The basic concept of man-in-the-middle (MiM) attacks was introduced in Chapter 4. In this section, we discuss the details of how exactly an attacker could establish a man-in-the-middle attack against your wireless network. There are two different methods to establish a man-in-the-middle attack in a wireless network. The first is using management frames and is specific to wireless networking, and the second is ARP spoofing, which is also a problem for wired networks.
Because the management frames lack any integrity protection, establishing a man in the middle with IEEE 802.11 based networks is easy (there's even a hacker tool that will do it for you, described in Chapter 16). MiMs can be established regardless of any protections (WPA, RSN, VPN, and so on) that you might be using but do not necessarily pose a threat if the security protocol is strong. MiM attacks are possible because there are no integrity guarantees provided at the link layer (layer 2), and MAC addresses are easily forged.
The attack begins (assuming that the target STA is already associated to an AP) by the attacker issuing a Deauthentication message to the target STA. This causes the STA to drop its association with its current AP and look to reassociate with another (possibly the old) AP. At the same time, the attacker establishes a malicious AP with the same ESSID and MAC address as an AP within range of the attacker but on a different channel than the valid AP. The target STA associates with the attacker's fake AP because it is denied service at the valid AP by the attacker's forged Deauthentication messages. Once the STA has associated with the bogus AP, the bogus AP immediately associates with the valid AP and begins forwarding all traffic so authentication (if used as in WPA or RSN) completes. This process is shown in Figure 15.6. The attacker now has complete control over the traffic stream between the STA and its valid AP. If encryption is not used, then the attacker can modify packets before forwarding. If encryption is used, packets can be denied or delayed. They can also be modified to assist in other attacks, as we'll see later in this chapter.
Figure 15.6. Example Man-in-the-Middle Attack
ARP spoofing has been a plague on wired networks for some time; and while there are some limited countermeasures available to prevent and identify ARP attacks, an ARP attack can still succeed more often than not. ARP identifies the MAC address for a given IP address. A client or STA wanting to communicate with a specific IP address issues an ARP-Request as a broadcast packet on the LAN asking to learn the MAC address of the given IP address. Because ARP packets do not have any integrity protection, anyone (even attackers with access to LAN) can respond with incorrect or malicious information, effectively poisoning the ARP cache of the requestor. Thus, from that point until the cache entry times out, the client uses an improper MAC address for the given IP address, causing all traffic to go to the attacker rather than the real recipient.
There is an important distinction between using management frames (as described in the previous section) and using ARP spoofing for establishing MiM attacks. With ARP spoofing, the attacker must have access to the link layer, whereas using management frames does not have this requirement. If encryption is being used, the attacker must first break the encryption (or be able to forge packets) before he can perform a successful ARP spoofing attack. With WEP-based networks, breaking the encryption, as we have seen, is a small problem. But with WPA- or RSN-based networks, this is a significant (and hopefully impossible) hurdle.