Spanning Tree Protocol Attack and Mechanism of STP Attacks

Spanning Tree Protocol Attack

By altering the network topology, Spanning Tree Protocol (STP) attacks aim to take advantage of flaws in the STP, which functions at the OSI Layer 2. The overall objective is to abuse STP behavior, usually by transmitting fake Bridge Protocol Data Units (BPDUs), to affect forwarding pathways, disrupt the network, or intercept communications.

Overview and Mechanism of STP Attacks

By choosing a Root Bridge and establishing a forwarding tree devoid of loops, STP avoids switching loops, which is essential for network stability. However, because STP lacks an authentication mechanism to confirm the source of BPDUs and trusts all devices, it is vulnerable. Any device that supports BPDU, such as a switch, Unix host, or Linux computer, can take part in and modify the spanning tree process.

The switch designated as the Root Bridge is the one with the lowest Bridge ID.

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Common Attack Techniques

Root Bridge Takeover (BPDU Spoofing/Hijack):

• Many STP assaults have this as their primary objective. An attacker creates BPDUs with a superior (extremely low) Bridge ID by plugging a device into an access port and using a program like Ettercap or Yersinia.

• By accepting this fake communication, other switches are compelled to recognize the attacker’s device as the new Root Bridge.

• Traffic is rerouted through the attacker’s device as a result of the network recalculating the topology, which makes packet sniffing and Man-in-the-Middle (MITM) attacks possible.

BPDU Flood / Topology Change Notification (TCN) Flood (Denial of Service–DoS):

• The root bridge receives an excessive volume of BPDUs or TCNs (Topology Change Notification messages) from the attacker.

• This results in unreliable MAC address tables, high CPU usage on switches, and continuous network reconvergence.

• For authorized users, this may result in network congestion, instability, sluggish speeds, outages, or a full denial of service.

Switch Spoofing / Rogue Switch:

• By advertising BPDUs and negotiating a trunk, an attacker manipulates the topology by making a host act like a switch.

• Because the root bridge is no longer accessible, a denial of service attack is initiated if the attacker’s device crashes after successfully identifying itself as the Root Bridge.

STP Information Disclosure:

• Because access ports are not protected by security measures, attackers are able to read STP traffic (BPDU-conf frames).

• When these frames are analyzed using tools like Wireshark, useful information is revealed, including the root ID, flags, and BPDU type. This information can be used to plan additional attack paths.

Real-World Impacts

When STP is exploited, the consequences can be severe:

• Traffic Interception: When traffic is rerouted via the malicious device, man-in-the-middle (MITM) attacks take place.

• Service Disruption: During continuous reconvergence, network services (such as voice or apps) are disrupted.

• Network Failure: Partial network failure may result from excessive CPU consumption on the control plane.

• Loops: If STP is turned off or successfully deceived, broadcast storms or network loops may result.

• Security Bypass: Modified forwarding routes can make segmentation and Access Control Lists (ACLs) less effective.

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Detection Signals

Network administrators should monitor for the following indicators:

• Unexpected modifications to the root bridge are recorded in switch events.
• Repeated STP recalculations or logs displaying numerous TCN/BPDU events, as well as frequent topology changes.
• Abnormalities in the origin of BPDUs, including BPDUs received on access ports or ports that are only supposed to connect to hosts.
• BPDUs that advertise a low Bridge ID or come from unidentified or unreliable devices.
• Increased switch CPU utilization.
• Oscillations of the spanning-tree state or interface flaps.
• An indication of a potential rogue switch is the negotiation of a new trunk on a normally host-only access port.
• Broadcast traffic surges or slower network speeds.

According to the response strategy, if an attack is detected, you should confirm the symptoms, use CAM/MAC tables to identify the offending port or device, isolate the danger by shutting down the port, gather evidence (logs, captures), and look for follow-on activities like ARP/DHCP tampering.

Preventive Controls and Mitigation

Defending against STP attacks relies heavily on applying port-level controls (especially at the access layer) and strict configuration hygiene.

Advanced alternatives to classic STP, such EVPN/VXLAN, MSTP, RSTP (Rapid STP), TRILL, or SPB, are available for multi-tier networks. Nonetheless, protecting the edge ports is still essential even with more recent technologies.

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