CVE-2023-47537: Fortinet FortiOS Information Disclosure

CVE-2023-47537 Overview

CVE-2023-47537 is an improper certificate validation vulnerability affecting Fortinet FortiOS across multiple versions. This security flaw allows a remote and unauthenticated attacker to perform a Man-in-the-Middle (MitM) attack on the FortiLink communication channel between FortiOS devices and FortiSwitch. The vulnerability stems from inadequate certificate validation mechanisms, which could enable attackers to intercept, modify, or eavesdrop on sensitive network management communications.

Critical Impact

Remote unauthenticated attackers can intercept and potentially manipulate communications between FortiOS and FortiSwitch devices via Man-in-the-Middle attacks, compromising network infrastructure integrity.

Affected Products

• Fortinet FortiOS 7.4.0 through 7.4.1
• Fortinet FortiOS 7.2.0 through 7.2.6
• Fortinet FortiOS 7.0.0 through 7.0.15
• Fortinet FortiOS 6.4 all versions

Discovery Timeline

• 2024-02-15 - CVE-2023-47537 published to NVD
• 2026-01-14 - Last updated in NVD database

Technical Details for CVE-2023-47537

Vulnerability Analysis

This vulnerability falls under CWE-295 (Improper Certificate Validation), a classification for security weaknesses where software fails to properly validate certificates during secure communication establishment. In the context of FortiOS, the FortiLink protocol is used for communication between FortiGate devices and FortiSwitch units, enabling centralized management of switch infrastructure.

The improper certificate validation allows attackers positioned on the network path between FortiOS and FortiSwitch to intercept the FortiLink communication channel without being detected. This attack requires the attacker to be in a network position where they can intercept traffic—a high attack complexity requirement that somewhat mitigates the overall risk.

Successful exploitation could allow attackers to gain limited access to confidential information transmitted over the FortiLink channel and potentially modify data in transit. The attack does not require any privileges or user interaction, making it exploitable by any remote attacker with the appropriate network positioning.

Root Cause

The root cause of this vulnerability is the failure of FortiOS to properly validate SSL/TLS certificates during the establishment of the FortiLink communication channel. When FortiOS connects to FortiSwitch devices, it should verify that the certificate presented by the switch is trusted, valid, and issued to the correct entity. The improper validation allows attackers to present fraudulent certificates that are incorrectly accepted as legitimate.

Certificate validation failures can occur due to several implementation issues, including: not checking certificate revocation status, accepting expired certificates, not validating the certificate chain of trust, or not verifying that the certificate's subject matches the expected identity.

Attack Vector

The attack vector for CVE-2023-47537 is network-based, requiring the attacker to position themselves between the FortiOS device and FortiSwitch in order to intercept and manipulate the FortiLink communications. This is a classic Man-in-the-Middle scenario where:

1. The attacker gains a position on the network segment between FortiOS and FortiSwitch
2. The attacker intercepts the TLS handshake during FortiLink session establishment
3. The attacker presents their own certificate to both endpoints
4. Due to improper certificate validation, FortiOS accepts the attacker's certificate
5. The attacker can now decrypt, view, and potentially modify all FortiLink traffic

This type of attack is commonly executed through ARP spoofing, DNS poisoning, or by compromising network infrastructure devices. For detailed technical information, see the FortiGuard Security Advisory.

Detection Methods for CVE-2023-47537

Indicators of Compromise

• Unexpected certificate warnings or changes in TLS certificates observed during FortiLink communications
• ARP table anomalies indicating potential ARP spoofing on network segments between FortiOS and FortiSwitch
• Unexplained changes in FortiSwitch configurations that were not authorized
• Network traffic analysis showing FortiLink traffic routing through unexpected network paths

Detection Strategies

• Deploy network monitoring tools to detect ARP spoofing and DNS poisoning attempts on management network segments
• Implement certificate pinning verification and alert on any certificate mismatches in FortiLink communications
• Enable comprehensive logging on both FortiOS and FortiSwitch devices to capture authentication and connection events
• Use network intrusion detection systems (IDS) to monitor for Man-in-the-Middle attack patterns

Monitoring Recommendations

• Monitor FortiLink session establishment events and log all certificate exchanges
• Implement network segmentation alerts to detect unauthorized traffic flow changes between FortiOS and FortiSwitch
• Review FortiOS and FortiSwitch logs regularly for failed or suspicious authentication attempts
• Deploy SentinelOne Singularity to monitor for lateral movement and network-based attacks targeting infrastructure devices

How to Mitigate CVE-2023-47537

Immediate Actions Required

• Upgrade FortiOS to the latest patched version as specified in the FortiGuard Security Advisory
• Isolate FortiLink management traffic on dedicated, secured network segments
• Implement network access controls to restrict who can access the network path between FortiOS and FortiSwitch
• Enable additional authentication mechanisms for FortiLink where available

Patch Information

Fortinet has released security updates to address this vulnerability. Organizations should consult the FortiGuard Security Advisory FG-IR-23-301 for specific patched versions and upgrade instructions. The advisory provides detailed guidance on which FortiOS versions contain the fix for this improper certificate validation issue.

For FortiOS 7.4.x, upgrade to versions beyond 7.4.1 as specified in the advisory. For FortiOS 7.2.x, upgrade beyond 7.2.6. For FortiOS 7.0.x, upgrade beyond 7.0.15. Organizations running FortiOS 6.4.x should consult the advisory for migration paths to supported versions.

Workarounds

• Implement strict network segmentation to isolate FortiLink traffic from potentially compromised network segments
• Deploy additional network monitoring and anomaly detection on management network infrastructure
• Use out-of-band management networks for FortiLink communications where possible
• Implement 802.1X port-based authentication on network segments carrying FortiLink traffic to prevent unauthorized device connections

# Example FortiOS configuration for network segmentation
# Isolate FortiLink interface on dedicated VLAN
config system interface
    edit "fortilink"
        set vdom "root"
        set allowaccess ping https ssh
        set type aggregate
        set member "port1" "port2"
        set lldp-reception enable
        set lldp-transmission enable
    next
end

# Configure dedicated management VLAN for FortiLink
config switch-controller managed-switch
    edit "FortiSwitch-Serial"
        config ports
            edit "port1"
                set vlan "management-vlan"
            next
        end
    next
end