CVE-2023-46715: Fortinet FortiOS Auth Bypass Vulnerability

CVE-2023-46715 Overview

An origin validation error vulnerability (CWE-346) affects Fortinet FortiOS IPSec VPN, allowing an authenticated IPSec VPN user with dynamic IP addressing to send packets spoofing the IP address of another user. This vulnerability enables attackers to craft network packets that appear to originate from a different user's IP address, potentially facilitating unauthorized network activities while masking the true source of the traffic.

Critical Impact

Authenticated IPSec VPN users can spoof the IP addresses of other VPN users, potentially enabling unauthorized network access, bypassing IP-based access controls, and masking malicious activity attribution.

Affected Products

• Fortinet FortiOS version 7.4.0 through 7.4.1
• Fortinet FortiOS version 7.2.6 and below
• FortiOS IPSec VPN with dynamic IP addressing enabled

Discovery Timeline

• January 14, 2025 - CVE-2023-46715 published to NVD
• January 31, 2025 - Last updated in NVD database

Technical Details for CVE-2023-46715

Vulnerability Analysis

This vulnerability stems from an origin validation error in the FortiOS IPSec VPN implementation. The flaw specifically affects VPN users configured with dynamic IP addressing, which is a common configuration in enterprise environments where users connect from varying network locations.

The vulnerability allows an authenticated attacker to craft network packets with forged source IP addresses, effectively impersonating another user's network identity. While the attacker can send spoofed packets, the one-way nature of this attack means they cannot receive responses to the spoofed traffic. This limitation somewhat constrains the attack surface but still enables several concerning attack scenarios.

The network-based attack vector requires low complexity to exploit and only requires low privileges (an authenticated VPN session). No user interaction is necessary for exploitation.

Root Cause

The root cause lies in insufficient origin validation within the FortiOS IPSec VPN packet processing logic. When handling outbound traffic from VPN clients with dynamically assigned IP addresses, the system fails to properly validate that the source IP address in the packet header matches the IP address legitimately assigned to the sending VPN user. This allows authenticated users to forge the source address field in their outbound packets, bypassing the expected network isolation between VPN clients.

Attack Vector

The attack requires an authenticated IPSec VPN session with dynamic IP addressing. Once connected, the attacker crafts network packets with a spoofed source IP address belonging to another VPN user. These packets are transmitted through the VPN tunnel and processed by the FortiGate device, which forwards them to their destination without validating that the source IP matches the sender's assigned address.

This enables various attack scenarios including evading IP-based access control lists, bypassing network monitoring that tracks activity by source IP, and potentially framing other VPN users for malicious activity. The attack is limited to send-only operations since response traffic would be routed to the spoofed IP address rather than back to the attacker.

Detection Methods for CVE-2023-46715

Indicators of Compromise

• Unusual traffic patterns from IPSec VPN users where source IP addresses don't match assigned VPN addresses
• Multiple VPN sessions appearing to originate from the same source IP address simultaneously
• Network logs showing traffic from VPN IP addresses that don't correlate with active VPN sessions
• IP address conflicts or anomalies in VPN connection logs

Detection Strategies

• Enable comprehensive logging on FortiGate devices for IPSec VPN traffic and correlate source IPs with assigned addresses
• Implement network traffic analysis to detect packets with source IPs that deviate from expected VPN pool assignments
• Deploy intrusion detection systems (IDS) with rules to flag potential IP spoofing within VPN traffic
• Monitor for discrepancies between VPN authentication logs and observed network traffic patterns

Monitoring Recommendations

• Configure FortiGate devices to log all VPN session establishment and termination events with IP assignments
• Implement SIEM correlation rules to match VPN user activity with their assigned IP addresses
• Enable real-time alerting for traffic anomalies that could indicate IP spoofing attempts
• Regularly audit VPN configurations to ensure dynamic IP addressing pools are properly segmented

How to Mitigate CVE-2023-46715

Immediate Actions Required

• Review the FortiGuard Security Advisory FG-IR-23-407 for the latest guidance and patches
• Upgrade FortiOS to a patched version as recommended by Fortinet
• Audit current IPSec VPN configurations to identify users with dynamic IP addressing
• Implement additional network access controls that don't rely solely on source IP validation

Patch Information

Fortinet has released security updates to address this vulnerability. Organizations running affected versions of FortiOS (7.4.0 through 7.4.1 or 7.2.6 and below) should consult the FortiGuard Security Advisory for specific patch versions and upgrade instructions. Apply updates during scheduled maintenance windows following your organization's change management procedures.

Workarounds

• Consider using static IP addressing for VPN users where operationally feasible to reduce the attack surface
• Implement additional egress filtering to validate that VPN traffic source addresses match assigned IP pools
• Enable strict source IP validation policies on network segments accessible via VPN
• Segment VPN users into separate network zones with dedicated access control lists

# Example: Review current IPSec VPN phase1 interface configuration
config vpn ipsec phase1-interface
    show
end

# Verify VPN user IP assignment settings
config user local
    show
end

# Review firewall policies for VPN traffic
config firewall policy
    show | grep -i vpn
end