Introduction

In today's rapidly evolving networking environment, security is a top priority for both individuals and organizations. As network infrastructures become more complex, vulnerabilities within those systems are increasingly being exploited. One such vulnerability that has come to light in recent years is the VLAN double-tagging attack. A VLAN double-tagging attack can target switches, often taking advantage of certain features or configurations that can make them vulnerable to exploitation.

This attack method, which manipulates the VLAN tags within Ethernet frames, can pose serious risks if left unchecked. Organizations and network engineers must be vigilant in understanding which configurations or features make switches susceptible to VLAN double-tagging attacks. This blog will explore the technical details behind VLAN double-tagging, how certain switch configurations can create vulnerabilities, and how these vulnerabilities can be mitigated.

As we delve deeper into this topic, it is important to focus on how to configure switches securely to prevent such attacks. By following the guidance in this blog, professionals can ensure their networks remain secure and resilient against VLAN double-tagging threats.

What is VLAN Double-Tagging?

VLAN (Virtual Local Area Network) is a network protocol that allows logical grouping of devices across different physical networks. It enables segmentation, management, and enhanced performance of networks. VLAN tags, which are used to identify and separate traffic within a network, are inserted into Ethernet frames. These tags consist of 12 bits and are placed between the Ethernet frame's destination address and its payload.

A VLAN double-tagging attack involves adding two VLAN tags to an Ethernet frame, one from the attacking source and one from the victim's VLAN. When a switch processes the frame, it typically removes the outer VLAN tag and forwards the frame with the remaining inner tag. However, in a double-tagging attack, the outer tag is used to bypass security measures, allowing the attacker to gain access to resources they would not normally be able to reach.

VLAN double-tagging attacks can cause a range of security issues, including unauthorized access to sensitive data, network disruptions, and even the potential to completely compromise the network. For this reason, it is vital to understand how switch configurations may leave networks vulnerable to such attacks.

Key Switch Configurations that Make Devices Vulnerable to VLAN Double-Tagging Attacks

While VLAN double-tagging attacks are a concern, the vulnerabilities associated with them are not necessarily inherent in the VLAN protocol itself. Instead, these vulnerabilities arise due to the misconfiguration of switches or other network devices. Let’s look at the specific features or configurations that can expose a switch to VLAN double-tagging attacks.

1. Port Trunking Misconfiguration

Trunking is a feature on switches that allows the transmission of multiple VLANs across a single physical link. It is achieved by configuring the port to allow traffic for multiple VLANs, and this is especially used in inter-switch communications.

However, improper trunking configurations can expose a network to VLAN double-tagging attacks. Specifically, when a trunk port allows VLAN 1 or other default VLANs on the trunk, it may inadvertently forward double-tagged packets without any additional scrutiny.

In an attack scenario, the attacker can send a double-tagged frame, where the outer VLAN tag corresponds to the VLAN of the trunk port, and the inner VLAN tag corresponds to a VLAN that the attacker wishes to target. The switch will process the outer VLAN tag, then strip it off, forwarding the inner VLAN tag as if it were the original, vulnerable VLAN.

Mitigation:

To prevent this, network engineers should configure trunk ports to block VLAN 1 and other default VLANs from being allowed on trunk links. They should also ensure that all trunks are configured with a strict VLAN range and only allow specific VLANs to be carried on trunk links.

2. Native VLAN Misconfiguration

A native VLAN is used for untagged traffic on a trunk port. By default, the native VLAN is often set to VLAN 1, but in many cases, it is left unchanged during configuration.

This can be a significant security risk in the case of VLAN double-tagging attacks. Since untagged frames (which are implicitly assigned to the native VLAN) are forwarded to the trunk port, attackers can exploit this vulnerability by sending frames with double VLAN tags. By manipulating the native VLAN, attackers can use the outer VLAN tag to get access to a different VLAN, bypassing security measures.

Mitigation:

To protect against this risk, network administrators should configure trunk ports with a dedicated VLAN for the native VLAN that is not the default VLAN. Additionally, using the "native vlan" command to specify an unused VLAN for native traffic will help avoid exploitation of the native VLAN.

3. Unrestricted VLAN Tagging on Access Ports

Access ports are configured to carry traffic for only a single VLAN, typically to a single device or endpoint. When a switch port is improperly configured to allow more than one VLAN, it can potentially carry double-tagged frames that are not properly checked, allowing attackers to gain access to sensitive resources in another VLAN.

A common misconfiguration here is when an access port is mistakenly configured as a trunk or is part of a range of ports where VLAN double-tagging is not explicitly filtered.

Mitigation:

To prevent double-tagging attacks from exploiting access ports, network administrators should ensure that access ports are not configured to accept traffic from multiple VLANs. Configuring access ports to accept only specific VLAN traffic and using features like "BPDU Guard" or "Port Security" can help protect against these types of vulnerabilities.

4. VLAN Database Inconsistencies

In some instances, a switch may experience issues with its VLAN database or VLAN configuration, leading to incorrect mappings of VLAN IDs to ports. When these mappings are inaccurate, attackers can exploit these inconsistencies to launch a double-tagging attack.

The VLAN database is a critical component for managing VLANs, and any corruption or misalignment of this database can lead to incorrect VLAN tagging behavior, creating an opportunity for attackers.

Mitigation:

It is important for network administrators to regularly review and update VLAN configurations, ensuring that VLAN IDs are properly mapped to their corresponding ports. This can be done through routine audits of the VLAN configuration on all switches within the network.

Prevention of VLAN Double-Tagging Attacks: Best Practices

To safeguard against VLAN double-tagging attacks, network administrators must implement robust security practices. Some of the best practices include:

Disable unused VLANs: Any VLANs that are not actively being used should be removed from the network configuration.
VLAN pruning: Ensure that VLAN pruning is enabled on trunk links, limiting the number of VLANs transmitted across trunk ports.
Strict port security: Enforce port security policies, restricting the number of devices that can connect to each port and disabling unused ports.
Utilize access control lists (ACLs): By using ACLs, network administrators can control which VLANs can communicate with each other, reducing the likelihood of an attacker gaining unauthorized access.
Enable spanning tree protocol (STP): To protect against the risk of network loops and to ensure network stability, enable STP on all switches in the network.

Conclusion

VLAN double-tagging attacks exploit specific vulnerabilities in the configuration of network switches, particularly in relation to trunking, native VLAN settings, and the handling of VLAN tags. By understanding the mechanisms behind these attacks and implementing the recommended configurations and best practices, network engineers can significantly reduce the risk of such attacks.

At DumpsQueen, we emphasize the importance of securing network infrastructure, and through this blog, we hope you have gained valuable insights into preventing VLAN double-tagging attacks. Proper network security requires continuous education, configuration management, and regular audits to stay ahead of emerging threats.