CVE-2021-1379 - Critical Vulnerabilities in Cisco IP Phone Discovery Protocols — Exploit Details, Analysis, and Patches

*Published: June 2024*

Introduction

In early 2021, security researchers uncovered a set of dangerous vulnerabilities affecting Cisco IP Phone Series 68xx, 78xx, and 88xx. Registered as CVE-2021-1379 (among others), these flaws could let any nearby attacker send a malicious network packet that might crash your IP phone or even take it over completely.

This post will break down exactly how these vulnerabilities work — in plain English — what an attacker needs to exploit them, and what you need to do to stay protected.

Quick Background

- Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP) help network devices introduce themselves and tell each other what type they are, what features they have, and so on.
- Cisco IP Phones use these protocols to organize themselves within a company network, primarily over Layer 2 (the local broadcast domain).

The Vulnerability

Cisco was found to be missing some critical security checks when their phones processed incoming CDP or LLDP packets. In other words:
The phones trusted packets too much. If a nearby gadget pretended to be another device and sent a specially crafted Discovery packet, the IP phone would process invalid data — either:

Running attacker-provided code (remote code execution, RCE).

No authentication or special access is required — just the ability to send packets in the same Layer 2 broadcast area.

Send Malicious Packet:

The attacker crafts a malformed CDP or LLDP packet. The packet's data fields are specially arranged to trigger a buffer overflow or memory corruption bug inside the phone's protocol handler.

Overwriting Memory:

Because the phone doesn’t properly check the packet contents, the attacker’s packet can overwrite certain areas of the phone’s memory.

Phone crashes and reloads (causing connectivity loss).

- OR, if the attacker is advanced, they inject code that the phone’s CPU executes — taking over the device, monitoring calls, or pivoting into your network.

Example Code Snippet

Below is a Python pseudo-code concept showing how someone could send a malicious CDP frame. _Note: This is educational only!_

import socket

def send_malicious_cdp(target_mac, interface):
    # CDP uses Ethernet Type x200
    ETH_P_CDP = x200

    # Example: Create fake CDP payload (buffer overflow)
    cdp_payload = b'\x02\x01\x04\x00' + b'A' * 1024  # Oversized field

    eth_header = target_mac + b'\xaa\xaa\xaa\xaa\xaa\xaa' + b'\x20\x00' # Target / Source / Type
    frame = eth_header + cdp_payload

    s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW)
    s.bind((interface, ))
    s.send(frame)
    s.close()

# Usage
# send_malicious_cdp(b'\x11\x22\x33\x44\x55\x66', 'eth')

In real-world, this payload would be crafted to cause a buffer overflow at the vulnerable parsing function found in Cisco's code.

Real-World Impact

- Device Downtime: All phones in the same network segment are vulnerable — a single employee (or visitor) with a laptop can knock them out.
- Network Pivoting: Attackers might use phone bugs to jump deeper into the network, attack other equipment, or spy on VoIP calls.

1. Update Your Cisco IP Phones

Cisco has released firmware updates for the 68xx, 78xx, and 88xx series.

For download and update instructions:

Cisco Security Advisory

2. Network Segmentation

Keep untrusted devices off the same VLAN/broadcast domain as your phones.
Use separate switch ports, enforce 802.1X authentication, and review physical access controls.

3. Monitor for Odd Behavior

Watch for unusual reboots, call drops, or appearance/disappearance of phones on your management system.

References

- Official Cisco Post: Cisco Security Advisory SA-cdp-rce-c6TSgBnP
- NIST NVD CVE-2021-1379
- Project Zero: Discovery Protocol Attacks (background on CDP vulnerabilities)

Summary

CVE-2021-1379 proves that even humble office phones can be gateways for attacks. Update your Cisco IP phones — and always suspect new devices on your network.

Still have questions? Leave a comment or reach out for help with Cisco patching.


*(This article is original and written in plain English for quick understanding. Please consult Cisco’s official advisories for the latest updates.)*

Timeline

Published on: 11/18/2024 16:15:09 UTC
Last modified on: 11/18/2024 17:11:17 UTC