CVE-2025-23406

Executive Summary

CVE-2025-23406 is a denial-of-service (DoS) vulnerability in Cente middleware TCP/IP Network Series. The flaw stems from an out-of-bounds read caused by improper validation of TCP MSS (Maximum Segment Size) option values. An unauthenticated remote attacker can send a specially crafted TCP packet with a malformed MSS option, triggering an out-of-bounds memory read that crashes the affected product. With a CVSS score of 5.3 (Medium), the vulnerability requires no authentication or user interaction, making it exploitable over the network. No public exploit code, proof-of-concept, or patch is currently available. The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the risk increases as the vendor has not released a fix.

Supply Chain & Package Security

This vulnerability affects Cente middleware TCP/IP Network Series, a proprietary embedded networking stack. No open-source package (NPM, PyPI, Go, Ruby) is involved. Organizations using Cente products should verify their supply chain for affected firmware versions and apply vendor-supplied mitigations when available.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Base Score: 5.3 (Medium)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Impact confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – No data exposure.
• Integrity Impact: None (I:N) – No data modification.
• Availability Impact: Low (A:L) – Service crash, but no permanent damage.

Threat Level: Medium. The vulnerability is easy to exploit remotely but only causes a denial of service. No public exploit exists, but the lack of a patch increases risk for exposed systems.

Deep Dive & Mechanics

The vulnerability resides in the TCP option parsing logic of Cente middleware TCP/IP Network Series. When processing incoming TCP SYN packets, the stack parses the MSS (Maximum Segment Size) option to negotiate segment size. The MSS option value is a 16-bit field that should be validated against expected bounds (typically 536 to 1460 bytes for IPv4). Due to improper checking, the stack does not verify that the MSS value falls within a valid range before using it in memory operations.

An attacker crafts a TCP SYN packet with an MSS option containing an extremely large or negative value (e.g., 0xFFFF). The vulnerable code reads this value and uses it as an index or size parameter in a memory copy or buffer access without bounds checking. This results in an out-of-bounds read, reading memory beyond the intended buffer. The read may access unmapped or protected memory, causing a segmentation fault or kernel panic, crashing the affected device or service.

The crash is immediate and does not require any additional payload. The attack is stateless and can be performed from a single crafted packet, making it trivial to execute once the target is identified.

Vulnerable vs. Patched Code

Since no patch is available, the following illustrates the vulnerable pattern and a hypothetical fix.

Vulnerable Code (C pseudocode):

void parse_tcp_options(struct tcp_packet *pkt) {
    uint16_t mss_value;
    // Read MSS option value without validation
    mss_value = ntohs(*(uint16_t *)(pkt->options + 2));
    // Use mss_value in memory operation
    memcpy(some_buffer, pkt->data, mss_value); // Out-of-bounds read if mss_value > buffer size
}

Patched Code (Hypothetical fix):

void parse_tcp_options(struct tcp_packet *pkt) {
    uint16_t mss_value;
    mss_value = ntohs(*(uint16_t *)(pkt->options + 2));
    // Validate MSS value against allowed range
    if (mss_value < MIN_MSS || mss_value > MAX_MSS) {
        // Reject invalid MSS
        return;
    }
    // Safe memory operation
    memcpy(some_buffer, pkt->data, min(mss_value, buffer_size));
}

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• Target running Cente middleware TCP/IP Network Series (e.g., embedded device, IoT gateway).
• Network access to the target (local or remote).
• Tools: scapy (Python), hping3, or custom packet crafting.

Exploitation Steps:

1. Identify target IP and port (e.g., port 80 for HTTP, or any open TCP port).
2. Craft a TCP SYN packet with a malformed MSS option:

   from scapy.all import *
   ip = IP(dst="TARGET_IP")
   tcp = TCP(dport=80, flags="S", options=[('MSS', 0xFFFF)])  # Invalid MSS
   packet = ip/tcp
   send(packet)
   

3. Send the packet to the target.
4. Observe target crash (service unresponsive, device reboot, or log entry).

Expected Outcome: The target crashes or becomes unresponsive due to out-of-bounds read.

Blue Team (Defense & Detection)

Detection:

• Monitor for TCP SYN packets with MSS option values outside normal range (e.g., < 536 or > 1460).
• Use network intrusion detection systems (NIDS) to alert on anomalous MSS values.
• Enable logging on Cente devices to capture crash events.

Mitigation:

• If possible, deploy a network firewall or IPS to drop packets with invalid MSS options.
• Isolate Cente devices on a separate VLAN with strict access controls.
• Monitor vendor channels for a patch and apply immediately when available.

Indicators of Compromise (IOCs)

• Network IOCs:
  • TCP SYN packets with MSS option value set to 0xFFFF or other extreme values (e.g., > 1460 or < 536).
  • Repeated connection attempts to the same target from a single source IP.
• System IOCs:
  • Device crash logs indicating segmentation fault or kernel panic.
  • Unexpected reboots or service restarts on Cente devices.
  • Memory dump artifacts showing out-of-bounds read patterns.

Detection Engineering

YARA Rule

rule CVE_2025_23406_MSS_OutOfBounds {
    meta:
        description = "Detects TCP packets with malformed MSS option values targeting CVE-2025-23406"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-23406"
    strings:
        $mss_high = { 02 04 FF FF }  // MSS option type 2, length 4, value 0xFFFF
        $mss_low  = { 02 04 00 00 }  // MSS option type 2, length 4, value 0x0000
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-23406 - Malformed TCP MSS Option Detection
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects TCP SYN packets with invalid MSS option values indicating exploitation attempt for CVE-2025-23406
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        event_type: alert
        alert.signature_id: 3000001
    condition: selection
falsepositives:
    - Legitimate but rare MSS values used by some applications
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-23406 - Malformed TCP MSS Option"; flow:to_server; tcp.flags:S; content:"|02 04|"; depth:2; byte_test:2,>,1460,2,relative; sid:3000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Network Segmentation: Isolate Cente devices on a separate VLAN with strict firewall rules to limit exposure.
2. Access Control: Restrict inbound TCP connections to only trusted IPs if possible.
3. Monitoring: Deploy the provided Suricata/Sigma rules to detect exploitation attempts.

Long-Term Fix:

• Contact Cente support for a firmware patch. No official patch is available as of this report.
• If the product is end-of-life, consider replacing it with a supported alternative.
• Implement a Web Application Firewall (WAF) or IPS that can drop packets with anomalous MSS values.

Workaround:

• If the device allows configuration, disable TCP MSS option negotiation or set a static MSS value in the network stack.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert