Adversaries may use Valid Accounts to log into remote machines using Secure Shell (SSH). The adversary may then perform actions as the logged-on user.
SSH is a protocol that allows authorized users to open remote shells on other computers. Many Linux and macOS versions come with SSH installed by default, although typically disabled until the user enables it. The SSH server can be configured to use standard password authentication or public-private keypairs in lieu of or in addition to a password. In this authentication scenario, the user’s public key must be in a special file on the computer running the server that lists which keypairs are allowed to login as that user.
|M1042||Disable or Remove Feature or Program||
Disable the SSH daemon on systems that do not require it. For macOS ensure Remote Login is disabled under Sharing Preferences.
Require multi-factor authentication for SSH connections wherever possible, such as password protected SSH keys.
|M1018||User Account Management||
Limit which user accounts are allowed to login via SSH.
|ID||Data Source||Data Component|
|DS0028||Logon Session||Logon Session Creation|
|DS0029||Network Traffic||Network Connection Creation|
Use of SSH may be legitimate depending on the environment and how it’s used. Other factors, such as access patterns and activity that occurs after a remote login, may indicate suspicious or malicious behavior with SSH. Monitor for user accounts logged into systems they would not normally access or access patterns to multiple systems over a relatively short period of time.
On macOS systems
log show --predicate 'process = "sshd"' can be used to review incoming SSH connection attempts for suspicious activity. The command
log show --info --predicate 'process = "ssh" or eventMessage contains "ssh"' can be used to review outgoing SSH connection activity.
On Linux systems SSH activity can be found in the logs located in
/var/log/secure depending on the distro you are using.