CVE-2023-43492

Executive Summary

CVE-2023-43492 is a stack-based buffer overflow vulnerability residing in the codesys.cgi binary of Weintek cMT3000 series Human-Machine Interface (HMI) web CGI devices. An anonymous, unauthenticated remote attacker can exploit this flaw to hijack control flow and completely bypass login authentication, effectively gaining full administrative access to the device. This vulnerability carries a critical CVSS score of 9.8 and affects a wide range of industrial HMI models, making it a high-priority target for industrial control system (ICS) environments where these devices are exposed to untrusted networks.

Technical Analysis

The vulnerability is classified as a stack-based buffer overflow, which places it within the CWE-121 classification. The flaw resides in the codesys.cgi executable, a Common Gateway Interface (CGI) binary deployed on Weintek cMT3000 series HMI devices. The HMI web interface relies on CGI scripts to handle HTTP requests for various device management and configuration functions.

A stack-based buffer overflow occurs when a program writes more data to a stack-allocated buffer than the buffer is statically allocated to hold. In the context of a CGI binary, the overflow is triggered when specific input parameters included in an HTTP request are copied into a fixed-size stack buffer without proper bounds checking. If an attacker crafts a request containing an overly long value for the vulnerable parameter, the excess data overwrites adjacent stack memory regions, including the saved return address.

The critical implication of this overflow is the ability to hijack control flow. By precisely calculating the offset to the saved return address and supplying a carefully constructed payload, the attacker can redirect execution to arbitrary code. In this case, the NVD description specifically notes that exploitation allows bypassing login authentication — this indicates that the attacker can redirect control flow to bypass the authentication check routine, or inject shellcode that grants immediate access to protected functionality. Given that the vulnerability is in a CGI handler, the injected code would execute with the privileges of the web server process — typically a high-privileged service account on embedded HMI devices.

The attack surface is the HTTP interface exposed by the HMI device's web server. The device processes codesys.cgi requests, which are reachable without authentication, making the vulnerability remotely exploitable by an anonymous attacker. No authentication is required to trigger the overflow; it only requires network access to the device's HTTP/HTTPS port. The NVD description states the attack can be performed by an "anonymous attacker," and CVSS score of 9.8 reflects the unauthenticated, network-based nature of exploitation (AV:N/AC:L/PR:N/UI:N), with high impact across confidentiality, integrity, and availability (C:H/I:H/A:H).

Insufficient technical detail is available in the NVD description to identify the exact vulnerable parameter name, the precise buffer size, or the memory offset required for precise exploitation. No public PoC exploit code has been identified as of the knowledge cutoff; the specific trigger mechanism must be reverse-engineered from the codesys.cgi binary.

Exploitation Scenarios

Since no public PoC is available, the attack chain is inferred from the vulnerability class (stack-based buffer overflow in a CGI endpoint).

Step 1 — Reconnaissance/Target Identification: An attacker would identify exposed Weintek cMT3000 series HMI devices using internet-wide scanning tools (e.g., Shodan, Censys) with queries for Weintek-branded web interfaces or specific device banners. Industrial environments often expose these HMIs directly to the internet or through misconfigured network segments, making them reachable from untrusted networks.

Step 2 — Triggering the Flaw: The attacker sends an HTTP request to the codesys.cgi endpoint containing an abnormally long value in one of the request parameters (query string, POST body field, or cookie). Because the CGI handler copies this input into a fixed-size stack buffer without bounds validation, the oversized input overflows the buffer and corrupts adjacent stack memory.

Step 3 — Payload Execution (Vulnerability-Class Level): A stack-based overflow typically follows one of two exploitation patterns at the class level: (a) Return address overwrite — the attacker precisely positions shellcode or a ROP chain at the location where the saved instruction pointer resides, redirecting execution upon function return to attacker-controlled code; (b) Authentication bypass via control-flow hijack — the attacker redirects execution to a code path that skips the authentication logic entirely, granting immediate access to protected administrative functions. The NVD description explicitly notes that control flow hijacking enables login bypass, suggesting the attacker can alter execution to reach privileged functionality without knowing valid credentials.

Step 4 — Post-Exploitation Impact: Once control flow is hijacked, the attacker achieves unauthenticated administrative access to the HMI device. From this position, the attacker can modify device configurations, alter HMI screens and control logic, exfiltrate sensitive engineering data, pivot into the OT network segment where the HMI resides, or use the device as a foothold for further lateral movement into the industrial control network.

The attack does not require physical access or any pre-existing credentials. The CVSS vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) confirms that the attack is straightforward (no special conditions required), completely unauthenticated, and compromises all three security pillars.

Impact Assessment

Business Impact: Successful exploitation grants an unauthenticated remote attacker complete control over the HMI device, which operates as a critical control interface between operators and industrial processes. Consequences include:

• Confidentiality breach (CVSS C:H): Attacker can read sensitive engineering data, screen configurations, alarm histories, and process parameters being displayed or managed by the HMI.
• Integrity compromise (CVSS I:H): An attacker can modify HMI logic, alter process setpoints, or manipulate displayed data to mislead operators, potentially causing unsafe operating conditions.
• Availability loss (CVSS A:H): The attacker could crash the CGI process, the web server, or the entire HMI device, disrupting operator visibility and control.

Affected Systems: All Weintek cMT3000 series HMI models are affected — cMT-FHD, cMT-HDM, cMT3071, cMT3072, cMT3103, cMT3090, and cMT3151 — across firmware versions prior to those listed in the vendor advisory. These devices are widely deployed in manufacturing, water treatment, energy, and other critical infrastructure sectors.

Blast Radius: The blast radius extends beyond the HMI device itself. In OT environments, HMIs are frequently interconnected with PLCs, RTUs, and other controllers via industrial protocols (Modbus, Ethernet/IP, etc.). A compromised HMI can be leveraged as a pivot point to attack downstream controllers, manipulate process values, or disrupt physical operations. The combination of internet-exposed HMIs and this high-severity flaw elevates risk significantly for asset owners who have not yet applied the vendor's patch.

Threat Context: The EPSS score of 0.00876 (55.6th percentile) suggests moderate exploitation likelihood in the wild, but this figure predates potential weaponization. The CVE is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog, and no in-the-wild exploitation evidence has been publicly documented. However, the vulnerability is in a class (unauthenticated stack overflow in network-facing services) that is highly attractive to both ransomware operators targeting industrial environments and ICS-specific threat actors.

Detection Guidance

Detection of this vulnerability requires both network-level monitoring of web requests targeting codesys.cgi and host-level monitoring of process anomalies on the HMI device itself.

Sigma Rule

title: Weintek cMT3000 codesys.cgi Stack Buffer Overflow Attempt
id: 5d4f3a2c-6c3e-4d4a-8e3f-1a2b3c4d5e6f
status: experimental
description: Detects HTTP requests to Weintek cMT3000 HMI codesys.cgi endpoint with abnormally long parameter values, indicative of stack-based buffer overflow exploitation attempts.
author: Valters IT Security Team
date: 2024/01/15
tags:
    - attack.t1190
    - attack.initial_access
    - cve.2023-43492
logsource:
    category: webserver
    product: apache
    definition: 'Web server access logs from environment hosting Weintek cMT3000 HMI devices or network IDS alerts.'
detection:
    selection:
        cs-uri-path:
            - '*codesys.cgi*'
    filter_normal:
        cs-uri-query|length: < 512
    condition: selection and not filter_normal
    timeframe: 5m
level: high
falsepositives:
    - None expected; legitimate requests to codesys.cgi are typically short configuration queries.

YARA Rule

rule Weintek_CVE_2023_43492_codesys_cgi_overflow {
    meta:
        author = "Valters IT Security"
        description = "Detects HTTP request payloads targeting Weintek cMT3000 codesys.cgi endpoint with characteristics of stack buffer overflow exploitation attempt"
        reference = "CVE-2023-43492"
        date = "2024-01-15"
        cve = "CVE-2023-43492"
    strings:
        $endpoint = "codesys.cgi" ascii wide nocase
        $pattern1 = "A" * 256
        $pattern2 = "\x90\x90\x90" ascii
        $shellcode_hint = "GET " ascii nocase
        $http_hint = "POST " ascii nocase
    condition:
        ($endpoint at 0..1024) and
        (any of ($pattern*)) and
        (filesize > 1500)
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (
    msg:"ET EXPLOIT Weintek cMT3000 codesys.cgi Stack Buffer Overflow Attempt (CVE-2023-43492)";
    flow:to_server,established;
    http.uri;
    content:"codesys.cgi";
    http.uri;
    content:"=";
    isdataat:1000,relative;
    threshold:type limit, track by_src, count 1, seconds 60;
    classtype:attempted-admin;
    sid:2023434;
    rev:1;
    metadata:created_at 2024_01_15, updated_at 2024_01_15, cve CVE_2023_43492;
)

Elastic Detection

sequence by source.ip with maxspan=30s
  [network where event.category == "network" and destination.port == 80
   and network.protocol == "http"
   and http.request.uri contains "codesys.cgi"
   and wildcard(http.request.bytes, ">=1000")
  ]
  [process where event.action == "exec" and process.name == "codesys.cgi"
   and event.outcome == "success"]

Splunk SPL Query

index=web sourcetype=access_combined uri_path="*/codesys.cgi*"
| eval uri_length=len(uri_query)
| where uri_length > 1000
| stats count by src_ip, uri_path, uri_query, _time
| where count > 1
| table _time, src_ip, uri_path, uri_query, uri_length, count

Wazuh OSSEC Rule

<group name="weintek_cmt, cve-2023-43492,">
  <rule id="100434" level="12">
    <decoded_as>json</decoded_as>
    <field name="url">\.*codesys\.cgi\.*</field>
    <regex>(\?|&)[^=&]{1,30}=[^&]{256,}</regex>
    <description>Weintek cMT3000 codesys.cgi - possible stack buffer overflow attempt (CVE-2023-43492)</description>
    <group>exploit,gci,web_attack</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 (CLAIMED) — The vendor (Weintek) has published a security bulletin and upgrade instructions claiming patches for all affected models. This claim is based on a single source (the vendor's own CNA disclosure) and has not yet been independently verified by third-party researchers or firm analysis of the patched binaries.

Immediate Mitigations (before patch is applied):

1. Network segmentation: Isolate cMT3000 HMI devices from untrusted networks. Remove direct internet exposure; place devices behind firewalls restricting access to authorized engineering workstations only.
2. Access control: If the web interface is not required for operational use, disable the web server or restrict HTTP/HTTPS access to specific management VLANs via ACLs.
3. Authentication enforcement: Ensure existing authentication is in place (though the vulnerability allows bypassing it, defense-in-depth still applies).
4. Monitor for anomalous traffic: Deploy the detection rules above (Suricata/Sigma/Splunk) to watch for oversized requests to codesys.cgi as an early warning signal.

Patch/Upgrade Steps:

Follow Weintek's official upgrade instructions (referenced below) to update affected cMT3000 series devices to the following OS versions:

• cMT-FHD: OS version 20210211
• cMT-HDM: OS version 20210205
• cMT3071: OS version 20210219
• cMT3072: OS version 20210219
• cMT3103: OS version 20210219
• cMT3090: OS version 20210219
• cMT3151: OS version 20210219

The upgrade procedure is documented in Weintek's official OS update instructions PDF (see References). After patching, verify the OS version on each device to confirm the update was applied successfully. Note that patch availability has only been claimed by the vendor; asset owners should validate that the update fully addresses the overflow and consider performing independent verification (e.g., re-testing the codesys.cgi endpoint with fuzzing payloads post-patch).

Long-Term Hardening:

1. Inventory and exposure management: Maintain an accurate inventory of all HMI devices in the OT environment and regularly verify that none are directly reachable from the internet or untrusted zones.
2. Patch management program: Integrate Weintek security advisories into the organization's OT patch management cycle. HMI firmware updates should be tested in a qualification environment before production deployment.
3. Network visibility: Deploy passive network monitoring within OT segments to detect anomalous traffic patterns toward HMI devices, including unusual HTTP request sizes and unexpected destinations.
4. Vendor relationship: Actively subscribe to Weintek's security bulletin distribution and track CVEs affecting OT assets via the ICS-CERT/ CISA advisories.
5. Incident response readiness: Given the critical severity of this vulnerability, ensure incident response playbooks cover OT device compromise scenarios, including procedures for safely isolating and forensically capturing HMI devices.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2023-43492
2. Weintek Security Bulletin (TEC23005E): https://dl.weintek.com/public/Document/TEC/TEC23005E_cMT_Web_Security_Update.pdf
3. Weintek cMT Series OS Update Instructions (UM018010E): https://dl.weintek.com/public/Document/UM0/UM018010E_cMT_Series_OS_Update_Instructions_eng.pdf
4. MITRE ATT&CK Technique: https://attack.mitre.org/techniques/T1190/

Threat Actor Activity: As of this report, no known threat actor campaigns, exploit-in-the-wild evidence, or public exploit code have been identified for CVE-2023-43492. The CVE is not listed on the CISA KEV catalog. However, the combination of unauthenticated remote exploitation, critical severity scoring, and prevalence of HMI devices in industrial environments makes this a high-value target for future exploitation. Asset owners are strongly advised to treat this vulnerability as actively exploitable and prioritize patching accordingly.