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Topic 1: System Hardening and Availability

1.2 Understanding Management Plane Security Technologies and Core Concepts Covering Security Features Available to Protect the Management Plane.

The management plane consists of functions that achieve the management goals of the network. This includes interactive management sessions that use SSH, as well as statistics-gathering with SNMP or NetFlow. When you consider the security of a network device, it is critical that the management plane be protected. If a security incident is able to undermine the functions of the management plane, it can be impossible for you to recover or stabilize the network.

General Management Plane Hardening

The management plane is used in order to access, configure, and manage a device, as well as monitor its operations and the network on which it is deployed. The management plane is the plane that receives and sends traffic for operations of these functions. You must secure both the management plane and control plane of a device, because operations of the control plane directly affect operations of the management plane. This list of protocols is used by the management plane:

  • Simple Network Management Protocol
  • Telnet
  • Secure Shell Protocol
  • File Transfer Protocol
  • Trivial File Transfer Protocol
  • Secure Copy Protocol
  • TACACS+
  • RADIUS
  • NetFlow
  • Network Time Protocol
  • Syslog

Steps must be taken to ensure the survival of the management and control planes during security incidents. If one of these planes is successfully exploited, all planes can be compromised.

A)    Password Management

Passwords control access to resources or devices. As a security best practice, passwords must be managed with a TACACS+ or RADIUS authentication server. However, note that a locally configured password for privileged access is still needed in the event of failure of the TACACS+ or RADIUS services. A device can also have other password information present within its configuration, such as an NTP key, SNMP community string, or Routing Protocol key.

enable secret: The enable secret command is used in order to set the password that grants privileged administrative access to the Cisco IOS system. The enable secret command must be used, rather than the older enable password command. The enable password command uses a weak encryption algorithm.

If no enable secret is set and a password is configured for the console tty line, the console password can be used in order to receive privileged access, even from a remote virtual tty (vty) session. This action is almost certainly unwanted and is another reason to ensure configuration of an enable secret.

service password-encryption:The service password-encryption global configuration command directs the Cisco IOS software to encrypt the passwords, Challenge Handshake Authentication Protocol (CHAP) secrets, and similar data that are saved in its configuration file. Such encryption is useful in order to prevent casual observers from reading passwords, such as when they look at the screen over the muster of an administrator.

The enable secret command and the Enhanced Password Security feature use Message Digest 5 (MD5) for password hashing. This algorithm has had considerable public review and is not known to be reversible. However, the algorithm is subject to dictionary attacks.

B)   Enhanced Password Security

The feature Enhanced Password Security, introduced in Cisco IOS Software Release 12.2(8)T, allows an administrator to configure MD5 hashing of passwords for the username command. Prior to this feature, there were two types of passwords: Type 0, which is a cleartext password, and Type 7, which uses the algorithm from the Vigen re cipher. The Enhanced Password Security feature cannot be used with protocols that require the cleartext password to be retrievable, such as CHAP.

In order to encrypt a user password with MD5 hashing, issue the username secret global configuration command.

!
 
 username <name> secret <password>
 
 !

C)   Login Password Retry Lockout

The Login Password Retry Lockout feature, added in Cisco IOS Software Release 12.3(14)T, allows you to lock out a local user account after a configured number of unsuccessful login attempts. Once a user is locked out, their account is locked until you unlock it. An authorized user who is configured with privilege level 15 cannot be locked out with this feature. The number of users with privilege level 15 must be kept to a minimum.

Note that authorized users can lock themselves out of a device if the number of unsuccessful login attempts is reached. Additionally, a malicious user can create a denial of service (DoS) condition with repeated attempts to authenticate with a valid username.

This example shows how to enable the Login Password Retry Lockout feature:

!
 
 aaa new-model
 aaa local authentication attempts max-fail <max-attempts>
 aaa authentication login default local
 
 !
 
 username <name> secret <password>
 
 !

This feature also applies to authentication methods such as CHAP and Password Authentication Protocol (PAP).

D)  No Service Password-Recovery

In Cisco IOS Software Release 12.3(14)T and later, the No Service Password-Recovery feature does not allow anyone with console access to insecurely access the device configuration and clear the password. It also does not allow malicious users to change the configuration register value and access NVRAM.

!
 
 no service password-recovery
 
 !

The current password recovery procedure enables anyone with console access to access the device and its network. The No Service Password-Recovery feature prevents the completion of the Break key sequence and the entering of ROMMON during system startup.

If no service password-recovery is enabled on a device, it is recommended that an offline copy of the device configuration be saved and that a configuration archiving solution be implemented. If it is necessary to recover the password of a Cisco IOS device once this feature is enabled, the entire configuration is deleted.

E)    Disable Unused Services

As a security best practice, any unnecessary service must be disabled. These unneeded services, especially those that use User Datagram Protocol (UDP), are infrequently used for legitimate purposes, but can be used in order to launch DoS and other attacks that are otherwise prevented by packet filtering.

The TCP and UDP small services must be disabled. These services include:

  • echo (port number 7)
  • discard (port number 9)
  • daytime (port number 13)
  • chargen (port number 19)

Although abuse of the small services can be avoided or made less dangerous by anti-spoofing access lists, the services must be disabled on any device accessible within the network. The small services are disabled by default in Cisco IOS Software Releases 12.0 and later. In earlier software, the no service tcp-small-servers and no service udp-small-servers global configuration commands can be issued in order to disable them.

This is a list of additional services that must be disabled if not in use:

  • Issue the no ip finger global configuration command in order to disable Finger service. Cisco IOS software releases later than 12.1(5) and 12.1(5)T disable this service by default.
  • Issue the no ip bootp server global configuration command in order to disable Bootstrap Protocol (BOOTP).
  • In Cisco IOS Software Release 12.2(8)T and later, issue the ip dhcp bootp ignore command in global configuration mode in order to disable BOOTP. This leaves Dynamic Host Configuration Protocol (DHCP) services enabled.
  • DHCP services can be disabled if DHCP relay services are not required. Issue the no service dhcp command in global configuration mode.
  • Issue the no mop enabled command in interface configuration mode in order to disable the Maintenance Operation Protocol (MOP) service.
  • Issue the no ip domain-lookup global configuration command in order to disable Domain Name System (DNS) resolution services.
  • Issue the no service pad command in global configuration mode in order to disable Packet Assembler/Disassembler (PAD) service, which is used for X.25 networks.
  • The HTTP server can be disabled with the no ip http server command in global configuration mode, and Secure HTTP (HTTPS) server can be disabled with the no ip http secure-server global configuration command.
  • Unless Cisco IOS devices retrieve configurations from the network during startup, the no service config global configuration command must be used. This prevents the Cisco IOS device from an attempt to locate a configuration file on the network with TFTP.
  • Cisco Discovery Protocol (CDP) is a network protocol that is used in order to discover other CDP enabled devices for neighbor adjacency and network topology. CDP can be used by Network Management Systems (NMS) or during troubleshooting. CDP must be disabled on all interfaces that are connected to untrusted networks. This is accomplished with the no cdp enable interface command. Alternatively, CDP can be disabled globally with the no cdp run global configuration command. Note that CDP can be used by a malicious user for reconnaissance and network mapping.
  • Link Layer Discovery Protocol (LLDP) is an IEEE protocol that is defined in 802.1AB. LLDP is similar to CDP. However, this protocol allows interoperability between other devices that do not support CDP. LLDP must be treated in the same manner as CDP and disabled on all interfaces that connect to untrusted networks. In order to accomplish this, issue the no lldp transmit and no lldp receive interface configuration commands. Issue the no lldp run global configuration command in order to disable LLDP globally. LLDP can also be used by a malicious user for reconnaissance and network mapping.

F)    EXEC Timeout

In order to set the interval that the EXEC command interpreter waits for user input before it terminates a session, issue the exec-timeout line configuration command. The exec-timeout command must be used in order to logout sessions on vty or tty lines that are left idle. By default, sessions are disconnected after 10 minutes of inactivity.

!
 
 line con 0
  exec-timeout <minutes> [seconds]
 line vty 0 4
  exec-timeout <minutes> [seconds]
 !

G)  Keepalives for TCP Sessions

The service tcp-keepalives-in and service tcp-keepalives-out global configuration commands enable a device to send TCP keepalives for TCP sessions. This configuration must be used in order to enable TCP keepalives on inbound connections to the device and outbound connections from the device. This ensures that the device on the remote end of the connection is still accessible and that half-open or orphaned connections are removed from the local Cisco IOS device.

!
 
 service tcp-keepalives-in
 service tcp-keepalives-out
 !

H)  Management Interface Use

The management plane of a device is accessed in-band or out-of-band on a physical or logical management interface. Ideally, both in-band and out-of-band management access exists for each network device so that the management plane can be accessed during network outages.

One of the most common interfaces that is used for in-band access to a device is the logical loopback interface. Loopback interfaces are always up, whereas physical interfaces can change state, and the interface can potentially not be accessible. It is recommended to add a loopback interface to each device as a management interface and that it be used exclusively for the management plane. This allows the administrator to apply policies throughout the network for the management plane. Once the loopback interface is configured on a device, it can be used by management plane protocols, such as SSH, SNMP, and syslog, in order to send and receive traffic.

!
 interface Loopback0
  ip address 192.168.1.1 255.255.255.0
 !

I)      Memory Threshold Notifications

The feature Memory Threshold Notification, added in Cisco IOS Software Release 12.3(4)T, allows you to mitigate low-memory conditions on a device. This feature uses two methods in order to accomplish this: Memory Threshold Notification and Memory Reservation.

Memory Threshold Notification generates a log message in order to indicate that free memory on a device has fallen lower than the configured threshold. This configuration example shows how to enable this feature with the memory free low-watermark global configuration command. This enables a device to generate a notification when available free memory falls lower than the specified threshold, and again when available free memory rises to five percent higher than the specified threshold.

!
 
 memory free low-watermark processor <threshold>
 memory free low-watermark io <threshold>
 !

Memory Reservation is used so that sufficient memory is available for critical notifications. This configuration example demonstrates how to enable this feature. This ensures that management processes continue to function when the memory of the device is exhausted.

!
 memory reserve critical <value>
!

J)      CPU Thresholding Notification

Introduced in Cisco IOS Software Release 12.3(4)T, the CPU Thresholding Notification feature allows you to detect and be notified when the CPU load on a device crosses a configured threshold. When the threshold is crossed, the device generates and sends an SNMP trap message. Two CPU utilization thresholding methods are supported on Cisco IOS software: Rising Threshold and Falling Threshold.

This example configuration shows how to enable the Rising and Falling Thresholds that trigger a CPU threshold notification message:

!
 
 snmp-server enable traps cpu threshold
 !
 
 snmp-server host <host-address> <community-string> cpu 
 !
 
 process cpu threshold type <type> rising <percentage> interval <seconds> 
      [falling <percentage> interval <seconds>]
 process cpu statistics limit entry-percentage <number> [size <seconds>]
 !

K)   Reserve Memory for Console Access

In Cisco IOS Software Release 12.4(15)T and later, the Reserve Memory for Console Access feature can be used in order to reserve enough memory to ensure console access to a Cisco IOS device for administrative and troubleshooting purposes. This feature is especially beneficial when the device runs low on memory. You can issue the memory reserve console global configuration command in order to enable this feature. This example configures a Cisco IOS device to reserve 4096 kilobytes for this purpose.

!
 memory reserve console 4096
 !

L)    Memory Leak Detector

Introduced in Cisco IOS Software Release 12.3(8)T1, the Memory Leak Detector feature allows you to detect memory leaks on a device. Memory Leak Detector is able to find leaks in all memory pools, packet buffers, and chunks. Memory leaks are static or dynamic allocations of memory that do not serve any useful purpose. This feature focuses on memory allocations that are dynamic. You can use the show memory debug leaks EXEC command in order to detect if a memory leak exists.

M) Buffer Overflow: Detection and Correction of Redzone Corruption

In Cisco IOS Software Release 12.3(7)T and later, the Buffer Overflow: Detection and Correction of Redzone Corruption feature can be enabled by on a device in order to detect and correct a memory block overflow and to continue operations.

These global configuration commands can be used in order to enable this feature. Once configured, the show memory overflow command can be used in order to display the buffer overflow detection and correction statistics.

!
 exception memory ignore overflow io
 exception memory ignore overflow processor
 !

N)  Enhanced Crashinfo File Collection

The Enhanced Crashinfo File Collection feature automatically deletes old crashinfo files. This feature, added in Cisco IOS Software Release 12.3(11)T, allows a device to reclaim space in order to create new crashinfo files when the device crashes. This feature also allows configuration of the number of crashinfo files to be saved.

!
 exception crashinfo maximum files <number-of-files>
 !

O)  Network Time Protocol

The Network Time Protocol (NTP) is not an especially dangerous service, but any unneeded service can represent an attack vector. If NTP is used, it is important to explicitly configure a trusted time source and to use proper authentication. Accurate and reliable time is required for syslog purposes, such as during forensic investigations of potential attacks, as well as for successful VPN connectivity when depending on certificates for Phase 1 authentication.

  • NTP Time Zone – When you configure NTP, the time zone needs to be configured so that timestamps can be accurately correlated. There are usually two approaches to configure the time zone for devices in a network with a global presence. One method is to configure all network devices with the Coordinated Universal Time (UTC) (previously Greenwich Mean Time (GMT)). The other approach is to configure network devices with the local time zone. More information on this feature can be found in ?clock timezone? in the Cisco product documentation.
  • NTP Authentication – If you configure NTP authentication, it provides assurance that NTP messages are exchanged between trusted NTP peers.

 

Next Topic: Understanding Management Plane Security Technologies and Core Concepts Covering Security Features Available to Protect the Management Plane:- Limit Access to the Network with Infrastructure ACLs

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