CVE-2025-13406

Executive Summary

CVE-2025-13406 is a NULL Pointer Dereference vulnerability in the Softing Industrial Automation GmbH smartLink SW-HT (Webserver modules). This flaw allows an unauthenticated, remote attacker to cause a Denial of Service (DoS) condition on the HTTP server by sending a specially crafted request that triggers a NULL pointer dereference. The vulnerability affects version 1.43 of the smartLink SW-HT product. No patch is currently available, and no public exploit code has been released. The EPSS score is 0.0%, indicating a low probability of exploitation in the wild at this time, but the risk remains significant for industrial environments relying on this device.

CVSS Breakdown & Threat Level

The CVSS vector is not provided in the NVD data. However, based on the description (NULL Pointer Dereference leading to HTTP DoS), a reasonable assessment would be:

• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): None (N)
• Integrity (I): None (N)
• Availability (A): High (H)

Estimated CVSS v3.1 Score: 7.5 (High) – AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Threat Level: High. The vulnerability is remotely exploitable without authentication and directly impacts availability, which is critical in industrial control system (ICS) environments.

Deep Dive & Mechanics

The vulnerability resides in the HTTP server module of the Softing smartLink SW-HT. A NULL Pointer Dereference (CWE-476) occurs when the software attempts to dereference a pointer that has not been initialized or has been set to NULL. In the context of an HTTP server, this can be triggered by sending a malformed HTTP request that causes the server to access a memory location through a NULL pointer.

Mechanism:

1. The smartLink SW-HT HTTP server receives an incoming HTTP request.
2. The server parses the request headers and body.
3. Due to a missing or insufficient validation of a specific field or parameter, the server creates a pointer that remains NULL.
4. When the server later attempts to use this pointer (e.g., to copy data, process a header, or generate a response), it dereferences the NULL pointer.
5. This causes a segmentation fault or access violation, crashing the HTTP server process.
6. The crash results in a Denial of Service, making the web interface and any HTTP-based services unavailable.

The vulnerability is specific to version 1.43. No patch details are available, indicating that the vendor may not have released a fix yet.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison cannot be made. However, based on the vulnerability type, the following hypothetical code snippets illustrate the issue.

Vulnerable Code (C/C++ example):

void handle_request(http_request *req) {
    char *header_value = NULL;
    // Missing validation: if header "X-Custom" is not present, header_value remains NULL
    header_value = get_header(req, "X-Custom");
    // Dereference without NULL check
    process_header_value(header_value);
}

Patched Code (Hypothetical):

void handle_request(http_request *req) {
    char *header_value = NULL;
    header_value = get_header(req, "X-Custom");
    if (header_value == NULL) {
        // Handle missing header gracefully
        log_error("Missing X-Custom header");
        return;
    }
    process_header_value(header_value);
}

The fix would involve adding a NULL check before dereferencing any pointer obtained from request parsing.

Exploitation & Lab Setup

Red Team (Exploitation):

Since no public exploit code is available, exploitation would require reverse engineering the smartLink SW-HT firmware to identify the exact request that triggers the NULL pointer dereference. A likely approach:

1. Fuzzing: Use a tool like boofuzz or wfuzz to send malformed HTTP requests to the device.
2. Targeted Requests: Focus on missing or malformed headers, unusual HTTP methods, or oversized payloads.
3. Trigger: A request missing a specific header (e.g., Host, Content-Length, or a custom header) might cause the crash.

Example fuzzing command (using curl):

# Send a request without a Host header
curl -H "User-Agent: " --http1.0 http://target-ip/

Blue Team (Detection & Hardening):

1. Network Segmentation: Isolate the smartLink SW-HT device on a separate VLAN with strict access controls.
2. Web Application Firewall (WAF): Deploy a WAF to inspect and filter malformed HTTP requests.
3. Monitoring: Monitor device logs for crashes or unexpected restarts.
4. Rate Limiting: Implement rate limiting to reduce the impact of repeated DoS attempts.

Lab Setup:

• Target: Softing smartLink SW-HT (firmware version 1.43)
• Attacker Machine: Kali Linux with tools like nmap, curl, burpsuite, and boofuzz.
• Network: Isolated lab network with mirrored traffic for analysis.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Repeated HTTP requests with missing or malformed headers (e.g., no Host header, invalid Content-Length).
  • Sudden increase in HTTP 400/500 error responses followed by connection timeouts.
  • Device web interface becoming unresponsive.
• System IOCs:
  • Device logs showing segmentation faults or NULL pointer dereference errors.
  • Unexpected restart of the HTTP server process.
  • Memory dumps or crash logs indicating a NULL pointer access.

Detection Engineering

YARA Rule:

rule CVE_2025_13406_NULL_Pointer_Dereference {
    meta:
        description = "Detects potential NULL pointer dereference in Softing smartLink SW-HT HTTP server"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-13406"
    strings:
        $s1 = "NULL pointer" nocase
        $s2 = "segmentation fault" nocase
        $s3 = "access violation" nocase
    condition:
        any of them
}

Sigma Rule:

title: CVE-2025-13406 - NULL Pointer Dereference in Softing smartLink SW-HT
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP requests that may trigger a NULL pointer dereference in Softing smartLink SW-HT version 1.43.
author: Valters IT
date: 2025-04-10
tags:
    - attack.t1499
    - cve.2025.13406
logsource:
    category: webserver
    product: softing_smartlink
detection:
    selection:
        http.response.status_code: 500
        http.response.body|contains: "NULL pointer"
    condition: selection
falsepositives:
    - Legitimate server errors
level: high

Suricata Rule:

alert http any any -> any any (msg:"CVE-2025-13406 - Potential NULL Pointer Dereference in Softing smartLink SW-HT"; flow:to_server; content:"|00|"; http_client_body; depth:1; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the Device: Place the smartLink SW-HT behind a firewall or VPN, limiting access to trusted IPs only.
2. Disable HTTP if Unnecessary: If the web interface is not required, disable the HTTP server module.
3. Monitor Logs: Enable detailed logging and monitor for crash events.

Long-Term Fix:

• Vendor Patch: Await an official firmware update from Softing Industrial Automation GmbH. Contact the vendor for a timeline.
• Workaround: If possible, upgrade to a newer version of the smartLink SW-HT that is not affected (if available).
• Compensating Controls: Deploy a reverse proxy or WAF that can sanitize HTTP requests before they reach the device.

Valters IT Signature:

Valters IT
Senior DevSecOps Engineer
Cyber Threat Intelligence Expert