Download William Stallings, Cryptography and Network Security 3/e

Device Security
• A device is a node helping to form the topology
of the network.
• A compromised device may be used by the
attacker as a jumping board.
• A DoS attack may be launched against a device.
• Device security is an important and required
step in ensuring infrastructure security in a
network.
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Layered view of device security
(Figure 3-1)
• Physical security
– confidentiality, integrity, availability
– Placing the device in a secure location ?
– Power got cut off ?
• Logical security
– Securing the device against nonphysical attacks
1. Static core configuration of the device
2. Dynamic configuration and performance
3. Network traffic flow through the device
• The security of each layer depends on the security of the
layers within.
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Physical security
Considerations:
•
Using redundant devices?
•
Network topology (serialized, star, fully meshed?)
•
Where to place the network devices?
•
Media security (wire tapping, physical eavesdropping)
•
Adequate/uninterrupted power supply
•
disasters
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Device Redundancy
•
A backup device (router, switch, gateway, …)
is configured to take over the functionality of a
failed active device.
•
Means of achieving redundancy:
A. Use routing to enable redundancy
B. Use a redundancy protocol
– Hot Standby Router Protocol (HSRP)
– Virtual Router Redundancy Protocol (VRRP)
– Failover protocols: a feature of Cisco PIX
firewalls
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Security of major devices
• How to protect the device against attacks aimed
at compromising the device itself
–
–
–
–
–
–
Routers
Firewalls
Switches
Authentication servers
wireless access points
…
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Steps to secure a device (hardening):
• Physical security
• Password management
• ROMmon (ROM monitor, or
the bootstrap program)
• Controlling access to the
device (tty, vty ports)
• Securing access to the
device (via SSH)
• Backup of configuration files
and the device software
• Logging events on the
device
• Disabling unnecessary
services
• Login banner messages
• Controlling SNMP as a
management protocol
• Controlling HTTP as a
management protocol
• Using CEF as a switching
mechanism
• Setting up the scheduler from
a security perspective
• Using the Network Time
Protocol (NTP)
• Capturing core dumps
• Using service nagle to
improve Telnet access during
high CPU events
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Password Management
•
Passwords stored on the router should be
properly encrypted.
•
The default password-encryption is either type
0 (clear text passwords) or type 7 (weak
encryption).
•
service password-encryption
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ROMmon
• ROM monitor (aka the bootstrap program)
• Source:
http://www.cisco.com/en/US/docs/routers/access/800/801/software/configur
ation/guide/rommon.html#wp1013650
– “The ROM monitor firmware runs when the router is powered up
or reset.”
– “The firmware helps to initialize the processor hardware and boot
the operating system software.”
– “You can use the ROM monitor to perform certain configuration
tasks, such as recovering a lost password or downloading
software over the console port.”
• The no service password-recovery is a security
enhancement feature that prevents the completion of the
break key sequence and the entering of ROMmon mode.
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Controlling access to the router
• A tty port (console port) is physically connected
to a terminal or workstation for local
administrative access to the router.
• A vty (virtual tty) port is used to allow remote inband connection sessions, via telnet, ssh, or
rlogin.
• An aux ports, similar to a tty port, is connected
to a modem for remote out-of-band
administrative access to the router.
– An aux port in most cases should be disabled.
– Out-of-band management (http://en.wikipedia.org/wiki/Out-of-band_management)
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Vulnerabilities of tty or aux ports
• A tty or aux port may suffer reverse telnet
attack, where the terminal server connected to
the tty port or the modem connected to the aux
port of the router is used by the attacker (as a
remote client) to access the router.
• Reverse Telnet (as defined in Wikipedia)
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Reverse Telnet (cont.)
• An example:
What is Reverse Telnet and how do I
configure it?
“Reverse Telnet gives you the ability to telnet to a device, and then
console to another device from there. For example, you could
telnet to a router, and then console into a switch, or a modem, or
anything that has a console port. There are a lot of devices out
there that don’t have remote access built into them, their only
option is a console session. Well, this will allow you to remotely
manage these devices.”
“You need a straight through cable going from the console port of
the console-only device to the AUX port on your router.”
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Normal telnet
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Reverse Telnet
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Vulnerabilities of tty or aux ports
• Solution?
– Disable the console port
Line con0
transport input none
– Allow only SSH access to a router’s console port (a
feature added to IOS v12.2 or higher)
Line con0
login authentication default
rotary 1
transport input ssh
ip ssh port 2001 rotary 1
• Requirement: The router must be set up as a SSH server.
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Controlling vty access
1.
Restricted access: Only allow the protocols that will be
used by the network admin
•
•
Only addresses in the ‘access list’ are allowed to
connect: access-class, access-list
Short timeouts:
2.
3.
•
4.
Since Cisco IOS v11.1, the default is none.
Example: To allow only telnet and ssh connections
line vty 0 4
transport input telnet ssh
The default timeout value is 10 minutes. To set it to 5 min. 30
sec.
line vty 0 4
exec-timeout 5 30
Authentication for vty access: either local or RADIUS
authentication (preferred).
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Securing access to the router
using encryption
•
IPsec VPN client (preferred; more details in Ch 13)
– Two cases:
A. The VPN client access a back-end LAN (the
destination) by building a tunnel between itself and
a router (the IPsec gateway), behind which the
LAN is located.
B. The VPN client is used to remotely administer the
router, which is both the gateway and the
destination.
•
SSH: Only SSH v1 is supported by Cisco IOS
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Logging events
• Advantages: Allows auditing and tracking
 forensics (in case of an attack)
performance tuning (maintenance)
• Requirement:
good time stamping  using NTP
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Disable unnecessary services
• If a service is not being actively used on a
device, it should be disabled.
• Otherwise it may be used as a back door
for the attacker to gain access to the
device.
• Sample services to be disabled:
TCP small servers, UDP small servers, Finger
server, …
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• Access control based on loopback
– Enable a block of IP addresses to be assigned to be
used as the loopback IPs of a block of routers.
– Router IDs: All routers can be forced to use these
loopback IP addresses as source addresses when
accessing the servers.
– Access Control: The servers can then also be locked
down to allow access only from this block of IP
addresses.
– Accesses from addresses outside this block are
denied.
• Examples (next)
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Examples of
access control based on loopback
source: http://ws.edu.isoc.org/data/2004/112350407740360107a09f9/loopback-1up.pdf
• TFTP Server Access
– TFTP is the most common tool for uploading and
downloading configurations.
– The TFTP server's security is critical, which means that
you should always use security tools with IP source
addresses.
– IOS Software allows TFTP to be configured to use specific
IP interfaces address. This allows a fixed ACL on the
TFTP server based on a fixed address on the router (for
example, the loopback interface).
ip tftp source-interface Loopback0
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Examples of access control
based on loopback (cont.)
• Source:
http://ws.edu.isoc.org/data/2004/112350407740360107a09f9/loopback1up.pdf (slide 12)
• TACACS+ distributed authentication system for
management access to routers
– Configure TACACS+ so that the loopback address is used
in packets originating from the router
– Configuration example:
ip tacacs source-interface Loopback0
tacacs-server host 215.17.1.1
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Banner messages
• Informational messages displayed to users who connect
to the device
• To warn the unauthorized users of their activity and to
warn them they are being monitored and logged
• Useful for law enforcement and system admin
• Sequence:
–
–
–
–
MOTD banner
Login banner
login session
EXEC banner (or incoming banner) -- for users to enter
commands; show the contexts
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Backup of the Device Software
• Cisco IOS Resilient Configuration feature
• Enables a router to secure a working copy of the
running image and configuration (the primary
bootset)
• Those files can withstand malicious attempts to
erase the contents of persistent storage
• Those secure files are protected by the IOS File
System (IFS); cannot be removed by the user
• secure boot-image
• secure boot-config
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Check the Default Settings
• Disable unnecessary services
– Finger, FTP, HTTP, …
• Disable potentially insecure services, which may
be used by the attacker to map or exploit the
network
– IP source routing
– IP mask reply
– IP redirects
• The Auto-Secure feature
– Disables common IP services that can be exploited
– Enables IP services and features that can aid in
defending the network
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Using NTP
• Network Time Protocol
• Critical for services requiring good time
stamping: logging, AAA, Kerberos, …
• Challenge: authentication between
devices exchanging NTP information
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Controlling SNMP
• An application layer protocol facilitating the exchange of
information between network devices
• can be used in read-only and ‘read and write’ modes
• Unless necessary, use read-only mode on routers.
• The ‘read and write’ mode allows the admin to modify
the router’s configurations via SNMP.
• Access into the network via SNMP should be blocked at
the network’s boundary.
• Security of SNMP:
– v1 and v2 use ‘community strings’ as the only authentication
mechanism. (Not secure)
– v3 is more secure by providing MD5 or SHA for authentication,
and DES for encryption.
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Controlling SNMP
• SNMP v3 threats vs protections
Threats
Protections
• modification of info
Message integrity
• masquerade
Message authentication
• disclosure
Encryption
• message stream modification
- Messages might be maliciously reordered,
delayed, or replayed to a greater extent than
can occur through the natural operation of a
subnetwork service to effect unauthorized
management operations.
Timestamped messages,
Session identifiers (sequence
numbers)
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Controlling HTTP
(as a management protocol)
• Unless necessary, HTTP access to the router should be
disabled.
• HTTP protocol provides little security.
• The default authentication sends the password as clear
text.
• Admin access to the router via HTTP should be secured,
by activating authentication.
• Ideally, a secure connection via VPN or SSL should be
used. (example: HTTPS)
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Using CEF as a switching mechanism
• Cisco Express Forwarding
• Routers using the traditional switching mechanisms need
to update routing caches when packets destined for new
addresses arrive.
• SYN floods and DDoS attacks use a large number of
random or pseudo-random IP addresses as ultimate
targets.
• CEF replaces the normal routing cache with a data
structure (Forwarding Information Base, or FIB, and
Adjacency Table) that mirrors the entire routing tables.
• It does away with the need to update the cache each
time a new IP address needs to be routed to.
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CEF Components
http://www.cisco.com/en/US/docs/ios/12_1/switch/configuration/guide/xcdcef.html#wp1000922
1. Forwarding Information Base
– CEF uses a FIB to make IP destination prefix-based switching decisions.
– The FIB is conceptually similar to a routing table or information base. It maintains
a mirror image of the forwarding information contained in the IP routing table.
– When routing or topology changes occur in the network, the IP routing table is
updated, and those changes are reflected in the FIB. The FIB maintains next-hop
address information based on the information in the IP routing table.
– Because there is a one-to-one correlation between FIB entries and routing table
entries, the FIB contains all known routes and eliminates the need for route
cache maintenance that is associated with switching paths such as fast switching
and optimum switching.
2. Adjacency Tables
– Nodes in the network are said to be adjacent if they can reach each other with a
single hop across a link layer.
– In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing
information. The adjacency table maintains Layer 2 next-hop addresses for all
FIB entries.
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Using the scheduler
• scheduler allocate
• scheduler interval
• To prevent the router from becoming too busy
responding to the interrupts on its interfaces due to the
large number of packets arriving  large-scale network
attack, esp. a DDoS attack
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Using the scheduler (cont.)
• Example (from: http://www.cymru.com/Documents/performance.html)
– scheduler allocate 4000 200 // The default values
– Where 4000 is the maximum number of microseconds to allocate to fast
switching any single network interrupt context, and 200 is the minimum
guaranteed number of microseconds to allocate to process level tasks
while network interrupts are masked.
– In cases where extremely high network load presents itself on the
interface of a router, it is possible that other tasks will not be able to run.
– By default, the Cisco IOS allocates 5% of the CPU time to the lower
priority tasks. During a high load event, such as a DDOS, this default
may be insufficient to ensure that other tasks acquire CPU time, such as
routing protocol updates and CEF table maintenance.
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Capturing core dumps
• In the event of system crash, the core
dump may provide useful info for tracking
the attack(s).
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Service nagle
• Nagle is an algorithm that can be enabled as a service
on a Cisco router, to allow the router to pace the TCP
connection for Telnet in a way that reduces the burden
on the CPU and generally improves the performance of
the Telnet session.
– service nagle
• More info (next)
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•
•
•
•
From: http://www.ciscopress.com/articles/article.asp?p=27137&seqNum=7
The Nagle congestion-control algorithm is something that many ISPs turn on to
improve the performance of their Telnet sessions to and from the router.
When using a standard TCP implementation to send keystrokes between machines,
TCP tends to send one packet for each keystroke typed. On larger networks, many
small packets use up bandwidth and contribute to congestion.
John Nagle's algorithm (RFC 896) helps alleviate the small-packet problem in TCP.
In general, it works this way:
– The first character typed after connection establishment is sent in a single
packet, but TCP holds any additional characters typed until the receiver
acknowledges the previous packet.
– The second, larger packet is sent, and additional typed characters are saved
until the acknowledgment comes back.
– The effect is to accumulate characters into larger chunks and pace them out to
the network at a rate matching the round-trip time of the given connection.
– This method is usually good for all TCP-based traffic and helps when
connectivity to the router is poor or congested or the router itself is busier than
normal.
– However, do not use the service nagle command when real-time traffic (like
voice over IP) are processed on the router — performance will become very
poor.
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Security of other devices
• Firewalls, switches, …
• Similar procedure
– Check the default settings
– ‘Harden’ the device before placing it into use
in the production network.
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Device Security Checklist
✓Device security policy written, approved, distributed, and reviewed on regular basis.
✓Facilities (room, building, area) housing the devices secured—physical security.
✓Password policies to ensure that good passwords are created that cannot be easily
guessed or hacked.
✓Password encryption used so that passwords are not visible when device configuration is
viewed.
✓Access methods such as Console, VTY, AUX using ACLs, and authentication
mechanisms secured.
✓Access methods such as SSH with AAA authentication chosen wisely.
✓Unneeded services and protocols to be disabled.
✓Unused interfaces shut down or disabled.
✓Configuration hardened for network services and protocols in use (for example, HTTP and
SNMP).
✓Port and protocol needs of the network and use access lists to limit traffic flow identified.
✓Access list for anti-spoofing and infrastructure protection and for blocking reserved and
private addresses considered.
✓Routing protocols established that use authentication mechanisms for integrity.
✓Appropriate logging enabled with proper time information.
✓Device’s time of day set accurately, maintained with NTP.
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Security Baseline Checklist
— Infrastructure Device Access
• http://www.cisco.com/en/US/docs/solution
s/Enterprise/Security/Baseline_Security/ap
pendxD.html
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