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SSH
On Linux systems and supported network devices, Nessus uses Secure Shell (SSH) protocol version 2 based programs (e.g., OpenSSH, Solaris SSH, etc.) for host-based checks.
This mechanism encrypts the data in transit to protect it from being viewed by sniffer programs. Nessus supports five types of authentication methods for use with SSH: username and password, public/private keys, digital certificates, and Kerberos.
Users can select SSH settings from the Credentials menu and enter credentials for scanning Linux systems.
These credentials are used to obtain local information from remote Linux systems for patch auditing or compliance checks.
Note: Non-privileged users with local access on Linux systems can determine basic security issues, such as patch levels or entries in the /etc/passwd file. For more comprehensive information, such as system configuration data or file permissions across the entire system, an account with root privileges is required

There are four settings for SSH credentials that apply to all SSH Authentication methods.
Option | Default Value | Description |
---|---|---|
known_hosts file |
none |
If an SSH known_hosts file is available and provided as part of the Global Credential Settings of the scan policy in the known_hosts file field, Nessus will only attempt to log into hosts in this file. This can ensure that the same username and password you are using to audit your known SSH servers is not used to attempt a log into a system that may not be under your control. |
Preferred port |
22 |
This option can be set to direct Nessus to connect to SSH if it is running on a port other than 22. |
Client version |
OpenSSH_5.0 |
Specifies which type of SSH client Nessus will impersonate while scanning. |
Attempt least privilege (experimental) |
Cleared |
Enables or disables dynamic privilege escalation. When enabled, Nessus attempts to run the scan with an account with lesser privileges, even if the Elevate privileges with option is enabled. If a command fails, Nessus will escalate privileges. Plugins 102095 and 102094 report which plugins ran with or without escalated privileges. Note: Enabling this option may increase scan run time by up to 30%. |

Public Key Encryption, also referred to as asymmetric key encryption, provides a more secure authentication mechanism by the use of a public and private key pair. In asymmetric cryptography, the public key is used to encrypt data and the private key is used to decrypt it. The use of public and private keys is a more secure and flexible method for SSH authentication. Nessus supports both DSA and RSA key formats.
Like Public Key Encryption, Nessus supports RSA and DSA OpenSSH certificates. Nessus also requires the user certificate, which is signed by a Certificate Authority (CA), and the user’s private key.
Note: Nessus supports the OpenSSH SSH public key format. Formats from other SSH applications, including PuTTY and SSH Communications Security, must be converted to OpenSSH public key format.
The most effective credentialed scans are when the supplied credentials have root privileges. Since many sites do not permit a remote login as root, Nessus can invoke su, sudo, su+sudo, dzdo, .k5login, or pbrun with a separate password for an account that has been set up to have su or sudo privileges. In addition, Nessus can escalate privileges on Cisco devices by selecting Cisco ‘enable’ or .k5login for Kerberos logins.
Note: Nessus supports the blowfish-cbc, aes-cbc, and aes-ctr cipher algorithms. Some commercial variants of SSH do not have support for the blowfish algorithm, possibly for export reasons. It is also possible to configure an SSH server to only accept certain types of encryption. Check your SSH server to ensure the correct algorithm is supported.
Nessus encrypts all passwords stored in policies. However, the use of SSH keys for authentication rather than SSH passwords is recommended. This helps ensure that the same username and password you are using to audit your known SSH servers is not used to attempt a log in to a system that may not be under your control.
Note: For supported network devices, Nessus will only support the network device’s username and password for SSH connections.
If an account other than root must be used for privilege escalation, it can be specified under the Escalation account with the Escalation password.
Option | Description |
---|---|
Username |
Username of the account which is being used for authentication on the host system. |
Private Key |
RSA or DSA Open SSH key file of the user. |
Private key passphrase |
Passphrase of the Private Key. |
Allows for increasing privileges once authenticated. |


CyberArk is a popular enterprise password vault that helps you manage privileged credentials. Nessus Manager can get credentials from CyberArk to use in a scan.

Kerberos, developed by MIT’s Project Athena, is a client/server application that uses a symmetric key encryption protocol. In symmetric encryption, the key used to encrypt the data is the same as the key used to decrypt the data. Organizations deploy a KDC (Key Distribution Center) that contains all users and services that require Kerberos authentication. Users authenticate to Kerberos by requesting a TGT (Ticket Granting Ticket). Once a user is granted a TGT, it can be used to request service tickets from the KDC to be able to utilize other Kerberos based services. Kerberos uses the CBC (Cipher Block Chain) DES encryption protocol to encrypt all communications.
Note: You must already have a Kerberos environment established to use this method of authentication.
The Nessus implementation of Linux-based Kerberos authentication for SSH supports the aes-cbc and aes-ctr encryption algorithms. An overview of how Nessus interacts with Kerberos is as follows:
- End-user gives the IP of the KDC
- nessusd asks sshd if it supports Kerberos authentication
- sshd says yes
- nessusd requests a Kerberos TGT, along with login and password
- Kerberos sends a ticket back to nessusd
- nessusd gives the ticket to sshd
- nessusd is logged in
In both Windows and SSH credentials settings, you can specify credentials using Kerberos keys from a remote system. Note that there are differences in the configurations for Windows and SSH.
Option | Description |
---|---|
Username |
The target system’s username. |
Password |
Password of the username specified. |
Key Distribution Center (KDC) |
This host supplies the session tickets for the user. |
KDC Port |
This option can be set to direct Nessus to connect to the KDC if it is running on a port other than 88. |
KDC Transport |
The KDC uses TCP by default in Linux implementations. For UDP, change this option. Note that if you need to change the KDC Transport value, you may also need to change the port as the KDC UDP uses either port 88 or 750 by default, depending on the implementation. |
Realm |
The Realm is the authentication domain, usually noted as the domain name of the target (e.g., example.com). |
Elevate privileges with |
Allows for increasing privileges once authenticated. |
If Kerberos is used, sshd must be configured with Kerberos support to verify the ticket with the KDC. Reverse DNS lookups must be properly configured for this to work. The Kerberos interaction method must be gssapi-with-mic.



Option | Description | Required |
---|---|---|
Username | The target system’s username. |
yes |
Lieberman host |
The Lieberman IP/DNS address. Note: If your Lieberman installation is in a subdirectory, you must include the subdirectory path. For example, type IP address or hostname / subdirectory path. |
yes |
Lieberman port | The port on which Lieberman listens. |
yes |
Lieberman API URL | The URL Nessus uses to access Lieberman. | no |
Lieberman user | The Lieberman explicit user for authenticating to the Lieberman RED API. |
yes |
Lieberman password | The password for the Lieberman explicit user. |
yes |
Use SSL |
If Lieberman is configured to support SSL through IIS, check for secure communication. |
no |
Verify SSL Certificate |
If Lieberman is configured to support SSL through IIS and you want to validate the certificate, check this option. Refer to Custom CA documentation for how to use self-signed certificates. |
no |
System Name | In the rare case your organization uses one default Lieberman entry for all managed systems, enter the default entry name. |
no |
Custom password prompt | The password prompt used by the target host. Only use this setting when an interactive SSH session fails due to Nessus receiving an unrecognized password prompt on the target host's interactive SSH shell. |
no |

Option | Default Value |
---|---|
Arcon host |
(Required) The Arcon IP address or DNS address. Note: If your Arcon installation is in a subdirectory, you must include the subdirectory path. For example, type IP address or hostname/subdirectory path. |
Arcon port |
The port on which Arcon listens. |
API User |
(Required) The API user provided by Arcon. |
API Key |
(Required) The API key provided by Arcon. |
Authentication URL | The URL Nessus Manager uses to access Arcon. |
Password Engine URL |
The URL Nessus Manager uses to access the passwords in Arcon. |
Username | (Required) The username to log in to the hosts you want to scan. |
Checkout Duration |
(Required) The length of time, in hours, that you want to keep credentials checked out in Arcon. Configure the Checkout Duration to exceed the typical duration of your Tenable.io scans. If a password from a previous scan is still checked out when a new scan begins, the new scan fails. Note: Configure the password change interval in Arcon so that password changes do not disrupt your Tenable.io scans. If Arcon changes a password during a scan, the scan fails. |
Use SSL | If enabled, Nessus Manager uses SSL through IIS for secure communications. You must configure SSL through IIS in Arcon before enabling this option. |
Verify SSL | If enabled, Nessus Manager validates the SSL certificate. You must configure SSL through IIS in Arcon before enabling this option. |

Option | Default Value |
---|---|
Centrify Host |
(Required) The Centrify IP address or DNS address. Note: If your Centrify installation is in a subdirectory, you must include the subdirectory path. For example, type IP address or hostname/subdirectory path. |
Centrify Port |
The port on which Centrify listens. |
API User | (Required) The API user provided by Centrify |
API Key |
(Required) The API key provided by Centrify. |
Tenant | The name of a specified team in a multi-team environment. |
Authentication URL |
The URL Nessus Manager uses to access Centrify. |
Password Engine URL | The name of a specified team in a multi-team environment. |
Username | (Required) The username to log in to the hosts you want to scan. |
Checkout Duration |
The length of time, in minutes, that you want to keep credentials checked out in Centrify. Configure the Checkout Duration to exceed the typical duration of your Nessus Manager scans. If a password from a previous scan is still checked out when a new scan begins, the new scan fails. Note: Configure the password change interval in Centrify so that password changes do not disrupt your Nessus Manager scans. If Centrify changes a password during a scan, the scan fails. |
Use SSL | If enabled, Nessus Manager uses SSL through IIS for secure communications. You must configure SSL through IIS in Centrify before enabling this option. |
Verify SSL | If enabled, Nessus Manager validates the SSL certificate. You must configure SSL through IIS in Centrify before enabling this option. |