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2020-11-21 14:21:40 +00:00

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Qubes Split SSH

Split SSH implements a concept similar to having a smart card with your private SSH keys, except that the role of the “smart card” is played by another Qubes AppVM. This Qubes setup allows you to keep your SSH private keys in a vault VM (vault) while using an SSH Client VM (ssh-client) to access your remote server. This is done by using Qubes's qrexec framework to connect a local SSH Agent socket from your SSH Client VM to the SSH Agent socket within the vault VM. This way the compromise of the domain you use to connect to your remote server does not allow the attacker to automatically also steal all your keys. (We should make a rather obvious comment here that the so-often-used passphrases on private keys are pretty meaningless because the attacker can easily set up a simple backdoor which would wait until the user enters the passphrase and steal the key then.)

Overview

Make sure the TemplateVM you plan to use is up to date.
Create vault and ssh-client AppVMs.
Create an ssh key in your vault AppVM and set up automatic key adding prompt.
Set up VM interconnection
(Strongly Encouraged) Create a KeePassXC Database and set up SSH Agent Integration in KeePassXC.

Preparing your system

Make sure the templates you plan to base your AppVMs on are up-to-date.

Creating AppVMs

If you’ve installed Qubes OS using the default options, a few qubes including a vault AppVM has been created for you. Skip the first step if you don't wish to create another vault.

Create a new vault AppVM (vault) based on your chosen template. Set networking to (none).
Create a SSH Client AppVM (ssh-client). This VM will be used to make SSH connections to your remote machine.

Setting up SSH

Perform the next steps in the AppVM vault.

Generate an SSH key pair. Skip this step if you already have your keys. Note that it is okay to not enter a password for your private keys since the vault AppVM has no networking. If you still want to encrpt your keys you must refer to the Securing Your Private Key section.

[user@vault ~]$ ssh-keygen -t ed25519 -a 500
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/user/.ssh/id_ed25519): 
Created directory '/home/user/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user/.ssh/id_ed25519
Your public key has been saved in /home/user/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:DBxSxZcp16d1NSVSid3m8HRipUDM2INghQ4Sx3jPEDo user@vault
The key's randomart image is:
+--[ED25519 256]--+
|    o==+++.@++o=*|
|    o==o+ B BoOoB|
|    Eoo* +   *.O.|
|     . o+   .   o|
|        S        |
|                 |
|                 |
|                 |
|                 |
+----[SHA256]-----+

-t: type
-a: num_trials

Please note that the key fingerprint and the randomart image will differ.

For more information about ssh-keygen, run man ssh-keygen.

Notice: Skip the following steps if you plan on using KeePassXC.

Make a new directory ~/.config/autostart

[user@fedora-32 ~]$ mkdir -p ~/.config/autostart

Create the file ~/.config/autostart/ssh-add.desktop
- Open the file with e.g. gedit
```
[user@fedora-32 ~]$ gedit ~/.config/autostart/ssh-add.desktop
```
- Paste the following contents:
```
[Desktop Entry]
Name=ssh-add
Exec=ssh-add
Type=Application
```
Note: If you've specified a custom name for your key using -f, you should adjust Exec=ssh-add to Exec=ssh-add <path-to-your-key-file>.

Setting Up VM Interconnection

In `dom0`:

Create and edit /etc/qubes-rpc/qubes.SshAgent.
- Open the file with e.g. nano.
```
[user@fedora-32 ~]$ sudo nano /etc/qubes-rpc/qubes.SshAgent
```
- If you want to explicitly allow only this connection, add the following line:
```
ssh-client vault ask
```
- If you want to allow all VMs to connect, add the following line:
```
@anyvm @anyvm ask
```
- If you want the input field to be "prefilled" by your vault VM, append default_target=vault so it looks like for example:
```
@anyvm @anyvm ask,default_target=vault
```
Note: There are many ways to fine-tune this policy. For more details see the Qubes qrexec documentation.

In the Template of Your AppVM `vault`:

Create and edit /etc/qubes-rpc/qubes.SshAgent.

Open the file with e.g. gedit

[user@fedora-32 ~]$ sudo gedit /etc/qubes-rpc/qubes.SshAgent

Paste the following contents:

#!/bin/sh
# Qubes App Split SSH Script

# safeguard - Qubes notification bubble for each ssh request
notify-send "[`qubesdb-read /name`] SSH agent access from: $QREXEC_REMOTE_DOMAIN"

# SSH connection
socat - UNIX-CONNECT:$SSH_AUTH_SOCK

In the AppVM `ssh-client`

Theoretically, you can use any AppVM but to increase security it is advised to create a dedicated AppVM for your SSH connections. Furthermore, you can set different firewall rules for each VM (i.e. for intranet and internet connections) which also provides additional protection.

Edit /rw/config/rc.local.

Open the file with e.g. gedit.

[user@ssh-client ~]$ sudo gedit /rw/config/rc.local

Add the following to the bottom of the file:

# SPLIT SSH CONFIGURATION >>>
# replace "vault" with your AppVM name which stores the ssh private key(s)
SSH_VAULT_VM="vault"

if [ "$SSH_VAULT_VM" != "" ]; then
  export SSH_SOCK="/home/user/.SSH_AGENT_$SSH_VAULT_VM"
  rm -f "$SSH_SOCK"
  sudo -u user /bin/sh -c "umask 177 && exec socat 'UNIX-LISTEN:$SSH_SOCK,fork' 'EXEC:qrexec-client-vm $SSH_VAULT_VM qubes.SshAgent'" 
fi
# <<< SPLIT SSH CONFIGURATION

Edit ~/.bashrc and add the following to the bottom of the file:

Open the file with e.g. gedit
```
[user@ssh-client ~]$ gedit ~/.bashrc
```

Add the following to the bottom of the file:

# SPLIT SSH CONFIGURATION >>>
# replace "vault" with your AppVM name which stores the ssh private key(s)
SSH_VAULT_VM="vault"

if [ "$SSH_VAULT_VM" != "" ]; then
  export SSH_AUTH_SOCK="/home/user/.SSH_AGENT_$SSH_VAULT_VM"
fi
# <<< SPLIT SSH CONFIGURATION

Securing Your Private Key

Although passwords wouldn't protect you against a full system compromise, it's possible for an adversary to gain read-only access to some of your files (e.g. file shares or offline backups of data) and not be able to modify anything. Passwords are advisable for mitigating these threats . You can either use the built-in password utility of your private key combined with a graphical prompt or prefer to use KeePassXC. Please note that since ssh-askpass prompt is displayed on vault VM boot, it is not possible to use both configurations simultaneously.

Using the Built-in Password Utility and `ssh-askpass`

Add a password to your private key with ssh-keygen -p. Note that the location and name of your private key may differ.

[user@vault ~]$ ssh-keygen -p 
Enter file in which the key is (/home/user/.ssh/id_rsa): /home/user/.ssh/id_ed25519
Key has comment 'user@vault'
Enter new passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved with the new passphrase.

Install ssh-askpass in the template of your vault VM.

For Fedora templates:

[user@fedora-32 ~]$ sudo dnf install openssh-askpass

For Debian templates:

user@debian-10:~$ sudo apt-get install ssh-askpass

Shutdown the template and restart your vault VM.

With this configuration you'll be prompted for entering your password every time you start your vault VM to be able to make use of your SSH key.

Using KeePassXC

Warning: This part is for setting up KeePassXC, not KeePassX or KeePass. See the KeePassXC FAQ.

KeePassXC should be installed by default in both Fedora and Debian TemplateVMs. If it’s not or you're using another template, you can install it manually.

Add KeepasXC to the Applications menu of the newly created AppVM for ease of access and launch it.

Note: Since the vault VM has no internet connection, you can safely deny automatic updates if prompted.
Create a new database.
Enter a name for your database and continue.
Adjust the encryption settings. Check the KeePassXC User Guide for more information about these settings.
Enter a password for your database. Take your time make a secure but also rememberable password. (hint)
Add a new entry.
Set password to your SSH key passphrase.
Go into the Advanced section and add your keys.

Note: Technically, you only need to add the private key (id_25519) for the following steps to work. If you add the public key here, too, you can later on backup your kdbx file and have everything in one place. You can even delete your keys (id_25519 and id_25519.pub) from your file system if you like.
Enable "SSH Agent Integration" within the Application Settings.
Restart KeePassXC
Check the SSH Agent Integration status.
Open the entry you created and select your private key in the "SSH Agent" section.

Testing the KeePassXC Setup

Close your KeePassXC database and run ssh-add -L. It should return The agent has no identities.
```
[user@vault ~]$ ssh-add -L
The agent has no identities.
```
Unlock your KeePassXC database and run ssh-add -L again. This time it should return your public key.
```
[user@vault ~]$ ssh-add -L
ssh-ed25519 <public key string> user@vault-keepassxc
```

Test Your Configuration

If you're using KeePassXC, shutdown KeePassXC on your vault VM. If not, make sure your private key is not added to the ssh-agent in your vault VM (Check with ssh-add -L). If it is, restart your vault VM and do not enter your password when it asks you to.
Try fetching your identities on the SSH Client VM.
```
[user@ssh-client ~]$ ssh-add -L
```
Allow operation execution. (If you don't see the below prompt, check your VM interconnection setup.)

It should return The agent has no identities..

If you're getting an error (e.g. error fetching identities: communication with agent failed), make sure your vault VM is running and check your VM interconnection setup.

Launch KeePassXC and unlock your database.
Try fetching your identities on the SSH Client VM.
```
[user@ssh-client ~]$ ssh-add -L
```
Allow operation execution. (If you don't see the below prompt, check your VM interconnection setup.)

Check if it returns ssh-ed25519 <public key string>

If you're getting an error (e.g. error fetching identities: communication with agent failed), make sure your vault VM is running and check your VM interconnection setup.

Security Benefits

In the setup described in this guide, even an attacker who manages to gain access to the ssh-client VM will not be able to obtain the user’s private key since it is simply not there. Rather, the private key remains in the vault VM, which is extremely unlikely to be compromised if nothing is ever copied or transferred into it. In order to gain access to the vault VM, the attacker would require the use of, e.g., a general Xen VM escape exploit or a signed, compromised package which is already installed in the TemplateVM upon which the vault VM is based.

Further Security tips

Regarding Your SSH Private Key

This goes without saying: keep your private keys private.
Tinkering with the user permissions is not necessary since it is assumed that an adversary who can find a Xen VM escape exploit is also capable of finding a user to root escalation exploit.

Regarding Your KeePassXC Database File

Although the database file is encrpyted with your password, if you haven't taken any protective measures, it can be bruteforced. Some tips for securing your keys against a vault compromise include:

Hide the *.kdbx file by simply renaming the file extension (e.g. *.zip). Keep in mind this is not likely to stop dedicated adversaries from finding your *.kdbx file.
Adjust the encrpytion settings in KeePassXC as per the KeePassXC documentation.

Current limitations

It is possible for a malicious VM to hold onto an ssh-agent connection for more than one use. Therefore, if you authorize usage once, assume that a malicious VM could then use it many more times. In this case, though, the SSH Agent will continue to protect your private keys; only usage of it would be available to the malicious VM until it was shut down.
It doesn’t solve the problem of allowing the user to know what is requested before the operation gets approved.

Want more Qubes split magic? Check out Split-GPG.

This guide has been inspired by:
Qubes Split SSH (Github: Jason Hennessey - henn) https://github.com/henn/qubes-app-split-ssh
Using split ssh in QubesOS 4.0 (Kushal Das) https://kushaldas.in/posts/using-split-ssh-in-qubesos-4-0.html
Using Split-SSH in Qubes 4 (Denis Zanin) https://deniszanin.com/using-split-ssh-gpg-in-qubes-os/
R.I.S.K.S. https://19hundreds.github.io/risks-workflow/ssh-split-setup

Contributor(s): @shaaati, @invalid-error, @deeplow, @santorihelix

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