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SSH Authorized Keys

Description from ATT&CK

Adversaries may modify the SSH authorized_keys file to maintain persistence on a victim host. Linux distributions and macOS commonly use key-based authentication to secure the authentication process of SSH sessions for remote management. The authorized_keys file in SSH specifies the SSH keys that can be used for logging into the user account for which the file is configured. This file is usually found in the user's home directory under <user-home>/.ssh/authorized_keys.(Citation: SSH Authorized Keys) Users may edit the system’s SSH config file to modify the directives PubkeyAuthentication and RSAAuthentication to the value “yes” to ensure public key and RSA authentication are enabled. The SSH config file is usually located under /etc/ssh/sshd_config.

Adversaries may modify SSH authorized_keys files directly with scripts or shell commands to add their own adversary-supplied public keys. In cloud environments, adversaries may be able to modify the SSH authorized_keys file of a particular virtual machine via the command line interface or rest API. For example, by using the Google Cloud CLI’s “add-metadata” command an adversary may add SSH keys to a user account.(Citation: Google Cloud Add Metadata)(Citation: Google Cloud Privilege Escalation) Similarly, in Azure, an adversary may update the authorized_keys file of a virtual machine via a PATCH request to the API.(Citation: Azure Update Virtual Machines) This ensures that an adversary possessing the corresponding private key may log in as an existing user via SSH.(Citation: Venafi SSH Key Abuse)(Citation: Cybereason Linux Exim Worm) It may also lead to privilege escalation where the virtual machine or instance has distinct permissions from the requesting user.

Where authorized_keys files are modified via cloud APIs or command line interfaces, an adversary may achieve privilege escalation on the target virtual machine if they add a key to a higher-privileged user.

SSH keys can also be added to accounts on network devices, such as with the 

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ip ssh pubkey-chain
Network Device CLI command.(Citation: cisco_ip_ssh_pubkey_ch_cmd)

Atomic Tests

Atomic Test #1 - Modify SSH Authorized Keys

Modify contents of <user-home>/.ssh/authorized_keys to maintain persistence on victim host. If the user is able to save the same contents in the authorized_keys file, it shows user can modify the file.

Supported Platforms: freebsd,macos,linux

auto_generated_guid: 342cc723-127c-4d3a-8292-9c0c6b4ecadc

Inputs:

None

Attack Commands: Run with sh!

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if [ -f ~/.ssh/authorized_keys ]; then ssh_authorized_keys=$(cat ~/.ssh/authorized_keys); echo "$ssh_authorized_keys" > ~/.ssh/authorized_keys; fi;

Cleanup Commands:

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unset ssh_authorized_keys

source