CVE-2023-27336

Executive Summary

CVE-2023-27336 is a critical Denial-of-Service (DoS) vulnerability in Softing edgeConnector Siemens OPC UA Server. The flaw resides in the handling of OPC client certificates, where a NULL pointer dereference occurs when processing a specially crafted certificate. An unauthenticated remote attacker can exploit this to crash the server, disrupting industrial control system (ICS) communications. With a CVSS score of 7.5 (High), the vulnerability poses a significant risk to operational technology (OT) environments, especially where the edgeConnector is exposed to untrusted networks. No public exploit code or proof-of-concept is currently available, and no patch has been released by the vendor.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• 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 human involvement.
• Scope: Unchanged (S:U) – impact confined to the vulnerable component.
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H) – complete denial of service.

Threat Level: High. The vulnerability is trivial to exploit (no auth, low complexity) and directly impacts availability of critical OT infrastructure.

Deep Dive & Mechanics

The vulnerability exists in the OPC UA server component of Softing edgeConnector Siemens. When a client initiates a connection, the server processes the client's certificate. Due to improper validation, a NULL pointer dereference occurs if the certificate is malformed or missing expected fields. This leads to a segmentation fault, crashing the server process.

Root Cause: The server code fails to check if a pointer returned from certificate parsing is NULL before dereferencing it. This is a classic CWE-476 (NULL Pointer Dereference) flaw.

Attack Vector: An attacker sends a crafted OPC UA connection request containing a malicious client certificate. The server parses the certificate, encounters a NULL pointer, and crashes.

Impact: The OPC UA server becomes unavailable, halting all OPC UA communications. This can disrupt monitoring, control, and data acquisition in Siemens-based industrial environments.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code pattern is described generically.

Vulnerable Code (C/C++ example):

// Vulnerable: No NULL check before dereference
void process_client_certificate(CERT *cert) {
    char *subject = cert->subject;  // Could be NULL
    printf("Subject: %s\n", subject);  // Dereference without check
}

Patched Code (Hypothetical fix):

// Patched: NULL check before dereference
void process_client_certificate(CERT *cert) {
    if (cert == NULL || cert->subject == NULL) {
        // Handle error gracefully
        return;
    }
    char *subject = cert->subject;
    printf("Subject: %s\n", subject);
}

The fix requires adding NULL pointer validation before any dereference of certificate fields.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Target running Softing edgeConnector Siemens OPC UA Server (default port 4840).
• Network access to the target.

Exploitation Steps:

1. Craft a malicious OPC UA client certificate that triggers a NULL pointer dereference. This can be done by omitting the subject field or providing an invalid ASN.1 structure.
2. Send a connection request to the target OPC UA server using the crafted certificate.
3. Observe the server crash – the service becomes unresponsive.

Example Python script (conceptual):

import socket
import ssl

# Craft a malformed certificate (simplified)
malformed_cert = b"\x30\x00"  # Invalid ASN.1

context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.load_cert_chain(certfile=malformed_cert)  # This would fail in real SSL

# Connect to target
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("target_ip", 4840))
# Send OPC UA hello with malformed cert
# ... (actual OPC UA protocol specifics omitted)

Note: No public exploit exists; this is a conceptual demonstration.

Blue Team (Defense & Detection)

Detection:

• Monitor for repeated connection attempts with invalid or malformed certificates.
• Alert on OPC UA server crashes or process terminations.
• Use network intrusion detection to flag anomalous OPC UA handshakes.

Mitigation:

• Restrict network access to the OPC UA server (firewall, VPN).
• Implement certificate validation at the network perimeter.
• Monitor logs for NULL pointer dereference errors (e.g., segmentation faults).

Indicators of Compromise (IOCs)

• Network:
  • OPC UA connections with malformed or incomplete client certificates.
  • Repeated connection attempts from a single source to port 4840.
• System:
  • OPC UA server process crash logs (segmentation fault, NULL pointer dereference).
  • Unexpected service restarts or downtime.
• File:
  • No specific file-based IOCs.

Detection Engineering

YARA Rule

rule CVE_2023_27336_NULL_PTR_DEREF {
    meta:
        description = "Detects malformed OPC UA client certificates triggering NULL pointer dereference"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2023-27336"
    strings:
        $null_deref = "NULL pointer dereference" nocase
        $segfault = "segmentation fault" nocase
        $opcua_cert = "OPC UA" nocase
    condition:
        any of ($null_deref, $segfault) and $opcua_cert
}

Sigma Rule

title: CVE-2023-27336 - OPC UA Server NULL Pointer Dereference
id: 3a1b2c3d-4e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects OPC UA server crashes due to NULL pointer dereference
logsource:
    product: windows
    service: application
detection:
    selection:
        EventID: 1000  # Application Error
        Provider_Name: 'Application Error'
        Description|contains: 'NULL pointer dereference'
    condition: selection
falsepositives:
    - Unknown
level: high

Suricata Rule

alert tcp any any -> any 4840 (msg:"CVE-2023-27336 - OPC UA Malformed Certificate DoS"; flow:to_server; content:"|30 00|"; depth:2; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the edgeConnector from untrusted networks (firewall, VLAN).
2. Implement network segmentation – restrict OPC UA traffic to trusted IPs only.
3. Monitor for crashes and anomalous connections.

Long-term Fix:

• Await an official patch from Softing. Contact vendor for updates.
• If possible, upgrade to a patched version or alternative OPC UA stack.
• Apply virtual patching via WAF/IPS rules (e.g., block malformed certificates).

Workaround:

• Disable client certificate validation if not required (not recommended for security).

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert