CVE-2023-20275: Cisco ASA Auth Bypass Vulnerability

CVE-2023-20275 Overview

A vulnerability exists in the AnyConnect SSL VPN feature of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software that could allow an authenticated, remote attacker to send packets with another VPN user's source IP address. This IP address spoofing vulnerability enables attackers to impersonate other VPN users, potentially leading to unauthorized access, audit log manipulation, and network reconnaissance within the VPN environment.

Critical Impact

An authenticated attacker can send crafted packets through the VPN tunnel with a spoofed source IP address belonging to another VPN user, enabling IP-based impersonation attacks within the protected network.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software versions 9.8.x through 9.19.x
• Cisco Firepower Threat Defense (FTD) Software versions 6.2.3 through 7.3.x
• Any deployment utilizing the AnyConnect SSL VPN feature

Discovery Timeline

• December 12, 2023 - CVE-2023-20275 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-20275

Vulnerability Analysis

This vulnerability stems from improper validation of the packet's inner source IP address after decryption within the AnyConnect SSL VPN tunnel processing logic. When a VPN user establishes a connection, the appliance assigns an IP address from the configured pool. The vulnerability allows an authenticated attacker to craft packets with a different source IP address—specifically, an IP address assigned to another legitimate VPN user—and transmit these packets through the tunnel without proper verification.

The attack requires the adversary to already have valid VPN credentials and an established SSL VPN session. While this limits the attack surface to authenticated users, it creates significant risk in environments where VPN users should not be able to impersonate each other. The vulnerability is particularly concerning in multi-tenant environments or organizations where different VPN users have different access levels based on their assigned IP addresses.

It is important to note that while the attacker can send packets with a spoofed source IP, they cannot receive return packets addressed to the spoofed IP, as those would be routed to the legitimate user. This limits the attack to one-way communication scenarios.

Root Cause

The root cause is classified under CWE-346 (Origin Validation Error). The ASA and FTD software fails to properly validate that the inner source IP address within decrypted VPN packets matches the IP address legitimately assigned to the sending user's VPN session. This missing validation check allows authenticated attackers to inject packets with arbitrary source IP addresses from the VPN address pool.

Attack Vector

The attack requires the following conditions:

1. The attacker must have valid credentials for the AnyConnect SSL VPN
2. The attacker must establish a legitimate VPN session
3. The attacker crafts packets with a modified inner source IP address matching another VPN user
4. These crafted packets are transmitted through the SSL VPN tunnel
5. The vulnerable appliance decrypts the packets but fails to validate the source IP, forwarding them to the internal network

The vulnerability is exploitable over the network by authenticated users. Since the attacker cannot receive return traffic, the attack is limited to scenarios involving one-way communication such as triggering actions, sending malicious requests, or polluting logs with spoofed source addresses.

Detection Methods for CVE-2023-20275

Indicators of Compromise

• VPN connection logs showing IP address anomalies where the same user appears to be connecting from multiple internal IP addresses simultaneously
• Internal firewall or IDS logs showing traffic from VPN-assigned IP addresses that does not correlate with active user sessions
• Audit logs showing activity attributed to a VPN user's IP address during periods when that user was not connected

Detection Strategies

• Implement deep packet inspection to correlate SSL VPN session state with observed internal traffic source IP addresses
• Deploy network behavior analytics to detect anomalous traffic patterns from VPN-assigned IP address ranges
• Cross-reference VPN session logs with internal network traffic logs to identify source IP address inconsistencies

Monitoring Recommendations

• Enable detailed logging on Cisco ASA/FTD devices for VPN session establishment and termination events
• Configure SIEM correlation rules to alert on VPN traffic where the source IP does not match the authenticated user's assigned IP
• Monitor for unusual one-way traffic patterns from the VPN address pool that may indicate spoofing attempts

How to Mitigate CVE-2023-20275

Immediate Actions Required

• Review the Cisco Security Advisory for patched software versions and upgrade paths
• Inventory all Cisco ASA and FTD deployments running AnyConnect SSL VPN to determine exposure
• Prioritize patching on appliances serving untrusted or multi-tenant VPN environments
• Implement additional network segmentation to limit the impact of potential IP spoofing within the VPN

Patch Information

Cisco has released software updates that address this vulnerability. Organizations should consult the Cisco Security Advisory for the specific fixed software releases applicable to their deployed versions. For Cisco ASA Software, fixed releases are available for the 9.8.x through 9.19.x branches. For Cisco FTD Software, fixed releases are available for the 6.2.3 through 7.3.x branches through the Firepower Management Center (FMC) upgrade process.

Workarounds

• There are no direct workarounds that completely mitigate this vulnerability; software upgrade is required
• Implement strict network access controls that do not solely rely on source IP address for authorization decisions
• Deploy additional authentication mechanisms for sensitive internal resources accessed via VPN
• Enable logging and monitoring as described above to detect potential exploitation attempts

# Verify current ASA software version
show version | include Version

# Check AnyConnect SSL VPN configuration
show running-config webvpn
show running-config tunnel-group

# Review active VPN sessions
show vpn-sessiondb anyconnect