In this article by Vyacheslav Fadyushin, the author of the book Building a Pentesting Lab for Wireless Networks, we will discuss various WLAN protection mechanisms and the flaws present in them. To be able to protect a wireless network, it is crucial to clearly understand which protection mechanisms exist and which security flaws do they have. This topic will be useful not only for those readers who are new to Wi-Fi security, but also as a refresher for experienced security specialists.

(For more resources related to this topic, see here.)

Hiding SSID

Let's start with one of the common mistakes done by network administrators: relying only on the security by obscurity. In the frames of the current subject, it means using a hidden WLAN SSID (short for service set identification) or simply WLAN name.

Hidden SSID means that a WLAN does not send its SSID in broadcast beacons advertising itself and doesn't respond to broadcast probe requests, thus making itself unavailable in the list of networks on Wi-Fi-enabled devices. It also means that normal users do not see that WLAN in their available networks list.

But the lack of WLAN advertising does not mean that a SSID is never transmitted in the air—it is actually transmitted in plaintext with a lot of packet between access points and devices connected to them regardless of a security type used. Therefore, SSIDs are always available for all the Wi-Fi network interfaces in a range and visible for any attacker using various passive sniffing tools.

MAC filtering

To be honest, MAC filtering cannot be even considered as a security or protection mechanism for a wireless network, but it is still called so in various sources. So let's clarify why we cannot call it a security feature.

Basically, MAC filtering means allowing only those devices that have MAC addresses from a pre-defined list to connect to a WLAN, and not allowing connections from other devices. MAC addresses are transmitted unencrypted in Wi-Fi and are extremely easy for an attacker to intercept without even being noticed (refer to the following screenshot):

An example of wireless traffic sniffing tool easily revealing MAC addresses

Keeping in mind the extreme simplicity of changing a physical address (MAC address) of a network interface, it becomes obvious why MAC filtering should not be treated as a reliable security mechanism.

MAC filtering can be used to support other security mechanisms, but it should not be used as an only security measure for a WLAN.

WEP

Wired equivalent privacy (WEP) was born almost 20 years ago at the same time as the Wi-Fi technology and was integrated as a security mechanism for the IEEE 802.11 standard.

Like it often happens with new technologies, it soon became clear that WEP contains weaknesses by design and cannot provide reliable security for wireless networks. Several attack techniques were developed by security researchers that allowed them to crack a WEP key in a reasonable time and use it to connect to a WLAN or intercept network communications between WLAN and client devices.

Let's briefly review how WEP encryption works and why is it so easy to break.

WEP uses so-called initialization vectors (IV) concatenated with a WLAN's shared key to encrypt transmitted packets. After encrypting a network packet, an IV is added to a packet as it is and sent to a receiving side, for example, an access point. This process is depicted in the following flowchart:

The WEP encryption process

An attacker just needs to collect enough IVs, which is also a trivial task using additional reply attacks to force victims to generate more IVs.

Even worse, there are attack techniques that allow an attacker to penetrate WEP-protected WLANs even without connected clients, which makes those WLANs vulnerable by default.

Additionally, WEP does not have a cryptographic integrity control, which makes it vulnerable not only to attacks on confidentiality.

There are numerous ways how an attacker can abuse a WEP-protected WLAN, for example:

• Decrypt network traffic using passive sniffing and statistical cryptanalysis
• Decrypt network traffic using active attacks (reply attack, for example)
• Traffic injection attacks
• Unauthorized WLAN access

Although WEP was officially superseded by the WPA technology in 2003, it still can be sometimes found in private home networks and even in some corporate networks (mostly belonging to small companies nowadays).

But this security technology has become very rare and will not be used anymore in future, largely due to awareness in corporate networks and because manufacturers no longer activate WEP by default on new devices.

In our humble opinion, device manufacturers should not include WEP support in their new devices to avoid its usage and increase their customers' security.

From the security specialist's point of view, WEP should never be used to protect a WLAN, but it can be used for Wi-Fi security training purposes.

Regardless of a security type in use, shared keys always add the additional security risk; users often tend to share keys, thus increasing the risk of key compromise and reducing accountability for key privacy.

Moreover, the more devices use the same key, the greater amount of traffic becomes suitable for an attacker during cryptanalytic attacks, increasing their performance and chances of success. This risk can be minimized by using personal identifiers (key, certificate) for users and devices.

WPA/WPA2

Due to numerous WEP security flaws, the next generation of Wi-Fi security mechanism became available in 2003: Wi-Fi Protected Access (WPA). It was announced as an intermediate solution until WPA2 is available and contained significant security improvements over WEP.

Those improvements include:

• Stronger encryption
• Cryptographic integrity control: WPA uses an algorithm called Michael instead of CRC used in WEP. This is supposed to prevent altering data packets on the fly and protects from resending sniffed packets.
• Usage of temporary keys: The Temporal Key Integrity Protocol (TKIP) automatically changes encryption keys generated for every packet. This is the major improvement over the static WEP where encryption keys should be entered manually in an AP config. TKIP also operates RC4, but the way how it is used was improved.
• Support of client authentication: The capability to use dedicated authentication servers for user and device authentication made WPA suitable for use in large enterprise networks.

The support of cryptographically strong algorithm Advanced Encryption Standard (AES) was implemented in WPA, but it was not set as mandatory, only optional.

Although WPA was a significant improvement over WEP, it was a temporary solution before WPA2 was released in 2004 and became mandatory for all new Wi-Fi devices.

WPA2 works very similar to WPA and the main differences between WPA and WPA2 is in the algorithms used to provide security:

• AES became the mandatory algorithm for encryption in WPA2 instead of default RC4 in WPA
• TKIP used in WPA was replaced by Counter Cipher Mode with Block Chaining Message Authentication Code Protocol (CCMP)

Because of the very similar workflows, WPA and WPA2 are also vulnerable to the similar or the same attacks and usually called and spelled in one word WPA/WPA2. Both WPA and WPA2 can work in two modes: pre-shared key (PSK) or personal mode and enterprise mode.

Pre-shared key mode

Pre-shared key or personal mode was intended for home and small office use where networks have low complexity. We are more than sure that all our readers have met this mode and that most of you use it at home to connect your laptops, mobile phones, tablets, and so on to home networks.

The general idea of PSK mode is using the same secret key on an access point and on a client device to authenticate the device and establish an encrypted connection for networking. The process of WPA/WPA2 authentication using a PSK consists of four phases and is also called a 4-way handshake. It is depicted in the following diagram:

WPA/WPA2 4-way handshake

The main WPA/WPA2 flaw in PSK mode is the possibility to sniff a whole 4-way handshake and brute-force a security key offline without any interaction with a target WLAN. Generally, the security of a WLAN mostly depends on the complexity of a chosen PSK.

Computing a PMK (short for primary master key) used in 4-way handshakes (refer to the handshake diagram) is a very time-consuming process compared to other computing operations and computing hundreds of thousands of them can take very long. But in case of a short and low complexity PSK in use, a brute-force attack does not take long even on a not-so-powerful computer. If a key is complex and long enough, cracking it can take much longer, but still there are ways to speed up this process:

• Using powerful computers with CUDA (short for Compute Unified Device Architecture), which allows a software to directly communicate with GPUs for computing. As GPUs are natively designed to perform mathematical operations and do it much faster than CPUs, the process of cracking works several times faster with CUDA.
• Using rainbow tables that contain pairs of various PSKs and their corresponding precomputed hashes. They save a lot of time for an attacker because the cracking software just searches for a value from an intercepted 4-way handshake in rainbow tables and returns a key corresponding to the given PMK if there was a match instead of computing PMKs for every possible character combination. Because WLAN SSIDs are used in 4-way handshakes analogous to a cryptographic salt, PMKs for the same key will differ for different SSIDs. This limits the application of rainbow tables to a number of the most popular SSIDs.
• Using cloud computing is another way to speed up the cracking process, but it usually costs additional money. The more computing power can an attacker rent (or get through another ways), the faster goes the process. There are also online cloud-cracking services available in Internet for various cracking purposes including cracking 4-way handshakes.

Furthermore, as with WEP, the more users know a WPA/WPA2 PSK, the greater the risk of compromise—that's why it is also not an option for big complex corporate networks.

WPA/WPA2 PSK mode provides the sufficient level of security for home and small office networks only when a key is long and complex enough and is used with a unique (or at least not popular) WLAN SSID.

Enterprise mode

As it was already mentioned in the previous section, using shared keys itself poses a security risk and in case of WPA/WPA2 highly relies on a key length and complexity. But there are several factors in enterprise networks that should be taken into account when talking about WLAN infrastructure: flexibility, manageability, and accountability.

There are various components that implement those functions in big networks, but in the context of our topic, we are mostly interested in two of them: AAA (short for authentication, authorization, and accounting) servers and wireless controllers.

WPA-Enterprise or 802.1x mode was designed for enterprise networks where a high security level is needed and the use of AAA server is required. In most cases, a RADIUS server is used as an AAA server and the following EAP (Extensible Authentication Protocol) types are supported (and several more, depending on a wireless device) with WPA/WPA2 to perform authentication:

• EAP-TLS
• EAP-TTLS/MSCHAPv2
• PEAPv0/EAP-MSCHAPv2
• PEAPv1/EAP-GTC
• PEAP-TLS
• EAP-FAST

You can find a simplified WPA-Enterprise authentication workflow in the following diagram:

WPA-Enterprise authentication

Depending on an EAP-type configured, WPA-Enterprise can provide various authentication options.

The most popular EAP-type (based on our own experience in numerous pentests) is PEAPv0/MSCHAPv2, which is relatively easily integrated with existing Microsoft Active Directory infrastructures and is relatively easy to manage. But this type of WPA-protection is relatively easy to defeat by stealing and bruteforcing user credentials with a rogue access point.

The most secure EAP-type (at least, when configured and managed correctly) is EAP-TLS, which employs certificate-based authentication for both users and authentication servers. During this type of authentication, clients also check server's identity and a successful attack with a rogue access point becomes possible only if there are errors in configuration or insecurities in certificates maintenance and distribution.

It is recommended to protect enterprise WLANs with WPA-Enterprise in EAP-TLS mode with mutual client and server certificate-based authentication. But this type of security requires additional work and resources.

WPS

Wi-Fi Protected Setup (WPS) is actually not a security mechanism, but a key exchange mechanism which plays an important role in establishing connections between devices and access points. It was developed to make the process of connecting a device to an access point easier, but it turned out to be one of the biggest wholes in modern WLANs if activated.

WPS works with WPA/WPA2-PSK and allows connecting devices to WLANs with one of the following methods:

• PIN: Entering a PIN on a device. A PIN is usually printed on a sticker at the back side of a Wi-Fi access point.
• Push button: Special buttons should be pushed on both an access point and a client device during the connection phase. Buttons on devices can be physical and virtual.
• NFC: A client should bring a device close to an access point to utilize the Near Field Communication technology.
• USB drive: Necessary connection information exchange between an access point and a device is done using a USB drive.

Because WPS PINs are very short and their first and second parts are validated separately, online brute-force attack on a PIN can be done in several hours allowing an attacker to connect to a WLAN.

Furthermore, the possibility of offline PIN cracking was found in 2014 which allows attackers to crack pins in 1 to 30 seconds, but it works only on certain devices.

You should also not forget that a person who is not permitted to connect to a WLAN but who can physically access a Wi-Fi router or access point can also read and use a PIN or connect via push button method.

Getting familiar with the Wi-Fi attack workflow

In our opinion (and we hope you agree with us), planning and building a secure WLAN is not possible without the understanding of various attack methods and their workflow. In this topic, we will give you an overview of how attackers work when they are hacking WLANs.

General Wi-Fi attack methodology

After refreshing our knowledge about wireless threats and Wi-Fi security mechanisms, let's have a look at the attack methodology used by attackers in the real world. Of course, as with all other types of network attack, wireless attack workflows depend on certain situations and targets, but they still align with the following general sequence in almost all cases:

1. The first step is planning. Normally, attackers need to plan what are they going to attack, how can they do it, which tools are necessary for the task, when is the best time and place to attack certain targets, and which configuration templates will be useful so that they can be prepared in advance. White-hat hackers or penetration testers need to set schedules and coordinate project plans with their customers, choose contact persons on a customer side, define project deliverables, and do some other organizational work if demanded. As with every penetration testing project, the better a project was planned (and we can use the word "project" for black-hat hackers' tasks), the more chances of a successful result.
2. The next step is survey. Getting as accurate as possible and as much as possible information about a target is crucial for a successful hack, especially in uncommon network infrastructures. To hack a WLAN or its wireless clients, an attacker would normally collect at least SSIDs or MAC addresses of access points and clients and information about a security type in use. It is also very helpful for an attacker to understand if WPS is enabled on a target access point. All that data allows attackers not only to set proper configs and choose the right options for their tools, but also to choose appropriate attack types and conditions for a certain WLAN or Wi-Fi client. All collected information, especially non-technical (for example, company and department names, brands, or employee names), can also become useful at the cracking phase to build dictionaries for brute-force attacks.
3. Depending on a type of security and attacker's luck, a data collected at the survey phase can even make the active attack phase unnecessary and allow an attacker to proceed directly with the cracking phase. The active attacks phase involves active interaction between an attacker and targets (WLANs and Wi-Fi clients). At this phase, attackers have to create conditions necessary for a chosen attack type and execute it. It includes sending various Wi-Fi management and control frames and installing rogue access points. If an attacker wants to cause a denial of service in a target WLAN as a goal, such attacks are also executed at this phase. Some of active attacks are essential to successfully hack a WLAN, but some of them are intended to just speed up hacking and can be omitted to avoid causing alarm on various wireless intrusion detection/prevention systems (wIDPS), which can possibly be installed in a target network. Thus, the active attacks phase can be called optional.
4. Cracking is another important phase where an attacker cracks 4-way-hadshakes, WEP data, NTLM hashes, and so on, which were intercepted at the previous phases. There are plenty of various free and commercial tools and services including cloud cracking services. In case of success at this phase, an attacker gets the target WLAN's secret(s) and can proceed with connecting to the WLAN, decrypt intercepted traffic, and so on.

The active attacking phase

Let's have a closer look to the most interesting parts of the active attack phase—WPA-PSK and WPA-Enterprise attacks—in the following topics.

WPA-PSK attacks

As both WPA and WPA2 are based on the 4-way handshake, attacking them doesn't differ—an attacker needs to sniff a 4-way handshake in the moment, establishing a connection between an access point and an arbitrary wireless client and bruteforcing a matching PSK. It does not matter whose handshake is intercepted, because all clients use the same PSK for a given target WLAN.

Sometimes, attackers have to wait long until a device connects to a WLAN to intercept a 4-way handshake and of course they would like to speed up the process when possible. For that purpose, they force already connected device to disconnect from an access point sending control frames (deauthentication attack) on behalf of a target access point. When a device receives such a frame, it disconnects from a WLAN and tries to reconnect again if the "automatic reconnect" feature is enabled (it is enabled by default on most devices), thus performing another one 4-way handshake that can be intercepted by an attacker.

Another possibility to hack a WPA-PSK protected network is to crack a WPS PIN if WPS is enabled on a target WLAN.

Enterprise WLAN attacks

Attacking becomes a little bit more complicated if WPA-Enterprise security is in place, but could be executed in several minutes by a properly prepared attacker by imitating a legitimate access point with a RADIUS server and by gathering user credentials for further analysis (cracking).

To settle this attack, an attacker needs to install a rogue access point with a SSID identical to a target WLAN's SSID and set other parameter (like EAP type) similar to the target WLAN to increase chances on success and reduce the probability of the attack to be quickly detected.

Most user Wi-Fi devices choose an access point for a connection to a certain WLAN by a signal strength—they connect to that one which has the strongest signal. That is why an attacker needs to use a powerful Wi-Fi interface for a rogue access point to override signals from legitimate ones and make devices around connect to the rogue access point.

A RADIUS server used during such attacks should have a capability to record authentication data, NTLM hashes, for example.

From a user perspective, being attacked in such way looks just being unable to connect to a WLAN for an unknown reason and could even be not seen if a user is not using a device at the moment and just passing by a rogue access point. It is worth mentioning that classic physical security or wireless IDPS solutions are not always effective in such cases. An attacker or a penetration tester can install a rogue access point outside of the range of a target WLAN. It will allow hacker to attack user devices without the need to get into a physically controlled area (for example, office building), thus making the rogue access point unreachable and invisible for wireless IDPS systems. Such a place could be a bus or train station, parking or a café where a lot of users of a target WLAN go with their Wi-Fi devices.

Unlike WPA-PSK with only one key shared between all WLAN users, the Enterprise mode employs personified credentials for each user whose credentials could be more or less complex depending only on a certain user. That is why it is better to collect as many user credentials and hashes as possible thus increasing the chances of successful cracking.

Summary

In this article, we looked at the security mechanisms that are used to secure access to wireless networks, their typical threads, and common misconfigurations that lead to security breaches and allow attacker to harm corporate and private wireless networks.

The brief attack methodology overview has given us a general understanding of how attackers normally act during wireless attacks and how they bypass common security mechanisms by abusing certain flaws in those mechanisms.

We also saw that the most secure and preferable way to protect a wireless network is to use WPA2-Enterprise security along with a mutual client and server authentication, which we are going to implement in our penetration testing lab.

Resources for Article:

Further resources on this subject:

• Pentesting Using Python [article]
• Advanced Wireless Sniffing [article]
• Wireless and Mobile Hacks [article]