Layer 2 protocols like CDP, VTP, DTP, and HSRP are vulnerable to attacks if not properly secured. An attacker can use tools like Yersinia to perform reconnaissance on layer 2 protocols to gain information about devices, protocols, and network topology. Common attacks include denial of service attacks, traffic hijacking, and bypassing network restrictions. To prevent attacks, companies should secure switches, use secure trunking configurations, disable unused ports and protocols, and deploy security features like DHCP snooping.

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PENTESTING LAYER 2 PROTOCOLS
By
Temmar Abdessamad
temmar.abdessamad@gmail.com

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Outline
1 Why Worry About Layer 2 Security ?
4 Conclusion
3 Pentesting Layer 2 methodology
2 Playing with Layer 2 protocols
1

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Why Worry About Layer 2 Security ?
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
Physical Links
MAC addresses
IP addresses
Initial Compromise
• Application Stream
POP3, IMAP, IM
SSL, SSH ...
Compromised
• OSI model was built to allow different layers to work without the knowledge of each other
• Unfortunately this means if one layer is hacked, communications are compromised without the other
layers being aware of the problem
• When it comes to networking ... layer 2 can be a very weak link !
• Security is only as strong as the weakest link

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Outline
1 Why Worry About Layer 2 Security ?
4 Conclusion
3 Pentesting Layer 2 methodology
Playing with Layer 2 protocols2

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LAYER 2 : EQUIPMENT, PROTOCOLS & ATTACKS
Categories
CDP (Cisco Discovery Protocol)
VTP (VLAN Trunking Protocol)
DTP (Dynamic Truncking protocol)
HSRP (Hot Standby Router Protocol)
DHCP (Dynamic Host Configuration Protocol)
Protocols
Reconnaissance Attacks : an attackers tries to learn information about the target network
(devices, protocols, topology ...) ;
DoS attacks : the objective is to interrupt or suspend normal network’s services functions
(routing, IP addressing)
Hijacking Attacks : hijack network’s traffic so the attacker will be able to sniff/intercept sensitive
data (MiTM) ;
Bypass Attacks : an attacker try to bypass network restriction in ordre to reach other VLAN ;
Topology Attacks : the main objective is to take control of the target network and alter his
topology.

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Cisco Discovery Protocol (CDP)
 Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol
 Allows Cisco devices to discover each other (IP address, software version, router model, etc)
 How it works : Each network entity broadcasts a CDP packet once per minute
 CDP does not run over IP : it runs directly over the data link layer.
Presentation Vulnerabilities Attacks Mitigation

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Cisco Discovery Protocol (CDP)
 CDP is clear text and unauthenticated
 Information leak :
 Software version and hardware platform
 specific release with a well-known bug that’s ready to be exploited.
 Auxiliary VLAN. An attacker can learn which VLAN is used by IP telephony
Presentation Vulnerabilities Attacks Mitigation
End Users

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Cisco Discovery Protocol (CDP)
 CDP Cache Pollution - CDP table becomes unusable because it contains a lot of false information
Presentation Vulnerabilities Attacks Mitigation
Network A
Switch> sh cdp neighbors
Port Device-ID Port-ID Platform
-------- ---------------- -------------------- ------------
2/16 2651e FastEthernet0/1 cisco 2651
2/21 inet3 FastEthernet0/0 cisco 2651
2/36 r2-7206 Ethernet2/0.1 cisco 7206VXR
2/47 00M55I1 Ethernet0 yersinia
2/47 00N55I1 Ethernet0 yersinia
2/47 00N66I1 Ethernet0 yersinia

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Cisco Discovery Protocol (CDP)
 Only enable CDP on ports to other network devices and uplinks, & disabling it to access ports
 But, CDP must remain enabled on ports to IP phones
 To turn off CDP :
Presentation Vulnerabilities Attacks Mitigation
CatOS> (enable) set cdp disable <mod>/<port> | all
IOS(config)#no cdp run
IOS(config-if)#no cdp enable

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Hot Standby Router Protocol (HSRP)
 It makes a group of adjacent routers appear as a single virtual router.
 Each physical router has its own MAC and IP addresses, but it shares one MAC and one IP address for the
virtual router.
 Routers exchanges HSRP messages to elect the active router. A standby router can becomes active when
 It receives no more HSRP hello messages from the active router
 The active router explicitly wants to become standby
Presentation Vulnerabilities Attacks Mitigation
Hosts with a Default Route to 192.168.0.8
Router A
IP : 192.168.0.7
MAC : From Hardware
Router B
IP : 192.168.0.7
MAC : 000.0C07.AC01
Router C
IP : 192.168.0.7
MAC : From Hardware
HSRP Group

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 HSRP is clear text : HSRP commits a slight information leackage by adverstising all the routers’IP addresses,
authentication Data ...
 There is a possibility for a standby router to immediatly take over the role of the active router :
 Standby routers used their own MAC addresses as source MAC
 The active router uses the virtual MAC addresses
Hot Standby Router Protocol (HSRP)
Presentation Vulnerabilities Attacks Mitigation

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Hot Standby Router Protocol (HSRP)
 DoS attack - an attacker send fake HSRP packet where the priority is set to the maximum value 255 & the
correct value for Authentication Data, Group virtual IP address. All trafic sent to a black hole.
Presentation Vulnerabilities Attacks Mitigation
Hosts with a Default Route to 192.168.0.7
Router A
IP : 192.168.0.7
MAC : From Hardware
Router B
IP : 192.168.0.7
MAC : 000.0C07.AC01
Router C
IP : 192.168.0.7
MAC : From Hardware
HSRP Group
Active Virtual Router
IP : 192.168.0.7
MAC : 000.0C07.AC01
Network A
Network B

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Hot Standby Router Protocol (HSRP)
 Man-In-The-Middle attack – attacker can intercept, listen & modify unprotected data
Presentation Vulnerabilities Attacks Mitigation
Hosts with a Default Route to 192.168.0.8
Router A
IP : 192.168.0.7
MAC : From Hardware
Router B
IP : 192.168.0.7
MAC : 000.0C07.AC01
HSRP Group
Active Virtual Router
IP : 192.168.0.7
MAC : 000.0C07.AC01
Router C
IP : 192.168.0.7
MAC : From Hardware

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Hot Standby Router Protocol (HSRP)
 The ways to mitigate these attacks rely on preventing :
 Forging valid authentication data. If the attacker is unable to present the correct credentials, all
other routers reject his packets.
 Sending HSRP packets. The network infrastructure blocks all HSRP packets except those sent by
authorized HSRP routers.
Presentation Vulnerabilities Attacks Mitigation
How to protect us from these attacks ?
Okey ... But How ?!
Using strong authentication : MD5 Key Chain to authenticate HSRP messages

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Others Attacks
This protocol gives an attacker the ability to add and remove VLAN from the network.
If a switch port has been configured to send and/or listen to DTP advertisements, a hacker can easily coerce the
port into becoming a trunk.
Hijacking Traffic Using DHCP Rogue Servers
DNS Server DHCP Server File Server
ClientAttacker
10.50.72.66
Attacker replies with
Fraudulent information.
This include his own
computer as the gateway,
so all packets from clients
pass through his server.
Hi may I please have IP, Gateway
& DNS @ ?
Client sends DHCP requests
packets for IP, DNS &
gateway addresses
IP : 10.50.72.0/24
GW :10.50.72.66
DNS : 10.50.72.66
VTP (VLAN Trunking Protocol)
DTP (Dynamic Trunking Protocol)
DHCP (Dynamic Host Configuration Protocol )

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Outline
1 Why Worry About Layer 2 Security ?
4 Conclusion
3 Pentesting Layer 2 methodology
2 Playing with Layer 2 protocols

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Pentesting Layer 2 - Methodology
Sniffing
(CDP, VTP, HSRP, DHCP ...)
NoAnalyze CDP packets
& pick your own IP @
Reconnaissance attacks
Yes
CDP packet analysis
HSRP packets
DHCP information
Become an active router
Introduce rogue DHCP server
MiTM
DNS Hijacking
DTP protocol analysis Enable truncking mode
Sniff network traffic of top layersHijacking attacks
DHCP Enabled ?

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Outline
1 Why Worry About Layer 2 Security ?
4 Conclusion
3 Pentesting Layer 2 methodology
2 Playing with Layer 2 protocols

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Conclusion
 According to our last pen test missions, 95 % of these attacks are successful, which prove that layer 2
security is always ignored by companies
 In general we recommend :
 Managing switches in as secure a manner as possible (SSH, permit lists, etc.)
 Using a dedicated VLAN ID for all trunk ports. Be paranoid: do not use VLAN 1 for anything.
 Setting users ports to a non trunking state.
 Deploying port-security whenever possible for user ports.
 Using private VLANS where appropriate to further divide L2 networks.
 Disabling all unused ports and put them in an unused VLAN.
 Disabling CDP whenever possible
 Ensuring DHCP attack prevention (DHCP snooping)

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REFERENCES
LAN Switch Security: What Hackers Know About Your Switches
Eric Vyncke, Christopher Paggen
Yersinia, a framework for layer 2 attacks - Black Hat
Berrueta Andres