A new vulnerability, CVE-2023-20109, has been identified in the GET VPN (Group Encrypted Transport VPN) feature of Cisco IOS and IOS XE software. This bug can let an attacker execute random code, take control of affected Cisco devices, or even crash them remotely. In other words: if you’re running GET VPN, this isn't a drill.

In this post, we’ll break down what CVE-2023-20109 is, how an attacker could exploit it, show direct code examples and traffic, and walk you through how to secure your systems.

What Is GET VPN and Why Is It Important?

GET VPN is a Cisco technology designed to securely send data between multiple sites using a single group encryption key. It relies on protocols like the Group Domain of Interpretation (GDOI) and Group-IKEv2, and uses *key servers* (KS) and *group members* (GMs). Think of the key server as the chief of a secured chat group, managing encryption for all the members.

What Is CVE-2023-20109?

CVE-2023-20109 is a vulnerability caused by improper validation of protocol attributes in both GDOI and G-IKEv2 within GET VPN. It’s rated critical because it can be exploited by anyone with admin access to a group member or key server, even remotely.

Here’s a simple breakdown

1. Attacker gains admin access to either a key server or group member (by phishing, leaked creds, or rogue insider).
2. Bad guy points a group member’s configuration to a malicious key server, or simply takes over a deployed key server.

Attacker sends crafted GDOI or G-IKEv2 responses with malicious attribute values.

4. Because the code fails to properly check these attributes, the payload can trigger either a crash or let the attacker run whatever command they want.

Proof-of-Concept (PoC): Simulated Attack

Below is a simplified PoC in Python using Scapy to send a crafted GDOI message. (Never run this on production devices! Test responsibly.) In reality, you’d need deeper knowledge of the protocol.

from scapy.all import *
# FYI: Scapy does not natively support GDOI, so this is conceptual
# Assume attacker is controlling key server

def send_malicious_gdoi(target_ip):
    # Over-simplified, PoC only for illustration
    pkt = IP(dst=target_ip)/UDP(sport=500, dport=848)/Raw(load=b'\xde\xad\xbe\xef' * 100)
    send(pkt)
    print("[*] Sent malformed GDOI to %s" % target_ip)

if __name__ == "__main__":
    victim = "192.168.123.100"  # Example group member IP
    send_malicious_gdoi(victim)

Real attacks would involve crafting the GDOI payload to overflow buffers or corrupt memory according to how Cisco parses attributes.

Send Payload:

- As key server, send crafted GDOI or G-IKEv2 messages with malformed attributes exploiting the validation flaw.

What’s the Impact?

- Full system compromise: Attacker can run any command, intercept all GET VPN traffic, create backdoors, or pivot further into your network.
- Denial-of-Service: Repeated exploitation causes device reboots, breaking encrypted site-to-site connectivity.
- Zero User Interaction Needed: Once the malicious key server is in the config, users don’t need to do anything.

Cisco has released fixes; apply them per your device model

- Cisco Security Advisory: CVE-2023-20109

Official References

- Cisco Security Advisory (CVE-2023-20109)
- NIST NVD - CVE-2023-20109

Conclusion

CVE-2023-20109 is a severe vulnerability in Cisco’s GET VPN feature that can let a remote attacker with access to a key server or group member execute arbitrary code or cause device crashes. The risk is real and can lead to network-wide breaches or outages.

Patch, verify your GET VPN settings, and never point group members at unknown key servers. If you’re responsible for a Cisco VPN, act now—malicious actors move fast, and proof-of-concept code is easy to write with only basic protocol knowledge.

Stay safe, patch often, and always audit your configurations!

© 2024 — Original content by ChatGPT. Share wisely.

Timeline

Published on: 09/27/2023 18:15:10 UTC
Last modified on: 10/05/2023 17:39:30 UTC