CVE-2023-30306

Executive Summary

CVE-2023-30306 describes a vulnerability in Mercury x30g and Mercury YR1800XG routers that allows an attacker to hijack TCP sessions. This can lead to a denial of service (DoS) condition by disrupting legitimate network connections. The vulnerability stems from insufficient validation of TCP session parameters, enabling an attacker to inject malicious packets and take over active sessions. No CVSS score, EPSS score, or public proof-of-concept (PoC) code has been released, and the vendor has not provided a patch. This report provides a technical analysis, detection strategies, and mitigation recommendations.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects embedded firmware on Mercury routers, which are consumer-grade networking devices. The supply chain risk is limited to the device firmware itself, which is not distributed through package registries.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating that no official CVSS vector has been assigned. Based on the description, a reasonable estimate would be:

• Attack Vector (AV): Network (AV:N) – The attack can be launched remotely.
• Attack Complexity (AC): Low (AC:L) – No special conditions are required.
• Privileges Required (PR): None (PR:N) – No authentication needed.
• User Interaction (UI): None (UI:N) – No user action required.
• Scope (S): Unchanged (S:U) – The vulnerable component is the router itself.
• Confidentiality (C): None (C:N) – No data exfiltration.
• Integrity (I): Low (I:L) – Session hijacking can alter traffic.
• Availability (A): High (A:H) – DoS impact is significant.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H Estimated Score: 7.5 (High)

Threat Level: High – The vulnerability is remotely exploitable without authentication and can cause a denial of service.

Deep Dive & Mechanics

The vulnerability lies in the TCP/IP stack implementation of the Mercury x30g and YR1800XG routers. TCP session hijacking typically involves an attacker sending spoofed TCP packets with a forged source IP address and sequence number to take over an existing connection. In this case, the router fails to properly validate the TCP sequence numbers or the authenticity of the packets, allowing an attacker to inject malicious data or reset the connection.

Attack Flow:

1. Reconnaissance: The attacker identifies a target router and monitors active TCP connections (e.g., via ARP spoofing or passive sniffing).
2. Session Hijacking: The attacker sends crafted TCP packets with a spoofed source IP (matching the legitimate client) and a guessed or observed sequence number.
3. Denial of Service: By injecting a RST (reset) packet or malicious data, the attacker terminates the legitimate session, causing a DoS for the client.

The root cause is likely a lack of proper TCP sequence number randomization or insufficient validation of incoming packets against the expected connection state.

Vulnerable vs. Patched Code

No patched code is available from the vendor. The vulnerable code is in the router's firmware, which is not publicly accessible. However, a typical vulnerable implementation might look like:

Vulnerable (pseudo-code):

void process_tcp_packet(packet *pkt) {
    // No validation of sequence number or source IP
    if (pkt->flags & RST) {
        reset_connection(pkt->src_ip, pkt->src_port);
    }
}

Patched (pseudo-code):

void process_tcp_packet(packet *pkt) {
    // Validate sequence number against expected window
    if (pkt->flags & RST) {
        if (is_sequence_valid(pkt->seq_num, pkt->src_ip, pkt->src_port)) {
            reset_connection(pkt->src_ip, pkt->src_port);
        } else {
            drop_packet(pkt);
        }
    }
}

The patch would involve implementing proper TCP sequence number validation and possibly using RFC 5961 (improving TCP's robustness against blind in-window attacks).

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: Mercury x30g or YR1800XG router with default firmware.
• Attacker: Kali Linux machine on the same network.
• Tools: scapy, nmap, tcpdump.

Exploitation Steps:

1. Identify a target connection: Use tcpdump to capture traffic and identify an active TCP session (e.g., a client browsing the web).
2. Craft a RST packet: Use scapy to send a spoofed TCP RST packet with the correct source IP and port, and a guessed sequence number.

   from scapy.all import *
   ip = IP(src="192.168.1.100", dst="192.168.1.1")
   tcp = TCP(sport=12345, dport=80, flags="R", seq=1000)
   send(ip/tcp)
   

3. Observe DoS: The client's connection to the router will be terminated.

Note: Without a public PoC, this is a theoretical approach. Sequence number guessing may require multiple attempts.

Blue Team (Defensive)

Detection:

• Monitor for unusual TCP RST packets with spoofed source IPs.
• Use network intrusion detection systems (NIDS) to flag anomalous TCP resets.

Mitigation:

• Implement TCP sequence number randomization (RFC 6528).
• Use IPsec or VPNs to encrypt traffic and prevent session hijacking.
• Apply vendor firmware updates if available.

Indicators of Compromise (IOCs)

• Network Traffic:

  • High volume of TCP RST packets from a single source.
  • TCP packets with invalid sequence numbers (outside expected window).
  • Spoofed source IP addresses in TCP packets.

• Logs:

  • Router logs showing unexpected connection resets.
  • Firewall logs indicating dropped packets with invalid TCP flags.

• File Hashes:

  • No specific file hashes available for this vulnerability.

Detection Engineering

YARA Rule

rule CVE_2023_30306_TCP_Hijack {
    meta:
        description = "Detects potential TCP session hijacking attempts targeting Mercury routers"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-30306"
    strings:
        $tcp_rst = { 00 00 00 00 00 00 00 00 00 00 00 00 08 00 45 00 } // Example pattern
    condition:
        $tcp_rst
}

Sigma Rule

title: TCP Session Hijacking Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects TCP RST packets with spoofed source IPs
author: Valters IT
logsource:
    category: network
    product: suricata
detection:
    selection:
        event_type: alert
        alert.signature_id: 2100001
    condition: selection
falsepositives:
    - Legitimate TCP resets from network devices
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2023-30306 - Potential TCP Session Hijack"; flow:to_server; flags:R; threshold: type both, track by_src, count 10, seconds 60; sid:2100001; rev:1;)

Remediation & Mitigation

1. Vendor Patch: No patch is available. Contact Mercury support for updates.
2. Workaround: Disable remote management and restrict access to the router's web interface.
3. Network Segmentation: Place the router behind a firewall that validates TCP sequence numbers.
4. Encryption: Use VPNs (e.g., WireGuard, OpenVPN) to encrypt all traffic, making session hijacking ineffective.
5. Monitoring: Deploy NIDS (e.g., Suricata) with rules to detect anomalous TCP resets.
6. Firmware Update: Check Mercury's website periodically for firmware updates.

Valters IT