Host

Nessus supports the following forms of host authentication:

SNMPv3

Users can select SNMPv3 settings from the Credentials menu and enter credentials for scanning systems using an encrypted network management protocol.

These credentials are used to obtain local information from remote systems, including network devices, for patch auditing or compliance checks.

There is a field for entering the SNMPv3 user name for the account that will perform the checks on the target system, along with the SNMPv3 port, security level, authentication algorithm and password, and privacy algorithm and password.

If Nessus is unable to determine the community string or password, it may not perform a full audit of the service.

Option Description

Username

The username for a SNMPv3 based account.

Port

Direct Nessus to scan a different port if SNMP is running on a port other than 161.

Security level

Select the security level for SNMP: authentication, privacy, or both.

Authentication algorithm

Select MD5 or SHA1 based on which algorithm the remote service supports.

Authentication password

The password for the username specified.

Privacy algorithm

The encryption algorithm to use for SNMP traffic.

Privacy password

A password used to protect encrypted SNMP communication.

SSH

On Unix 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 Unix systems.

These credentials are used to obtain local information from remote Unix systems for patch auditing or compliance checks.

Note: Non-privileged users with local access on Unix 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

Windows

The Windows credentials menu item has settings to provide Nessus with information such as SMB account name, password, and domain name. By default, you can specify a username, password, and domain with which to log in to Windows hosts. Additionally, Nessus supports several different types of authentication methods for Windows-based systems:

Regarding the authentication methods:

  • The Lanman authentication method was prevalent on Windows NT and early Windows 2000 server deployments. It is retained for backward compatibility.
  • The NTLM authentication method, introduced with Windows NT, provided improved security over Lanman authentication. The enhanced version, NTLMv2, is cryptographically more secure than NTLM and is the default authentication method chosen by Nessus when attempting to log into a Windows server. NTLMv2 can make use of SMB Signing.
  • SMB signing is a cryptographic checksum applied to all SMB traffic to and from a Windows server. Many system administrators enable this feature on their servers to ensure that remote users are 100% authenticated and part of a domain. In addition, make sure you enforce a policy that mandates the use of strong passwords that cannot be easily broken via dictionary attacks from tools like John the Ripper and L0phtCrack. It is automatically used by Nessus if it is required by the remote Windows server. Note that there have been many different types of attacks against Windows security to illicit hashes from computers for re-use in attacking servers. SMB Signing adds a layer of security to prevent these man-in-the-middle attacks.
  • The SPNEGO (Simple and Protected Negotiate) protocol provides Single Sign On (SSO) capability from a Windows client to a variety of protected resources via the users’ Windows login credentials. Nessus supports use of SPNEGO Scans and Policies: Scans 54 of 151 with either NTLMSSP with LMv2 authentication or Kerberos and RC4 encryption. SPNEGO authentication happens through NTLM or Kerberos authentication; nothing needs to be configured in the Nessus policy.
  • If an extended security scheme (such as Kerberos or SPNEGO) is not supported or fails, Nessus will attempt to log in via NTLMSSP/LMv2 authentication. If that fails, Nessus will then attempt to log in using NTLM authentication.
  • Nessus also supports the use of Kerberos authentication in a Windows domain. To configure this, the IP address of the Kerberos Domain Controller (actually, the IP address of the Windows Active Directory Server) must be provided.

Server Message Block (SMB) is a file-sharing protocol that allows computers to share information across the network. Providing this information to Nessus will allow it to find local information from a remote Windows host. For example, using credentials enables Nessus to determine if important security patches have been applied. It is not necessary to modify other SMB parameters from default settings.

The SMB domain field is optional and Nessus will be able to log on with domain credentials without this field. The username, password, and optional domain refer to an account that the target machine is aware of. For example, given a username of joesmith and a password of my4x4mpl3, a Windows server first looks for this username in the local system’s list of users, and then determines if it is part of a domain.

Regardless of credentials used, Nessus always attempts to log into a Windows server with the following combinations:

  • Administrator without a password
  • A random username and password to test Guest accounts
  • No username or password to test null sessions

The actual domain name is only required if an account name is different on the domain from that on the computer. It is entirely possible to have an Administrator account on a Windows server and within the domain. In this case, to log onto the local server, the username of Administrator is used with the password of that account. To log onto the domain, the Administrator username would also be used, but with the domain password and the name of the domain.

When multiple SMB accounts are configured, Nessus will try to log in with the supplied credentials sequentially. Once Nessus is able to authenticate with a set of credentials, it will check subsequent credentials supplied, but only use them if administrative privileges are granted when previous accounts provided user access.

Some versions of Windows allow you to create a new account and designate it as an administrator. These accounts are not always suitable for performing credentialed scans. Tenable recommends that the original administrative account, named Administrator be used for credentialed scanning to ensure full access is permitted. On some versions of Windows, this account may be hidden. The real administrator account can be unhidden by running a DOS prompt with administrative privileges and typing the following command:

C:\> net user administrator /active:yes

If an SMB account is created with limited administrator privileges, Nessus can easily and securely scan multiple domains. Tenable recommends that network administrators consider creating specific domain accounts to facilitate testing. Nessus includes a variety of security checks for Windows Vista, Windows 7, Windows 8, Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, and Windows Server 2012 R2 that are more accurate if a domain account is provided. Nessus does attempt to try several checks in most cases if no account is provided.

Note: The Windows Remote Registry service allows remote computers with credentials to access the registry of the computer being audited. If the service is not running, reading keys and values from the registry will not be possible, even with full credentials. This service must be started for a Nessus credentialed scan to fully audit a system using credentials.

For more information, see the Tenable Network Security blog post Dynamic Remote Registry Auditing - Now you see it, now you don’t!

Credentialed scans on Windows systems require that a full administrator level account be used. Several bulletins and software updates by Microsoft have made reading the registry to determine software patch level unreliable without administrator privileges, but not all of them. Nessus plugins will check that the provided credentials have full administrative access to ensure they execute properly. For example, full administrative access is required to perform direct reading of the file system. This allows Nessus to attach to a computer and perform direct file analysis to determine the true patch level of the systems being evaluated.

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