CVE-2023-20275 - Impersonating VPN Users in Cisco ASA and FTD SSL VPNs

---

When it comes to network security, vulnerabilities in critical infrastructure products like Cisco's Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) keep security teams on edge. In particular, CVE-2023-20275 stands out due to its ability to allow attackers to impersonate other VPN users using a clever trick involving packet manipulation.

In this long-read post, I'll explain the basics, show you how the vulnerability works, give you simple code examples, and present the relevant links you need to learn more.

What is CVE-2023-20275?

CVE-2023-20275 is a vulnerability in the SSL VPN feature of Cisco ASA and FTD software, specifically affecting AnyConnect VPN users. Here’s what makes it serious:

- An attacker with VPN access can craft packets that appear to come from another VPN user’s IP address.
- This is caused by the devices not properly checking the real source IP address inside the decrypted data, after the SSL VPN tunnel is established.

The attacker cannot receive reply packets, but they can send data as if it came from somebody else.

This vulnerability impacts network trust, can allow attackers to access internal resources, log actions under someone else's identity, or trigger other network-based attacks.

Attacker logs into VPN: The attacker needs valid VPN credentials to start.

2. Attacker crafts a packet: The attacker builds a packet that, when decrypted by the Cisco device, appears to come from another legitimate VPN user's internal assigned IP (for example, Alice’s IP).
3. Packet passes through device: The vulnerable Cisco device does not validate that the inner source IP matches the real VPN user.
4. Packet is routed into the network: Now the attacker can send data to internal resources, pretending to be Alice (or anyone else's IP).
5. Victim is not aware: The victim never receives data, but network logs and access controls may misattribute the attacker’s actions.

Visual Diagram

[ Attacker (10.2.1.12) ] --VPN--> [ Cisco ASA / FTD ]
       Packet crafted: SRC=10.2.1.22 (Alice)
                    |
                    v
         [ Internal Network Service ]
Service thinks Alice (10.2.1.22) is sending the packet, not the attacker!

Code Example: Crafting Spoofed Packets

While full proof-of-concept (PoC) code involves custom network utilities and specific VPN setups, here’s a conceptual Python snippet using scapy to send a packet with a spoofed source:

from scapy.all import *

# Attacker's real IP: 10.2.1.12 (assigned by VPN)
# Victim's IP: 10.2.1.22 (assigned by VPN)
# Target internal server: 192.168.100.10

# Prepare a packet with a fake source IP (impersonating Alice)
packet = IP(src="10.2.1.22", dst="192.168.100.10") / TCP(dport=80, sport=12345) / b"GET / HTTP/1.1\r\nHost: test\r\n\r\n"

# Send via the VPN tunnel
send(packet, iface="YOUR_VPN_INTERFACE")

Note:

iface must be set to the VPN network adapter.

- The remote server (192.168.100.10) will see the connection as coming from 10.2.1.22, even though it’s sent by 10.2.1.12.

Security Monitoring: Actions taken by an attacker may be falsely attributed to other users.

- Access Control: Internal systems may allow or deny access based on source IP; an attacker can try to bypass controls by impersonating trusted users.

Identity Confusion: Tracing malicious actions becomes harder.

> Limitations:
> The attacker cannot receive responses; this is a one-way attack. It makes data theft and session hijacking difficult, but internal manipulation (such as triggering automation, or exploiting trust relationships) is feasible.

References & Further Reading

- Cisco Security Advisory—CVE-2023-20275
- NIST NVD—CVE-2023-20275
- AnyConnect Secure Mobility Client
- Scapy Project (for packet crafting)

How to Protect Yourself

- Update: Patch your ASA / FTD devices to the latest versions Cisco has provided.

Monitor VPN traffic: Watch for traffic with unexpected source IP addresses.

- Restrict internal access: Use strict access controls that validate user identity, not just source IP.

Final Thoughts

*CVE-2023-20275 is a good reminder of why even authenticated users must be monitored with care—identity is more than just an IP address. Always patch critical infrastructure, segment trusted networks, and check your logs for suspicious behavior.*

Stay safe! If you liked this guide, share it, and keep watching for updates from Cisco and other security vendors.

Timeline

Published on: 12/12/2023 18:15:16 UTC
Last modified on: 12/12/2023 18:58:44 UTC