vault-ssh-helper Configuration

[Note]: This configuration is applicable for Ubuntu 14.04. SSH/PAM configurations differ with each platform and distribution.

vault-ssh-helper's configuration is written in HashiCorp Configuration Language (HCL). By proxy, this means that vault-ssh-helper's configuration is JSON-compatible. For more information, please see the HCL Specification.

Properties

Sample config.hcl:

vault_addr = "https://vault.example.com:8200"
ssh_mount_point = "ssh"
namespace = "my_namespace"
ca_cert = "/etc/vault-ssh-helper.d/vault.crt"
tls_skip_verify = false
allowed_roles = "*"

PAM Configuration

Modify the /etc/pam.d/sshd file as follows; each option will be explained below.

#@include common-auth
auth requisite pam_exec.so quiet expose_authtok log=/tmp/vaultssh.log /usr/local/bin/vault-ssh-helper -config=/etc/vault-ssh-helper.d/config.hcl
auth optional pam_unix.so not_set_pass use_first_pass nodelay

First, the previous authentication mechanism common-auth, which is the standard Linux authentication module, is commented out, in favor of using our custom configuration.

Next the authentication configuration for vault-ssh-helper is set.

The third line works around a bug between some versions of pam_exec.so and vault-ssh-helper that causes a successful authentication from vault-ssh-helper to fail due to some resources not being properly released. Because it is marked as optional, it is essentially a no-op that ensures that PAM cleans up successfully, avoiding the bug.

SSHD Configuration

Modify the /etc/ssh/sshd_config file. Note that for many distributions these are the default options; you may not need to set them explicitly but should verify their values if not.

ChallengeResponseAuthentication yes
UsePAM yes
PasswordAuthentication no