CVE-2023-40544

Executive Summary

CVE-2023-40544 is a cleartext-transmission vulnerability affecting the Westermo Lynx family of industrial networking devices. An attacker with access to the network segment where affected devices reside can passively sniff TCP communications and obtain sensitive information exchanged by the device. The vulnerability is rated medium severity (CVSS 5.7) and is a confidentiality-focused weakness typical of legacy industrial equipment, with no public exploit code and no evidence of in-the-wild exploitation reported to CISA as of the data provided.

Technical Analysis

The Westermo CNA classifies this issue with the title "Westermo Lynx Cleartext Transmission of Sensitive Information." Although the NVD entry does not assign a formal CWE identifier, the vulnerability class corresponds to CWE-319 — Cleartext Transmission of Sensitive Information. The NVD description states:

"An attacker with access to the network where the affected devices are located could maliciously actions to obtain, via a sniffer, sensitive information exchanged via TCP communications."

The wording indicates a passive, network-observable weakness rather than an actively exploitable memory-corruption or injection flaw. The affected Lynx devices send sensitive data across the network inside unencrypted TCP segments. This data may include, but is not limited to, authentication credentials, session tokens, configuration content, or operational parameters used by device management or control-plane functions.

Vulnerability mechanism. The core issue is an absence or misconfiguration of transport-layer encryption for TCP-based communications involving the device. Because the data is sent in cleartext, any host on the same Layer 2 broadcast domain or Layer 3 path — or an attacker who can coerce traffic onto an interception point — can recover the information without triggering any active flaw on the device. No authentication bypass, code execution, or malformed-packet handling is involved. The weakness is effectively a design property of the affected communications path.

Attack surface. The attack surface is the network segment on which the Lynx device operates. The attacker requires network access — either direct physical access to a switch port, a foothold on a compromised host within the OT segment, or the ability to place a tap/SPAN monitor upstream. Switched networks do not inherently mitigate the issue: an attacker can use ARP spoofing, ICMP redirects, or port mirroring abuse to direct traffic through an interception point.

Code-level analysis. The provided NVD description and CNA title contain no protocol identifiers, port numbers, function names, or file paths. Per the accuracy constraints of this report, no implementation-level details can be asserted. Insufficient technical detail is available to identify the specific TCP-based service that leaks information; the analysis is therefore limited to the vulnerability class and its observable network behavior.

Exploitation Scenarios

No public PoC or exploit code exists for CVE-2023-40544, so the attack chain below is inferred strictly from the vulnerability class (cleartext transmission of sensitive information) and the NVD description. The attack is passive in nature and does not require sending malicious payloads to the device.

1. Reconnaissance / target identification. The attacker first maps the OT network segment. Westermo Lynx devices are industrial Ethernet switches commonly deployed in rail, utility, and industrial automation environments. The attacker identifies candidate devices via MAC OUI lookups, LLDP/CDP neighbors, SNMP community queries, or banner grabbing on open management ports. Shodan dorks such as "Westermo Lynx" can assist in identifying internet-exposed instances where segmentation has failed.

2. Triggering the flaw. There is no active "trigger" — the vulnerability is the persistent cleartext behavior of the device. The attacker positions a sniffer on the target segment. This may be achieved by compromising an existing host, connecting an unauthorized device to a spare switch port, or using ARP spoofing to redirect TCP flows between the Lynx device and its management station or peers.

3. Payload / execution method. The attacker runs passive capture tooling (e.g., tcpdump, Wireshark, or tcpflow) and records TCP streams to or from the device. Sensitive information — credentials, configuration data, or operational telemetry — is then extracted from the captured plaintext payloads. Because the traffic is unencrypted, no decryption step is required.

4. Post-exploitation impact. Captured credentials or configuration data enable the attacker to authenticate to the device's management interface, alter switching/VLAN configuration, disrupt network availability, or use the device as a pivot point deeper into the ICS/OT environment. If the captured data includes credentials shared with other infrastructure, the blast radius can extend beyond the immediate network segment.

Impact Assessment

The CVSS base score of 5.7 places this vulnerability at the medium end of the severity scale. The primary impacted security property is confidentiality, since the flaw exposes sensitive information to passive observers. Availability and integrity are not directly compromised by the vulnerability itself, although captured credentials could enable later attacks with integrity and availability impact.

Affected systems. Westermo Lynx industrial switch/routers deployed in operational technology networks. The specific firmware versions are not disclosed in the available data, which complicates asset inventory and patch prioritization for defenders.

Business impact. In OT environments, a confidentiality breach of device management data can cascade into full network compromise. An attacker who harvests switch credentials gains the ability to disable monitoring, disrupt critical communications, or stage lateral movement into safety systems. The risk is amplified in sectors such as energy, transportation, and manufacturing where Westermo devices are commonly deployed and where network downtime carries direct safety and financial consequences.

Blast radius. The exposure is confined to network segments where affected devices send unencrypted traffic. However, the sensitive nature of the data — potentially including reusable credentials — can widen the blast radius across IT/OT boundaries if credentials are shared. Notably, the EPSS score (0.00168, 6.5% percentile) and absence from CISA's Known Exploited Vulnerabilities catalog indicate that criminals have not widely weaponized this CVE. This is consistent with a passive sniffing vulnerability that offers no remote code execution or direct device takeover.

Detection Guidance

Detecting passive network sniffing is inherently difficult because the attacker generates no malicious traffic toward the target. The rules below therefore focus on observable side effects: (1) cleartext management traffic that should not exist, (2) endpoint execution of sniffer tools, and (3) plaintext credential markers traversing the network. These are heuristic detections and should be tuned to each environment.

Sigma Rule

title: Cleartext Management Traffic to Westermo Lynx Device
id: 3c1e7f49-2a6b-4c9d-8e7f-5b2a1c0d9e4f
status: experimental
description: >
  Detects TCP connections to standard unencrypted management ports (Telnet, HTTP)
  which may expose sensitive information in cleartext on networks hosting
  Westermo Lynx industrial devices (CVE-2023-40544).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2023-40544
author: valtersit.com
date: 2025/01/01
logsource:
  product: windows
  category: network_connection
detection:
  selection:
    EventID: 3
    Initiated: 'true'
    DestinationPort:
      - 23
      - 80
  filter:
    DestinationPort: 80
    Image|endswith:
      - '\svchost.exe'
      - '\MsMpEng.exe'
  condition: selection and not filter
falsepositives:
  - Legacy management tools that do not support encryption
  - Normal internal HTTP traffic to web servers
level: medium
tags:
  - attack.collection
  - attack.t1040

YARA Rule

rule CVE_2023_40544_Cleartext_Sensitive_Data_PCAP {
    meta:
        author = "valtersit.com"
        description = "Matches packet capture files containing plaintext credential markers consistent with cleartext TCP transmission of sensitive information (CWE-319 / CVE-2023-40544)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2023-40544"
        date = "2025-01-01"
    strings:
        $user = /user(name)?\s*[=:]\s*\S+/ ascii nocase
        $pass = /pass(wd|word)?\s*[=:]\s*\S+/ ascii nocase
        $login = "login:" ascii nocase
        $pwd_prompt = "password:" ascii nocase
        $key  = /private\s+key\s*[=:]/ ascii nocase
    condition:
        // PCAP magic bytes (little-endian 0xa1b2c3d4 or nanosecond variant)
        (uint32(0) == 0xA1B2C3D4 or uint32(0) == 0x4D3C2B1A) and
        (2 of ($user, $pass, $login, $pwd_prompt) or $key)
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"ET TROJAN Possible Cleartext Sensitive Data Exfiltration in TCP Stream (CVE-2023-40544)"; flow:established; content:"password="; nocase; content:"user"; nocase; distance:0; within:64; classtype:policy-violation; sid:202340544; rev:1; metadata:created_at 2025_01_01, updated_at 2025_01_01, attack_target ICS, deployment Perimeter, signature_severity Major, former_category POLICY;)

Note: Because no specific Westermo string signatures or ports are disclosed, this rule alerts on plaintext authentication markers in TCP payloads common to the vulnerability class. Tune to your environment to reduce false positives.

Elastic Detection

EQL rule for endpoint sniffer-tool detection (passive collection of cleartext traffic):

process where event.type == "start" and
  process.name in ("tcpdump", "wireshark", "tshark", "dumpcap", "ettercap", "tcpflow", "ngrep", "tcpick")

EQL rule for network-level cleartext management connections:

network where network.protocol == "tcp" and network.transport == "tcp" and
  (destination.port == 23 or destination.port == 80) and
  not (source.ip is null or destination.ip is null)

Splunk SPL Query

(index=netflow OR index=ids OR index=pan_traffic) dest_port IN (23,80) 
| search (payload="*password=*" OR payload="*user=*" OR app="telnet" OR app="http") 
| stats count earliest(_time) latest(_time) by src_ip dest_ip dest_port app 
| where count >= 3 
| sort - count

Endpoint-based sniffer detection:

(index=windows EventCode=4688) OR (index=linux sourcetype=linux_audit) 
| search "tcpdump" OR "tshark" OR "wireshark" OR "dumpcap" OR "ettercap" OR "tcpflow" 
| stats values(CommandLine) by host, user, _time 
| sort - _time

Wazuh OSSEC Rule

<group name="network,sniffing,collection,">
  <rule id="100345" level="10">
    <if_sid>571</if_sid>
    <match>tcpdump|wireshark|tshark|dumpcap|ettercap|tcpflow|ngrep</match>
    <description>Network sniffing tool executed - possible passive collection of cleartext sensitive data (CVE-2023-40544)</description>
    <group>attack.collection,t1040,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 2/4 — CLAIMED. The CNA states that a fix exists, but the available data does not include a firmware version, advisory ID, or independent confirmation. Treat the "patch" as unverified and prioritize compensating controls.

1. Immediate mitigations (compensating controls). The vendor advises that "Westermo recommends following best practices for hardening, such as restricting access, disable unused services (attack surface reduction), etc., to mitigate the reported vulnerabilities." Concretely:

   • Segment OT networks so that management traffic for Lynx devices is isolated from general-purpose host traffic.
   • Restrict network access to switch management interfaces via ACLs, dedicated management VLANs, and firewall rules.
   • Disable all unused services and physical ports on the device.
   • Replace cleartext management protocols with encrypted alternatives where the device or adjacent management platform supports them.
   • Implement ARP spoofing protections (dynamic ARP inspection, port security) to raise the cost of intercepting traffic.

2. Patch / upgrade steps. Contact Westermo PSIRT (via the vendor support portal) to request the patched firmware version referenced by the CNA. Because the affected version range is not disclosed, inventory all Westermo Lynx devices and confirm with the vendor which firmware revision remediates CVE-2023-40544. Until a version is confirmed, do not assume the CNA "claimed" patch resolves the issue — validate the fix in a staging environment prior to OT deployment.

3. Long-term hardening.

   • Enforce encryption for all management and data-plane communications (SSH/TLS/IPsec) and retire any cleartext services.
   • Deploy network- and endpoint-detection controls tuned to flag plaintext credential material and sniffer tooling (see Detection Guidance).
   • Enable 802.1X and MAC-based port security to prevent unauthorized device attachment to OT switch ports.
   • Establish an OT asset inventory and firmware baseline so future Westermo advisories can be acted on promptly.
   • Conduct periodic red-team assessments of network segmentation and passive-capture exposure.

References

• NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2023-40544
• CVE.org Record: https://www.cve.org/CVERecord?id=CVE-2023-40544
• Westermo Vendor Portal / PSIRT Advisories: https://www.westermo.com
• CISA Known Exploited Vulnerabilities Catalog: CVE-2023-40544 is not listed (no confirmed exploitation in the wild as of the data provided).

No threat-actor campaigns, exploit kits, or in-the-wild exploitation evidence are associated with this CVE in the provided data. The EPSS probability (0.00168) and the absence of public PoC or Nuclei templates indicate that real-world exploitation is unlikely at this time; however, the vulnerability class remains valuable to attackers operating inside OT networks where passive observation yields high-value credentials.