CSS 211 - Introduction to Network Security

Lesson 4 - Wireless Network Security

Objectives:

This lesson covers chapter 6 in the text. It discusses problems specific to wireless networks. Objectives important to this lesson:

  1. IEEE 802.11 security
  2. Vulnerabilities of open wireless systems, WEP, and device authentication
  3. WPA and WPA2
  4. Implementing wireless security
Concepts:

Chapter 6 begins with a cautionary tale, of a large company that used an obsolete security protocol at a particular store, which led to an attacker accessing the company's main database, which led to millions of dollars of loss. The attack was made easier because the company used wireless networking and did not upgrade from a technology that had a well known exploit.

The text introduces the Institute of Electrical and Electronics Engineers (IEEE) as the main standards body for networking technologies. They have established many standards for networking. This chapter is concerned with the 802.11 series of standards that relate to wireless networking.

The text reviews some of the iterations of the 802.11 standard:

  • 802.11a - good bandwidth, high frequency (5 GHz), short range
  • 802.11b - bandwidth went down a bit, range went up a bit, suffers from interference from other devices (2.4 GHz)
  • 802.11g - combines some features of a and b, can still suffer from interference (2.4 GHz)
  • 802.11n - uses multiple antennas and multiple frequencies (2.4 and 5 GHz), can use multiple channels within bandwidth

Leaving the discussion of technology itself, the text turns to security issues. It describes three "protections" included in the original 802.11 standard:

  • controlling access - The text discusses this concept in terms of controlling access to the Wireless Access Point by allowed or blocked lists of MAC addresses; the text points out that this is easier to do with an allowed list.
  • Wired Equivalent Privacy (WEP) - The text seems a bit unreasonable in its praise for this protocol, given that it is the one that was hacked in the story at the beginning of the chapter. It was a reasonable system for the processing power available at the time it was invented. Note the use of a secret phrase that is used to create up to four shared keys used by devices to encrypt their transmissions. The faults of this system are discussed in a few pages.
  • device authentication - The point here is that a wireless device itself may be required to authenticate (prove it belongs) to a network, as well as the user of that device having to authenticate. The text describes two authentication methods in the 802.11 standard:
    • open system authentication - a devices sends an association request frame to an access point, that contains the Service Set Identifier (SSID) of the access point. The text refers to the SSID as identifying the network, but it is only a label that is configured in the access point. The access point compares the received SSID to the SSID it is using. This seems pointless, since most access points broadcast their SSID strings in periodic beacon signals. As the text will explain, the access point can be configured not to broadcast this label.
      Example:
      Bakernet... Bakernet...
      I want to attach to Bakernet...
      Sure, come on in... Bakernet... Bakernet
    • shared key authentication - This is a more secure method which has another step. The devices must exchange a sequence of messages, which the device requesting admission must encrypt with a key known to the access point
Vulnerabilities of IEEE 802.11

On page 198, the text begins a discussion of the vulnerabilities in the system described so far.

Open System Authentication

As noted above the broadcasting of the SSID in beacon frames is a security issue. The text points out that beaconing is needed for roaming from access point to access point. Another issue is a flaw in Windows XP. Devices running it prefer access points that beacon over those that do not. Even when the SSID is not broadcast, it is transmitted in clear text when a device sends an association request. Note the procedure in the text to force a device to send an association request by sending it a disassociation frame.

MAC Address Filtering

MAC addresses are sent in clear text when associating, so they are easily discovered, then the attacker pretends to be the same device. Controlling access by MAC address becomes more difficult, the more devices you allow to attach to the network. This is like the standard recommendation to use host files only if you have fewer than 10 hosts in your LAN. So this method becomes hard to manage as well as being less than secure.

WEP

This discussion goes on for several pages. Students should be aware of the major problems with WEP.

  • short key length - 64 or 128 bits total, including the 24 bit initialization vector, so the actual key is 40 or 104 bits
  • detectable patterns - examine the math in the text to get the idea that a system using WEP could be cracked in less than 7 hours, and probably less than 5.

The text turns to better methods of implementing wireless security.

WPA Personal Security

WPA is Wi-Fi Protected Access, developed in 2003. It contains two components to improve on WEP. They are:

  • PSK authentication - Preshared Keys are generated by running a passphrase through an algorithm that turns out a 64 bit hexadecimal number (the key). This key must be coded into every access point and device that will use this WPA route into the network.
  • TKIP encryption - Temporal Key Integrity Protocol uses a 128 bit key. A new key can be generated for each packet. Keys can be sent to the devices that will use them. Uses Message Integrity Check (MIC) instead of CRC for better assurance of data integrity.
WPA2 Personal Security

WPA2 is a 2004 revision of WPA. The text says it became mandatory for new equipment in 2006. One of its two components changed:

  • PSK authentication - Preshared Keys are still used. The text lists some areas of concern where this method is weak. The worst point, in my opinion, is the guest user having to use the same key as everyone else. A good administrator should change the key when the guest left the network, but how many would do it? Remember, when you change the key, you have to change every device's copy of it.
  • AES-CCMP encryption - The algorithm for this encryption method is intensive, incorporating several submethods. This is an improved level of encryption.
Enterprise Wireless Security

The text presents several topics under this heading.

IEEE 802.11i

This is one of the major models that enterprise wireless security may use. The text is critical of WEP's PRNG (which most Vulcan's might recall means Pseudo-Random Number Generator). This standard began development in 2001, and was not finished until 2004. In the same time frame, the WPA standard (which is not a standard of the IEEE) was also developed. IEEE 802.11i uses the port blocking methods found in the IEEE 802.1x standard (for wired LANs). Ports are not opened until a device authenticates as one allowed to join the network. The text lists two features. Key-caching saves a user's credentials to allow roaming away from the WLAN and reentering it without fully reauthenticating. Pre-authentication allows an AP the user is communicating with to hand off authentication for the session to the next AP, like a cell system, authorizing the user on the next AP before contact would otherwise be made.

Note that IEEE 802.11i only allows clients using AES-CCMP encryption.

WPA Enterprise Security

The personal version of WPA uses PSK for authentication, but this enterprise version uses 802.1x for authentication, and requires an authentication server. It uses TKIP for encryption.

Summary of Access Methods
Methodology Authentication Encryption
WEP open system;
MAC filtering

shared secret keys

WPA personal PSK TKIP
WPA2 personal PSK AES-CCMP
802.11i 802.1x AES-CCMP
WPA enterprise 802.1x TKIP
WPA2 enterprise 802.1x AES-CCMP;
(TKIP clients allowed)

The author ends the chapter with a discussion of wireless security devices.

Thin Access Points

Thin access points are simpler than regular access points. Thin access points are simple radios, and their usual gatekeeper functions are placed on a wireless switch instead. The wireless switch performs authentication and allows the administrator to remotely manage each access point.

Wireless VLANs

Wireless VLANs are described on page 210, and two methods are illustrated on page 211. In the first method, separate access points are created for users in different departments. Packets are passed from the access points to a switch which sends the packets to the appropriate VLAN. The switch separates the packets based on the access point of the connection. The text points out that this method could be used to restrict a particular wireless user to connecting through a particular access point, which inhibits roaming.

In the second method shown on page 211, two access points are set up and wireless users are allowed to connect through either one. Each access point has two SSIDs, which are used to separate the users of the two wireless VLANs. In this case, the access point separates the packets before sending them to the switch. Separate SSIDs on an access point allow separate encryption and authentication schemes for each one.

Rogue Access Point Discovery

The text describes the vulnerability that an unencrypted, unmanaged access point creates in a network. It is not just that an attacker can get access, but also that the attacker could intercept unencrypted traffic being passed by the access point. Consumer versions of access points, cell phones, and commercial equipment are all possible problems.

The text discusses network staff monitoring for rogue access points regularly, but decides this is not as effective as using a wireless probe to continuously monitor wireless traffic. Four types of wireless probes are described:

  • wireless device probe - a device (such as a wireless enabled laptop) is loaded with software and configured to listen for access points; it reports to a central database
  • desktop probe - same as above, but the desktop computer used is not meant to be mobile
  • access point probe - some access points can be used to monitor for other access points
  • dedicated probe - a dedicated device only listens and reports, it is not used for other purposes such as being a workstation or an access point itself

Network management software would be used to examine the reports to the database, to compare to a list on known, managed access points, and to disable any switch ports being used by rogue access points.