What Is Virtual Local Area Network Hopping (VLAN Hopping)?
VLAN hopping is a network security vulnerability where an attacker gains unauthorized access to a VLAN and uses it to break into other VLANs within a network. VLANs segment network traffic, improving security and performance by isolating different types of traffic. VLAN hopping allows attackers to bypass this segmentation, potentially gaining access to sensitive data and resources that should be isolated.
This type of attack exploits weaknesses in the way network switches handle VLAN tagging, enabling the attacker to traverse VLAN boundaries. As a result, the attacker can intercept or inject traffic into VLANs they should not have access to, posing significant security risks and compromising the integrity of the network.
This is part of a series of articles about cyber attack
What Are the Risks of VLAN Hopping?
VLAN hopping poses significant security threats as it allows attackers to bypass network segmentation. This can lead to unauthorized access to sensitive data systems normally isolated on different network segments. Once the segmentation is compromised, this escalates the risk of widespread network disruption and potential data breaches.
The risks extend beyond data theft to include potential impacts on service availability and network performance. By using VLAN hopping, malicious users can overload networks, introduce malware, or manipulate data traffic. This undermines the integrity and confidentiality of the network.
Types of VLAN Hopping Attacks
There are two main ways that attackers exploit VLAN hopping vulnerabilities.
Switch Spoofing Attacks
In a switch spoofing attack, attackers exploit the Dynamic Trunking Protocol (DTP) to negotiate a trunk link with a switch. DTP is a Cisco proprietary protocol that automates the creation of trunk links between switches.
Attackers can configure their device to appear as a switch and trick the network switch into establishing a trunk connection. Once this trunk link is established, the attackers gain access to multiple VLANs, bypassing the intended network segmentation.
After gaining access to a trunk link, an attacker can inject malicious traffic or eavesdrop on communications across various VLANs, leading to unauthorized access to sensitive data and potential disruption of network services. This is particularly dangerous because it allows the attacker to move laterally across the network.
Double Tagging Attacks
In a double tagging attack, the attacker sends packets with two VLAN tags. The outer tag corresponds to the VLAN of the attacker’s access port, and the inner tag corresponds to the target VLAN. When the packet reaches the first switch, it removes the outer tag and forwards the packet based on the inner tag, believing it has reached the appropriate VLAN. As a result, the packet is sent to the target VLAN, bypassing the intended network segmentation.
This attack exploits the way network switches handle VLAN tags, including the process of tag stripping. The inner tag remains intact, allowing the packet to traverse VLAN boundaries undetected.
Double tagging attacks are stealthy because the packet appears legitimate to network devices, making detection challenging. This method enables the attacker to inject malicious packets into a different VLAN without requiring direct access.
6 Ways to Prevent VLAN Hopping Attacks
Here are some of the measures that organizations can use to defend themselves against VLAN hopping attacks.
1. Disable DTP and Use Static Trunking
To prevent switch spoofing attacks, it’s crucial to disable Dynamic Trunking Protocol (DTP) on all switch ports that are not explicitly designated as trunk ports. DTP automatically negotiates trunk links between switches, making it a potential entry point for attackers.
Disabling DTP ensures that trunk links can only be established manually, blocking unauthorized devices from creating trunk links. Additionally, enabling port security features and limiting the number of MAC addresses that a port can learn helps to prevent unauthorized access.
Trunk ports should be configured statically using commands such as switchport mode trunk for trunk ports and switchport mode access, along with switchport nonegotiate for access ports. This minimizes the risk of an attacker successfully mimicking a switch to gain access to multiple VLANs.
2. Implement Layer 2 Mapping
Layer 2 mapping involves creating a visual representation of the network’s Layer 2 infrastructure, detailing VLAN assignments, switch port configurations, and interconnections between switches. This map enables network administrators to understand the existing VLAN topology and identify any misconfigurations or potential vulnerabilities.
Regularly updating this map ensures that changes in the network are accurately reflected, which helps in detecting anomalies and potential security gaps. Conducting periodic audits based on this map helps uncover discrepancies and areas of weakness that could be exploited by VLAN hopping attacks, allowing for proactive remediation.
3. Maintain a VLAN Device Inventory
A detailed inventory of devices connected to each VLAN enables effective network management and security. This inventory should include key information such as device types, IP addresses, MAC addresses, and the VLANs to which they are assigned. Regularly verifying this inventory against actual network traffic helps to identify unauthorized devices or unexpected changes in the network.
Discrepancies between the inventory and observed network activity can signal potential VLAN hopping attempts. Automated inventory management tools can simplify the process of keeping the inventory current and accurate, providing alerts for any inconsistencies and enabling rapid response to security threats.
4. Implement 802.1X Authentication
802.1X authentication enhances network security by requiring devices to authenticate before they can access the network. This protocol uses an authentication server to verify the credentials of devices attempting to connect. Enforcing 802.1X authentication on all access ports ensures that only authorized devices can communicate on the network.
In addition to preventing unauthorized access, this mitigates the risk of VLAN hopping attacks by ensuring that every device is authenticated before it can transmit data. Configuring 802.1X involves setting up the authentication server, configuring switch ports to use 802.1X, and ensuring that end devices support the protocol.
5. Monitor Network Traffic
Regularly monitoring network traffic is critical for detecting and preventing VLAN hopping attacks. Network monitoring tools and intrusion detection systems (IDS) can be used to continuously analyze traffic patterns and identify anomalies. It’s important to pay close attention to unusual traffic between VLANs, unexpected spikes in traffic volume, or any other irregularities that could indicate malicious activity.
By setting up alerts for suspicious activity, network administrators can quickly detect potential VLAN hopping attempts and respond promptly. Continuous monitoring also helps in identifying vulnerabilities in the network configuration and improving the organization’s overall security posture. This should be combined with logging and analysis mechanisms applied to network traffic.
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