CVE-2022-20849 - Breaking Down the Cisco IOS XR PPPoE Denial-of-Service Vulnerability

In September 2022, Cisco disclosed a critical vulnerability affecting the Broadband Network Gateway PPPoE feature in Cisco IOS XR Software. Tracked as CVE-2022-20849, this flaw gives attackers a way to force PPPoE processes into a continual crash and restart cycle, potentially making network services unavailable. Let’s explain how this vulnerability works, explore an example of the exploit, and discuss how to fix and avoid it.

What Is PPPoE and Why Does It Matter?

PPPoE (Point-to-Point Protocol over Ethernet) is widely used for broadband network connections. Internet providers rely on it to authenticate users and set up internet sessions, particularly for DSL and fiber subscribers. In large ISPs, devices like Cisco’s IOS XR Broadband Network Gateway (BNG) handle thousands or millions of these PPPoE sessions.

If PPPoE breaks, customers can’t connect to the internet. So, if there’s a way for someone to force PPPoE off over and over, that’s a serious risk for both the provider and all its users.

CVE-2022-20849: The Vulnerability Explored

CVE-2022-20849 impacts devices running Cisco IOS XR that have the PPPoE feature turned on. Here’s what Cisco found:

- Attack Vector: An unauthenticated attacker, located on the adjacent network (like a customer with access to the same LAN or VLAN), can send a specially crafted sequence of PPPoE packets.
- Flaw: The PPPoE service does NOT properly handle certain error conditions within these packets. The process can crash when encountering them.
- Effect: The PPPoE process restarts, but if the malicious packets keep coming, it never recovers, locking legitimate clients out—a denial of service (DoS) attack.

Step-by-Step Example: How Could It Be Exploited?

Let’s walk through a simple attack scenario, using generalized pseudo-code (no real harm—just to illustrate):

1. Attacker Controls a CPE Device:

The attacker is a customer or malware running on a customer's device. This gives physical or logical adjacency to the target BNG.

2. Attacker Crafts Malformed PPPoE Packets:

Here is a python-like snippet, using Scapy (a popular packet crafting tool), to send abnormal PPPoE packets on the local network:

from scapy.all import *

# Example: Malformed PADT (PPP Active Discovery Terminate) packet
# PPPoE code for PADT is xa7
malformed_packet = Ether(dst="ff:ff:ff:ff:ff:ff") / \
                   PPPoE(version=1, type=1, code=xa7, sessionid=x000, length=) / \
                   Raw(load=b"BAD_PAYLOAD" * 10)  # Payload that may cause error

sendp(malformed_packet, iface="eth")

Note: In the actual exploit, the packet sequence would be tailored to hit the specific error handling flaw in Cisco's PPPoE implementation, possibly involving unexpected order of discovery packets (PADI, PADO, PADR, PADS, PADT) or malformed PPP payloads.

3. Continuous Crash and Restart:

If the IOS XR device hits the bug in packet processing, the PPPoE process crashes and restarts automatically. If the attack keeps streaming in, the process is trapped in a restart (crash) loop.

4. Denial of Service Effect:

No new PPPoE sessions can be established. All customers connecting through this BNG would lose service until the attack stops and the device is rebooted with a fixed version.

Why Is This Dangerous?

This vulnerability is unauthenticated—the attacker does not need to log in or breach any password.

Mitigation and Patching

Cisco has released patches to fix this issue. There are no workarounds—turning off PPPoE is rarely an option, and filtering these packets in the core is not practical for most ISPs.

Immediate Actions

1. Upgrade to the latest fixed IOS XR version as advised by Cisco.

Monitor PPPoE process stability and network logs for looping crash symptoms.

3. Where possible, strictly limit physical/logical network adjacencies to only trusted zones and users.

Original Cisco advisory:
- Cisco Security Advisory: CVE-2022-20849

The full list of September 2022 advisories:
- Cisco IOS XR Software Security Advisory Bundled Publication – September 2022

Conclusion

CVE-2022-20849 is a classic example of how a seemingly small bug in packet parsing can topple major infrastructure for millions of users, even without any login credentials. While only adjacent attackers can exploit it, the risks are serious enough for every ISP or large network using PPPoE with Cisco IOS XR to patch immediately.

Stay updated, stay vigilant—and don’t assume your edge devices are safe just because they’re on “the customer side.”

Timeline

Published on: 11/15/2024 15:31:20 UTC