In October 2022, security researchers uncovered a serious flaw affecting Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) software. This vulnerability, tracked as CVE-2022-20928, poses a unique risk: an attacker with valid VPN credentials can establish a VPN connection as a different user — essentially bypassing user identity controls meant to keep your network safe.

In this breakdown, we’ll explain what CVE-2022-20928 is, how it can be exploited, walk through a simplified exploit scenario (with code snippets), and share how you can protect your systems. All in clear language — no jargon, no nonsense.

What is CVE-2022-20928?

CVE-2022-20928 is a vulnerability in how Cisco’s ASA and FTD software handle authentication and authorization for VPN connections. Normally, when you connect via VPN, the system checks both who you are (authentication) and what you’re allowed to do (authorization). Because of CVE-2022-20928, these checks can become separated, leading to a loophole.

What’s the Impact?

- Any remote attacker with legitimate VPN credentials may connect as if they are another user (with different access rights).

VPN authentication and authorization are not connected properly.

2. An attacker can craft VPN packets during the authentication, confusing the appliance into assigning them the role or privileges of a different user.
3. The attacker must already have valid VPN credentials, but they can “switch” who they appear to be at the last step.

Cisco’s bug arises from a mishandling of attributes during the VPN login phase. If you change user group information at the right stage, you can manipulate the backend into treating you as someone else.

Proof-of-Concept Exploit (Simplified)

While full-blown weaponized exploits aren’t public, researchers described the issue, and it’s possible to show how it could work in a simplified Python example, using the popular OpenConnect tool. Let’s walk step by step:

Have two sets of credentials:

- Attacker: user1/password1

Target user: user2 (has higher privileges)

2. The exploit involves sending a manipulated packet during phase 2 of IKEv2 negotiation (or RADIUS EAP if that’s used). This tricks the ASA/FTD into assigning user2’s privileges to user1’s active connection.

Here's a high-level code snippet showing the _idea_ (not a working exploit!)

import socket

# Placeholder for VPN negotiation steps
def start_vpn_session(my_username, my_password, spoofed_username):
    # Connect to VPN gateway
    vpn_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    vpn_sock.connect(('vpn.example.com', 443))

    # Step 1: Authenticate with real credentials
    send_auth_request(vpn_sock, username=my_username, password=my_password)
    # Receive challenge or proceed

    # Step 2: During group/attribute assignment, spoof identity
    send_spoofed_attributes(vpn_sock, username=spoofed_username)
    # Possibly triggers privilege escalation

    # ...Session established with spoofed user's access...

def send_auth_request(sock, username, password):
    # Simplified packet logic
    # In reality, this would need to conform to IKEv2 or VPN client's protocol
    packet = f"AUTH {username} {password}"
    sock.send(packet.encode())

def send_spoofed_attributes(sock, username):
    # Send like: "Hey, give me user2's privileges!"
    # Construct attribute packet with altered identity/group info
    spoof_packet = f"ATTRIBUTE USERNAME={username}"
    sock.send(spoof_packet.encode())

start_vpn_session('user1', 'password1', 'user2')

Note: The actual exploit is NOT this simple; real attacks would use low-level packet crafting and might leverage tools like Scapy or Python+C bindings to manipulate IKE packets directly.

Who’s at risk?

- Organizations exposing Cisco ASA/FTD VPN services to the internet.

They need to know the username (group identity) of their target.

- The exploit only works if the ASA/FTD software is unpatched.

Attacker logs in as themselves.

2. Attacker injects a malformed/group attribute referencing target user.
3. ASA/FTD creates a VPN session using the attacker's credentials — but with target user’s access.

Cisco Security Advisory:

CVE-2022-20928 - Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software Remote Authentication Bypass Vulnerability

Mitre CVE Listing:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-20928

Detailed Research:

https://seclists.org/fulldisclosure/2022/Nov/16

Monitor for suspicious privilege use in VPN logs.

Always keep network devices up-to-date. This bug shows that even devices supposedly designed for security can have critical identity flaws.

Conclusion

CVE-2022-20928 is a stark reminder: even trusted VPN systems can have dangerous gaps when authentication and authorization are decoupled. Understanding the mechanics of this flaw helps you protect your organization — patch your Cisco ASA/FTD now and review your remote access policies.

For more technical walk-throughs and vulnerability breakdowns, stay tuned, and always question if your network “knows” who its users really are.

Timeline

Published on: 11/15/2022 21:15:00 UTC
Last modified on: 11/21/2022 15:08:00 UTC