PAM-RSSH

This PAM module provides ssh-agent based authentication. The primary design goal is to avoid typing password when you sudo on remote servers. Instead, you can simply touch your hardware security key (e.g. Yubikey/Canokey) to fulfill user verification. The process is done by forwarding the remote authentication request to client-side ssh-agent as a signature request.

This project is developed in Rust language to minimize security flaws.

Development Status

It's ready for production use, and has been tested on production servers for over a year. More tests and feedback are welcome.

Currently supported SSH public key types:

• RSA (with SHA256 digest)
• DSA
• ECDSA 256/384/521
• ECDSA-SK (FIDO2/U2F)
• ED25519
• ED25519-SK (FIDO2)

Build and Install

Prerequisites:

• OpenSSL (>=1.1.1)
• libpam
• Rust (with Cargo)

Clone this repo with a submodule.

git clone --recurse-submodule https://github.com/z4yx/pam_rssh.git
cd pam_rssh

Then build it using Cargo.

cargo build --release
cp target/release/libpam_rssh.so <pam module path>

The <pam module path> is specific to certain Linux distributions.

OS	Destination
Arch Linux	/usr/lib/security/
Debian	/lib/x86_64-linux-gnu/security/
openSUSE	/lib/security/

Config

Add the following line to /etc/pam.d/sudo (place it before existing rules):

auth sufficient libpam_rssh.so

Then edit sudoers with visudo command. Add the following line: (It makes sudo keep the environment variable, so this module can communicate with ssh-agent)

Defaults        env_keep += "SSH_AUTH_SOCK"

Start a ssh-agent on your client, then add your keys with ssh-add.

Try to ssh to your server with forwarded agent (-A option), and make a sudo there.

Security Notice

The default public key authorization file used by pam_rssh is ~/.ssh/authorized_keys. Typically, this file is writable to users, meaning any program executed by the user can modify it. This enables malicious programs to exploit the file and gain root privileges via sudo without the user's knowledge.

To mitigate this risk, you may configure the auth_key_file to a file only writable by root. This prevents unauthorized modifications and enhances system security. For example:

auth sufficient libpam_rssh.so auth_key_file=/etc/authorized_keys/${user}.keys

Optional Arguments

The following arguments are supported:

• loglevel=<off|error|warn|info|debug|trace> Select the level of messages logged to syslog. Defaults to warn.
• debug Equivalent to loglevel=debug.
• ssh_agent_addr=<IP:port or UNIX domain address> The address of ssh-agent. Defaults to the value of SSH_AUTH_SOCK environment variable, which is set by ssh automatically.
• auth_key_file=<Path to authorized_keys> Public keys allowed for user authentication. Defaults to <home>/.ssh/authorized_keys. <home> is read from system configuration, usually it expands to /home/<username>.
• authorized_keys_command=<Path to command> A command to generate the authorized_keys. It takes a single argument, the username of the user being authenticated. The standard output of this command will be parsed as authorized_keys. The auth_key_file will be ignored if you specify this argument.
• authorized_keys_command_user=<Username> The authorized_keys_command will be run as the user specified here. If this argument is omitted, the authorized_keys_command will be run as the user being authenticated.
• cue Enable device interaction prompt. When enabled, displays a message reminding the user to touch their device during authentication.
• [cue_prompt=<message>] Set custom prompt message for device interaction. Default: "Please touch the device". Use square brackets to include spaces in the message.

Arguments should be appended to the PAM rule. For example:

auth sufficient libpam_rssh.so debug authorized_keys_command=/usr/bin/sss_ssh_authorizedkeys authorized_keys_command_user=nobody cue [cue_prompt=The prompt message here]

Use Variables in Arguments

Certain variables can be used in arguments. Supported formats are $var, ${var} and ${var:default value}. For example:

auth sufficient libpam_rssh.so auth_key_file=/data/${user}.keys

Variables are mapped to PAM items. Currently, the following variables are available:

• service - PAM_SERVICE. The service name (which identifies the PAM stack that will be used).
• user - PAM_USER. The username of the entity under whose identity service will be given.
• tty - PAM_TTY. The terminal name.
• rhost - PAM_RHOST. The requesting hostname.
• ruser - PAM_RUSER. The requesting entity.

For detailed descriptions of PAM items, read man page pam_get_item(3).