Greetings programs!

Last week I went down an interesting rabbit hole of MAC address spoofing. I found that while the problem was well defined and easily researched, there were no simple prescriptive recipes for a solution. I thought it might be helpful to share this solution in the hopes it could be useful to others.

I would like to acknowledge the contributions of Marvin Rhoads for technical vetting and proofreading, and Brad Johnson (web page) for climbing down into the rabbit hole with me and lending his considerable expertise.

Introduction

Media Access Control (MAC) Addresses commonly are used to identify endpoints for purposes of access control and authorization on access layer networks that have yet to implement 802.1x (dot1x) device authentication. The problem with this approach is MAC address spoofing is trivial to implement. However, with a defense in depth approach using basic tools and techniques, the risk and impact can be largely mitigated.

To explore the issues, we are going to evaluate the case of an organization that had recently implemented a network access control solution. A network penetration tester easily bypassed their access controls by cloning a mac address from an IP phone to a Linux laptop computer,

Background

The organization had recently implemented Cisco Systems Identity Services Engine (ISE) and had hired a pen-testing firm to evaluate its efficacy in preventing unwanted access to the network. In general, Network Access Control (NAC) implementations take a phased approach to control risk and get immediate value from the tool, and this was the case here.

At the time the penetration test was performed, some of the network was using 802.1x authentication with digital certificates, and some of the network was using MAC Authentication Bypass (MAB) combined with device profiling to determine the correct level of authorization for a connecting device.

Additionally, the Authorization policies weren’t fully implemented, so effectively authorization was a simple ‘yes/no’ result where full access is granted based on a device profile match.

When the Pentester did her work, she grabbed the MAC address off the back of an IP phone in a common area, applied it to her Linux laptop computer, and used the network cable from the IP phone to connect to the network. ISE recognized her computer as the IP phone, and she was granted unrestricted access to the network.

The information security team had the impression that ISE was able to handle a basic access layer attack like MAC address spoofing, and wanted some answers regarding how this happened and what could be done to mitigate it until they were able to roll out dot1x authentication

Analysis

Before getting into the details, we will set the stage by briefly reviewing how the ISE profiler works. Then we will:

1. Dive into what happens when Windows and Ubuntu Linux devices connect to the network with the same MAC address as a test IP phone
2. Review the ISE Anomalous Endpoint Detection (AED) feature and explain why it is ineffective in this case

ISE profiler primer

The ISE profiler has 11 modules that ingest information from a variety of sources to build a database of endpoints and endpoint attributes. The primary key for this data structure is the MAC address of the endpoint. The ISE user interface provides an interface to view the endpoint database and inspect individual endpoints through the Context VisibilityEndpoints Menu.

Endpoint Database and Endpoint Attributes

Figure – endpoint database

By clicking on the MAC address of the endpoint, we can view detailed information from the database

Figure – Summary information about the endpoint

In figure 2, we can conclude that Media Access Control (MAC) Authentication bypass is being employed because the username is the same value as the MAC address. Ultimately this means we are not doing authentication. However, we can use the profiling information to authorize a specific level of access, which we will review later.

In the following image, we can see some attributes that ISE learned, as well as a value called Total Certainty Factor (TCF)

Figure TCF and attributes learned from Device Sensor

So how is this information employed? ISE evaluates the attributes against a set of profiling policies, the policy with the highest TCF is assigned as the endpoint policy for that device. We then use that endpoint policy to decide how much access (if any) to authorize.

ISE profiling policies

Profiler polices assign point values to matching attributes. The highest Total Certainty Factor (TCF) score wins. The policies are arranged in a tree-like structure from coarse to finer-grained. The minimum score at each level of the tree has to be met before the child nodes will be evaluated.

Figure – Profiler policy for a 7965 IP phone

Logical polices

Logical policies are used to group like devices together where a collective policy decision would be made for them. It is the functional equivalent of putting users into groups for granting access to files and folders on a computer.

Figure – Logical profile

Policy Set Authorization (AuthZ) rule

Finally, we use the logical profile in an authorization rule, which then directs the network device to apply the authorization we have defined.

Figure – Authorization rule for IP Phones

So how does mac address spoofing bypass this system? Now that we have set the stage, we can start to talk about that.

Effect of MAC address spoofing on the profiler

Now we will see what happens when we try a Windows and then a Linux Computer using the MAC address of our previously profiled phone

Windows laptop

For our first, we will connect a Windows 10 computer to the switch, with the same mac address as the test phone, and we will see what changes.

Figure – Windows 10 laptop

Reviewing figure 7, There are 4 items highlighted:

1. Authorization profile
2. Total Certainty Factor
3. Dhcp-class-identifier
4. Host-name

The reason why we have an authorization result of DenyIP is that I had configured Anomalous endpoint detection. The dhcp-class-identifier change triggered the Anomalous Endpoint flag on the endpoint to true. I then used this as a condition in a rule to return the DenyIp authorization result. We will dive into the details in the AED section.

There are two main takeaways here:

1. ISE was able to respond to the MAC address spoofing attempt and flagged the endpoint.
2. The attributes from when the phone was profiled are still present, even though we can be reasonably sure the laptop did not send them.

It is the second point that’s important to understand. The absence of a value being sent (that was sent prior) does not equal a change as far as the profiler is concerned. The TCF changed by -30 because the dhcp-class-identifier value is scored in two locations for 10 and 20 points, respectively.

Figure – how attribute matches are scored

It is essential to grasp that except for DHCP, when endpoint attributes accumulate, they remain until there is a change. Usually, this is not too much of a problem….

Once a device has had the AED flag set, the only way to clear the condition is to delete the endpoint. I am going to unplug the laptop, delete the endpoint, and next, we will try the Ubuntu Laptop.

Linux laptop

The endpoint was deleted, the phone plugged back in, and our endpoint has been recreated with the phone accurately identified. Now let us plug in the Linux laptop and take a look.

Figure – Linux laptop

The only informational DHCP attribute Ubuntu sends in its DHCP discovery request is host-name. The result is that the laptop received the IP_phones Authorization Profile. Why didn’t AED fire and block the endpoint?

Because we are not getting anything actionable, there is nothing to trigger ISE. If the residual attributes left from when the phone was plugged in were not persistent, this would trigger a reprofile and we would be able to do something. The reasons the attributes are cached are understandable, but it presents a difficulty here.

The main takeaways here are:

1. In default DHCP configuration, the Ubuntu laptop doesn’t give up any useful information.
2. We just got p0wned.

Anomalous Endpoint Detection (AED)

We saw that AED worked in the case of a Windows machine but not in the case of the Ubuntu Linux machine. Let us take a closer look at AED and see how it works.

According to the Documentation (Cisco, 2018), AED can trigger on a change in one of three things.

1. NAS-Port-Type (i.e. wired or wireless)
2. DHCP-class-identifier
3. Endpoint policy change

If any of these conditions occur with AED enabled, the endpoint will be flagged. It should be noted that not getting the class-id when it was received prior will not trigger AED. And this is why we got p0wned by Linux.

While we are here, let us walk through how to configure AED.

1. Enable it
2. Create an Authorization exception rule that matches on the AnomalousBehaviour endpoint flag
3. Apply an authorization profile that allows the device to receive an address (optional, I’ll explain)

Enabling AED

In the profiler configuration settings, check off the AED boxes.

Figure – Enable AED

Define a DACL that only allows DHCP/Bootp

Reference the DACL in an authorization profile:

In the policy set, create an Exception rule that conditions on AnomalousBehaviour Endpoint Attribute. In this case, I also conditioned on IP phones because I only want the scope to cover the specific assets of interest. Assign the AuthZ profile.

Why the access-accept, and why give out an IP address? The simple answer is visibility. If we give the device an IP address, ISE can ingest the DHCP attributes and we will be able to tell from the ISE UI that a different device has been plugged in even though the endpoint was flagged by AED. If we were to prevent all access, we might not receive any new information until the endpoint was deleted and recreated.

Proposed Solution

Now we have discussed the limitations of AED and the inability to act with the Linux laptop because ISE is not receiving any actionable information. Even if we rolled out 802.1x, there might still be devices that require using MAB, so we cannot sidestep the issue.

There is a straightforward solution: Implement first-hop security and require the client to send a class-identifier in its DHCP requests. If the class-identifier does not match, or there is not one sent, then the device will not obtain an IP Address.

There are three tools we will employ to accomplish this:

1. DHCP Snooping (which you have already enabled on your network, right?)
2. IP source guard (depends on DHCP snooping, prevents user-assigned static addresses)
3. DHCP policy on the DCHP Server

DHCP Snooping

DHCP snooping listens to DHCP traffic and creates a table of IP MAC and Interface bindings. Configuring DHCP snooping is very straightforward. However, consult the documentation and make to understand how to scope it to specific VLANS and trust the uplinks leading towards the DHCP server. (Cisco Security Configuration Guide, no.date.)

Sample configuration:

Binding example:

IP Source Guard

IP source guard will check the DHCP snooping binding table as well as the static binding table for a matching entry when IP packets are received on the switchport. If there no match, the traffic will be dropped. This prevents an attacker from statically applying an IP address.

Ip source guard is a one-line command:

Verification output:

DHCP Policy

To close the loop, we will create a DHCP policy that will only offer addresses to devices that send the correct class-id in the DHCP Discover or Request packets. (Microsoft Docs, 2016). Creating the policy consists of the following steps:

1. Create a vendor Class
2. Create a policy that references the vendor class

Creating the Vendor Class.

In the DHCP management tool, right-click on the IP Address family icon and select define vendor classes. When defining the class, the beginning or the end of the string can be used for wildcard matches, but the characters must be exact. An easy way to get this is to copy it from the endpoint attribute in ISE.

Creating the policy that references the vendor class

1. In the policies folder, right-click and select new policy. This will launch the new policy wizard.
2. On the configure conditions window, click add to add a condition.
3. Select vendor class, operator, the vendor class defined earlier, then wildcard if desired. Select add, and click ok.

1. Use all of the addresses in the scope. NOTE: once done, the policy will reserve 100% of the addresses even if disabled. The policy must be deleted to release the reservation.

1. Click through the rest of the wizard and select finish.

Conclusion

Strong authentication using 802.1x will protect against identity spoofing, but it is a significant undertaking for existing networks, and it takes time to roll out. Additionally, there may be devices that cannot perform 802.1x authentication, but they require network access. Therefore, we have to deploy a defense-in-depth strategy to secure the access network when device authentication is not possible.

Recommendations

• Use 802.1x with certificate authentication whenever possible.
• If you have a NAC such as ISE, employ least privilege authorization to mitigate impact.
• Deploy First Hop Security (FHS), primarily DHCP snooping and IP Source Guard.
• DHCP servers are a good control point to perform policy enforcement. Take advantage.
• Take operations security practices seriously and follow the cycle.

Bibliography

Cisco. (2018, May). Configure Anomalous Endpoint Detection and Enforcement on ISE 2.2. Retrieved from Cisco.com: https://www.cisco.com/c/en/us/support/docs/security/identity-services-engine-22/200973-configure-anomalous-endpoint-detection-a.html

Cisco Security Configuration Guide. (no.date.). Security Configuration Guide, Cisco IOS XE Fuji 16.9.x, Configuring DHCP. Retrieved from Cisco.com: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-9/configuration_guide/sec/b_169_sec_9300_cg/configuring_dhcp.html?bookSearch=true&arrowback=true

Microsoft Docs. (2016, August 31). Scenario: Secure a subnet to a specific set of clients. Retrieved from https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/dn425039(v%3Dws.11)