Nessus_UNIX和WINDOWS扫描白皮书

Tenable Network Security, Inc. • 7063 Columbia Gateway Drive, Suite 100, Columbia, MD 21046 • 410.872.0555 • sales@tenable.com • www.tenable.com Copyright © 2011. Tenable Network Security, Inc. All rights reserved. Tenable Network Security and Nessus are registered trademarks of Tenable Network Security, Inc. The ProfessionalFeed is a trademark of Tenable Network Security, Inc. All other products or services are trademarks of their respective owners. NNeessssuuss CCrreeddeennttiiaall CChheecckkss ffoorr UUnniixx aanndd WWiinnddoowwss April 21, 2011 (Revision 23) Copyright © 2002-2011 Tenable Network Security, Inc. 2 TTaabbllee ooff CCoonntteennttss Introduction ............................................................................................................................... 4 Standards and Conventions ....................................................................................................... 4 Overview of Nessus Credential Checks ..................................................................................... 4 Purpose ................................................................................................................................. 4 Access Level ......................................................................................................................... 5 Technologies Used ................................................................................................................ 5 Unix Systems ..................................................................................................................................... 5 Windows Systems .............................................................................................................................. 6 Credential Checks on Unix Platforms ...................................................................................... 7 Prerequisites .............................................................................................................................. 8 Configuration Requirements for SSH ..................................................................................... 8 User Privileges ...................................................................................................................... 8 Configuration Requirements for Kerberos .............................................................................. 8 Enabling SSH Local Security Checks on Unix ............................................................................ 8 Generating SSH Public and Private Keys .............................................................................. 8 Creating a User Account and Setting up the SSH Key ........................................................... 9 Example ...............................................................................................................................10 Configuring Nessus for SSH Host-Based Checks .....................................................................10 Nessus User Interface ..........................................................................................................10 Nessus Unix Command Line ................................................................................................13 Using .nessus Files ....................................................................................................................... 13 Using .nessusrc Files ................................................................................................................... 13 Using SSH Credentials with the Tenable SecurityCenter ..........................................................14 Credential Checks on Windows Platforms .............................................................................15 Prerequisites .............................................................................................................................15 User Privileges .....................................................................................................................15 Enabling Windows Logins for Local and Remote Audits ............................................................15 Configuring a Local Account .................................................................................................15 Configuring a Domain Account for Local Audits ....................................................................15 Configuring Windows XP and 2003 ......................................................................................16 Configuring Windows 2008 ...................................................................................................16 Configuring Nessus for Windows Logins ...................................................................................17 Nessus User Interface ..........................................................................................................17 Nessus Unix Command Line ................................................................................................18 Using .nessus Files ....................................................................................................................... 18 Using .nessusrc Files ................................................................................................................... 18 Detecting when Credentials Fail..............................................................................................18 Troubleshooting .......................................................................................................................19 Securing Your Scanner ............................................................................................................21 Why should I secure my scanner? ............................................................................................21 What does it mean to lock down a scanner? .............................................................................21 Secure Implementation of Unix SSH Audits ..............................................................................21 Copyright © 2002-2011 Tenable Network Security, Inc. 3 Secure Windows Audits ............................................................................................................21 For Further Information ...........................................................................................................22 About Tenable Network Security .............................................................................................23 Copyright © 2002-2011 Tenable Network Security, Inc. 4 INTRODUCTION This paper describes how to perform authenticated network scans with Tenable Network Security’s Nessus vulnerability scanner. Authenticated network scans allow a remote network audit to obtain “host-based” data such as missing patches and operating system settings. Please share your comments and suggestions with us by emailing them to support@tenable.com. Nessus leverages the ability to log into remote Unix hosts via Secure Shell. For Windows hosts, Nessus leverages a variety of Microsoft authentication technologies. Note that Nessus also uses the Simple Network Management Protocol to make version and information queries to routers and switches. Although this is a form of “local checks”, it is not covered in this document. This document also makes extensive references to “Nessus”, but the basic concepts are also true for Tenable’s SecurityCenter. STANDARDS AND CONVENTIONS Throughout the documentation, filenames, daemons and executables are indicated with a courier bold font such as setup.exe. Command line options and keywords are also printed with the courier bold font. Command line options may or may not include the command line prompt and output text from the results of the command. Often, the executed command will be boldfaced to indicate what the user typed. Below is an example running of the Unix pwd command: # pwd /home/test/ # Important notes and considerations are highlighted with this symbol and grey text boxes. Tips, examples and best practices are highlighted with this symbol and white on blue text. OVERVIEW OF NESSUS CREDENTIAL CHECKS Tenable’s Nessus scanner is a very effective network vulnerability scanner with a comprehensive database of plugins that check for a large variety of vulnerabilities that could be remotely exploited. In addition to remote scanning, the Nessus scanner can also be used to scan for local exposures. Purpose External network vulnerability scanning is useful to obtain a snapshot in time of the network services offered and the vulnerabilities they may contain. However, it is only an external perspective. It is important to determine what local services are running and to identify security exposures from local attacks or configuration settings that could expose the system to external attacks that may not be detected from an external scan. Copyright © 2002-2011 Tenable Network Security, Inc. 5 In a typical network vulnerability assessment, a remote scan is performed against the external points of presence and an onsite scan is performed from within the network. Neither of these scans can determine local exposures on the target system. Some of the information gained relies on the banner information displayed, which may be inconclusive or incorrect. By using secured credentials, the Nessus scanner can be granted local access to scan the target system without requiring an agent. This can facilitate scanning of a very large network to determine local exposures or compliance violations. The most common security problem in an organization is that security patches are not applied in a timely manner. A Nessus credentialed scan can quickly determine which systems are out of date on patch installation. This is especially important when a new vulnerability is made public and executive management wants a quick answer regarding the impact to the organization. Another major concern for organizations is to determine compliance with site policy, industry standards (such as the Center for Internet Security (CIS) benchmarks) or legislation (such as Sarbanes-Oxley, Gramm-Leach-Bliley or HIPAA). Organizations that accept credit card information must demonstrate compliance with the Payment Card Industry (PCI) standards. There have been quite a few well-publicized cases where the credit card information for millions of customers was breached. This represents a significant financial loss to the banks responsible for covering the payments and heavy fines or loss of credit card acceptance capabilities by the breached merchant or processor. Access Level Credentialed scans can perform any operation that a local user can perform. The level of scanning is dependant on the privileges granted to the user account that Nessus is configured to use. 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. Credentialed scans on Windows systems require that an 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. 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. On Windows XP Pro, this file access will only work with a local administrator account if the “Network access: Sharing and security model for local accounts” policy is changed to “Classic – local users authenticate as themselves”. Technologies Used The challenge in running a credentialed scan is to automatically provide the privileged credentials to the scanner in a secure manner. It would certainly defeat the purpose of scanning for security exposures if doing so would open an even greater exposure! Nessus supports the use of several secure methods to solve this problem on both Unix and Windows platforms. Unix Systems On Unix systems, 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 Copyright © 2002-2011 Tenable Network Security, Inc. 6 transit to protect it from being viewed by sniffer programs. Nessus supports three types of authentication methods for use with SSH; username and password, public/private keys and Kerberos. Username and Password Although supported, Tenable does not recommend using a username and password for authentication with SSH. Static passwords are subject to “man in the middle” and brute force attacks when they have been in use over a long period of time. Public/Private Keys 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. Kerberos 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. The Nessus implementation of 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 ticket granting ticket, along with login and password > Kerberos sends a ticket back to nessusd > nessusd gives the ticket to sshd > nessusd is logged in Windows Systems Nessus supports several different types of authentication methods for Windows-based systems. Each of these methods takes a username, password and domain name (sometimes optional for authentication). LANMAN The Lanman authentication method was prevalent on Windows NT and early Windows 2000 server deployments. It is not really used on newer Windows deployments, but is retained for backwards compatibility. NTLM and NTLMv2 The NTLM authentication method, introduced with Windows NT, provided improved security over Lanman authentication. However, the enhanced version, NTLMv2, is cryptographically Copyright © 2002-2011 Tenable Network Security, Inc. 7 more secure than NTLM and is the default authentication method chosen by Nessus when attempting to log into a Windows server. 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. It is automatically used by Nessus if it is required by the remote Windows server. SPNEGO 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 with either NTLMSSP with LMv2 authentication or Kerberos and RC4 encryption. Kerberos 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. NTLMSSP (NT Lan Manager Security Support Provider) and LMv2 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. Windows Usernames, Passwords and Domains The SMB domain field is optional and Nessus will be able to logon 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 in there. 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” user name would also be used, but with the domain password and the name of the 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 Guests accounts > No username or password to test Null sessions CREDENTIAL CHECKS ON UNIX PLATFORMS The process described in this section enables you to perform local security checks on Unix based systems. The SSH daemon used in this example is OpenSSH. If you have a commercial variant of SSH, your procedure may be slightly different. Copyright © 2002-2011 Tenable Network Security, Inc. 8 To enable local security checks, there are two basic methods that can be used: 1. Use of a SSH private/public key pair 2. User credentials and sudo access or credentials for su access PREREQUISITES Configuration Requirements for SSH Nessus 4.x supports the blowfish-cbc, aesXXX-cbc (aes128, aes192 and aes256), 3des-cbc and aes-ctr 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. User Privileges For maximum effectiveness, the SSH user must have the ability to run any command on the system. On Unix systems, this is known as “root” privileges. While it is possible to run some checks (such as patch levels) with non-privileged access, full compliance checks that audit system configuration and file permissions require root access. For this reason, it is strongly recommended that SSH keys be used instead of credentials when possible. Configuration Requirements for Kerberos If Kerberos is used, sshd must be configured with Kerberos support to verify the ticket with t