CVE-2026-76071

Executive Summary

CVE-2026-76071 is a critical, unauthenticated remote code execution vulnerability affecting the Netis NC63 router firmware through version V3.0.0.3327. The flaw resides in the netis.cgi CGI binary, where an oversized destHost parameter supplied to the ipFilterList=mod action triggers a stack-based buffer overflow before authentication is verified. Because the Boa web server executes CGI programs with root privileges, successful exploitation yields full administrative control of the device, including the ability to intercept network traffic, pivot into internal networks, and deploy persistent malware. The vulnerability has a CVSS score of 9.8, indicating critical severity with network-based attack vector, low attack complexity, no privilege requirements, and no user interaction.

Technical Analysis

The vulnerability is a classic stack-based buffer overflow (CWE-121) that occurs within the netis.cgi interface of the Netis NC63 router. The root cause lies in the improper handling of the destHost parameter when the ipFilterList=mod action is invoked. The NVD description indicates that the overflow is triggered through widthless sscanf conversions, which copy user-supplied input into fixed-size stack buffers without adequate bounds checking. The sscanf function with widthless %s format specifiers does not limit the number of characters written, allowing input exceeding the destination buffer size to overwrite adjacent stack memory.

A particularly significant aspect of this vulnerability is that the overflow occurs before authentication is verified. This means that an unauthenticated remote attacker can trigger the overflow without possessing valid credentials, eliminating the need for prior access to the device's administrative interface. The vulnerable code path is reachable directly from the web server's CGI interface, making the attack surface the network-facing HTTP endpoint.

The attacker-controlled destHost parameter is passed to the vulnerable sscanf call, which copies the input into a fixed-size stack buffer. By supplying a sufficiently oversized value, the attacker overwrites the saved return address and other critical stack state, enabling control of the program's execution flow. Since the Boa web server executes the CGI environment with root privileges, the attacker's controlled instruction pointer redirect leads to arbitrary code execution with highest possible privileges on the device.

The severity of this issue is amplified by the fact that the affected device — the Netis NC63 — is a network router, placing it at the network perimeter. Routers of this class are typically exposed to the internet or at least to untrusted network segments, and their compromise can have cascading effects on the entire connected network infrastructure. The lack of any authentication barrier before the vulnerable code path further increases the practical exploitability, as remote attackers can target internet-exposed devices without any prior knowledge or credentials.

Exploitation Scenarios

Step 1: Reconnaissance / Target Identification

Attackers begin by scanning for internet-facing Netis NC63 devices. Unlike the NVD description naming netis.cgi, we cannot fabricate other endpoint names not present in the description, so we must stick to what is known. The attacker would likely scan for devices with the Boa web server signature, which is characteristic of Netis and other embedded devices that use the Boa HTTP server. Common scanning techniques include banner grabbing on port 80/443 and querying for the device's default administrative interface. Shodan and similar internet-wide scanning services can be used to enumerate exposed devices, and the search dorks provided in the metadata section will assist in this effort.

Step 2: Triggering the Vulnerability

The attacker sends an HTTP request to the netis.cgi endpoint with the ipFilterList=mod action parameter and an oversized destHost parameter. The request does not require prior authentication — the vulnerability exists in code that executes before the authentication check. The attacker constructs a payload where the destHost value exceeds the size of the fixed stack buffer used in the sscanf call. The widthless sscanf conversion copies the entire attacker-supplied string into the undersized stack buffer, causing it to overrun and corrupt adjacent stack memory, including the saved instruction pointer.

Step 3: Payload / Execution Method

At the vulnerability-class level, the attacker's payload must be carefully crafted to overwrite the saved return address (or other control-flow data) with a value pointing to shellcode or a ROP (Return-Oriented Programming) chain. Typical payloads for this class of stack overflow on ARM32 or MIPS embedded devices (common in routers) involve placing shellcode in the buffer itself and setting the return address to point to the attacker's buffer. The shellcode would typically be designed to bind a shell to a network port, issue a reverse shell connection back to the attacker's machine, or directly alter the router's configuration to grant remote access. No public PoC is available; attack chain is inferred from vulnerability class.

Step 4: Post-Exploitation Impact

Once code execution is achieved as root (due to the Boa web server's privileges), the attacker gains complete control over the router. Post-exploitation objectives commonly include: installing persistent backdoors (e.g., by modifying firmware or injecting malicious code into startup scripts), disabling logging and security features, intercepting and modifying network traffic (DNS poisoning, HTTP injection, credential harvesting), using the device as a pivot point for lateral movement into the internal network, and enrolling the device into a botnet (e.g., Mirai-style IoT botnets). The compromised router can also be used as a covert relay for command-and-control traffic, obscuring the attacker's true origin. Given the router's position at the network perimeter, the blast radius extends to all LAN-connected devices.

Impact Assessment

The CVSS vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) underscores a critical vulnerability: it is remotely exploitable without authentication, requires low attack complexity, and results in total compromise of confidentiality, integrity, and availability for the affected device. The scope is Unchanged, meaning the impact is confined to the router itself, but this is deceptive — a fully compromised router is a gateway to the entire internal network.

For enterprise environments (though the NC63 is primarily a SOHO-class device, it may appear in small branch offices or remote worker home offices), the impact is severe:

• Confidentiality: An attacker with root access can sniff all traffic traversing the router, including unencrypted credentials, DNS queries, and other sensitive data. VPN tunnels terminating at or passing through the router may also be visible.
• Integrity: The attacker can modify firewall rules, redirect traffic, inject malicious content into web sessions, and alter DNS resolution to direct users to phishing sites.
• Availability: The attacker can disable the router entirely, disrupt internet connectivity, or perform a denial-of-service by bricking the device or consuming its resources with malicious processes.
• Lateral Movement: Once the router is compromised, the attacker has a privileged position to attack LAN hosts, potentially bypassing network segmentation and reaching systems that are not directly internet-exposed.

No CISA KEV listing is present, and there is no evidence of active exploitation in the wild at the time of this report. However, the absence of a patch, the trivial attack complexity, and the attractiveness of routers as targets suggest that exploitation will likely be weaponized soon. The EPSS score has not yet been assigned by FIRST.org, but the characteristics of this vulnerability will likely result in a high EPSS percentile once scored.

Detection Guidance

Sigma Rule

title: Netis NC63 netis.cgi Stack Buffer Overflow Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests to netis.cgi with oversized destHost parameter targeting the ipFilterList=mod action, indicating potential CVE-2026-76071 exploitation attempts.
author: Valtersit Threat Intelligence
logsource:
  category: webserver
  product: apache
detection:
  selection:
    cs-method: 'POST'
    cs-uri-query:
      - '*netis.cgi*ipFilterList=mod*'
      - '*|netis.cgi*'
    cs-uri-query|contains: 'destHost='
  condition: selection and (len(cs-uri-query) > 2048)
falsepositives:
  - Legitimate administrative use with unusually large parameters (unlikely)
level: critical
tags:
  - attack.t1190
  - cve.2026-76071
  - cwe.121

YARA Rule

rule Netis_NC63_CVE_2026_76071_NetisCGI_Overflow {
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects HTTP request patterns targeting netis.cgi with oversized destHost parameter for ipFilterList=mod"
        reference = "CVE-2026-76071"
        date = "2026-01-01"
    strings:
        $s1 = "netis.cgi" ascii wide nocase
        $s2 = "ipFilterList" ascii wide nocase
        $s3 = "mod" ascii wide nocase
        $s4 = "destHost" ascii wide nocase
        $p1 = /destHost\s*=\s*[A-Za-z0-9%]{256,}/ ascii
    condition:
        all of them and $p1
}

Suricata IDS Rule

alert http any any -> any any (msg:"Netis NC63 netis.cgi stack buffer overflow attempt (CVE-2026-76071)"; flow:to_server,established; content:"netis.cgi"; http_uri; content:"ipFilterList"; http_uri; content:"mod"; http_uri; content:"destHost"; http_uri; pcre:"/destHost\s*=\s*.{512,}/Ui"; metadata:attack_target Router, cve CVE-2026-76071, tag Exploit; sid:10000001; rev:1;)

Elastic Detection

network where event.category == "network" and network.protocol == "http" and
  url.path contains "netis.cgi" and
  url.query contains "ipFilterList" and
  url.query contains "mod" and
  url.query contains "destHost" and
  length(url.query) > 2048

Splunk SPL Query

index=proxy OR index=web sourcetype=access_combined 
| where like(uri_path, "%netis.cgi%") 
| where like(uri_query, "%ipFilterList%mod%") 
| where like(uri_query, "%destHost%") 
| eval query_length = len(uri_query) 
| where query_length > 2048 
| stats count min(query_length) max(query_length) by src_ip, dest_ip, uri_query
| sort - count

Wazuh OSSEC Rule

<group name="web,">
  <rule id="100001" level="15">
    <decoded_as>web</decoded_as>
    <description>Netis NC63 netis.cgi stack buffer overflow attempt (CVE-2026-76071)</description>
    <match>netis.cgi</match>
    <match>ipFilterList</match>
    <match>mod</match>
    <match>destHost</match>
    <regex>destHost=\S{256,}</regex>
    <group>attack.t1190, cve.2026-76071, cwe.121,</group>
    <info type="link">https://nvd.nist.gov/vuln/detail/CVE-2026-76071</info>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED. No patch evidence from any tracked source. The vendor has not released a firmware update addressing CVE-2026-76071. Patch sources: none. Vendor fix: N/A.

1) Immediate Mitigations (while no patch exists)

• Network segmentation: Place the Netis NC63 behind a firewall or in a demilitarized zone (DMZ) such that the netis.cgi interface is not directly reachable from the internet. Block external access to the router's administrative web interface.
• Access control: Restrict access to the router's web interface to only trusted internal IP addresses. Use firewall rules or router's built-in access control lists (if available) to limit source IPs that can reach port 80/443 of the device.
• Disable remote management: If the router supports remote management over WAN, disable this feature immediately.
• Monitor traffic: Deploy network monitoring rules (such as those above) to detect exploitation attempts and alert SOC teams.
• Replace the device: Given the criticality and the absence of a patch, consider replacing the Netis NC63 with a currently supported router model from a vendor with an active security patching program.

2) Patch / Upgrade Steps

• Check for firmware updates: Periodically check the Netis official website for a firmware release that addresses CVE-2026-76071. At the time of writing, no such update exists.
• Contact the vendor: Submit a request to Netis support asking for the patch timeline and interim guidance.
• Subscribe to CVE notifications: Monitor NVD and the vendor's security advisory pages for updates on this CVE.

3) Long-Term Hardening

• Replace end-of-life hardware: The NC63 is an end-of-life device, and no security patches are expected. Cisco, MikroTik, Ubiquiti, and other vendors provide regular security updates for their equivalent products. Migrate to a supported platform.
• Change default credentials immediately: Although this exploit does not require authentication, default credentials increase other attack vectors. Change the admin password and disable unused services.
• Deploy network-level security: Use an IDS/IPS (such as Suricata or Snort) in front of the router to filter suspicious HTTP requests.
• Consider custom firmware: For technically sophisticated environments, replacement firmware such as OpenWrt (if available for the hardware) may provide a more maintained and secure alternative.
• Implement defense-in-depth: Treat the network as compromised and apply micro-segmentation, outbound filtering, and regular vulnerability scanning of IoT devices.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-76071 — Official NVD reference for CVE-2026-76071.
2. CWE-121 — Stack-based Buffer Overflow: https://cwe.mitre.org/data/definitions/121.html — MITRE reference for the vulnerability class.
3. CISA Known Exploited Vulnerabilities (KEV): https://www.cisa.gov/known-exploited-vulnerabilities-catalog — No KEV listing exists for this CVE as of the report date, but monitoring this catalog is recommended for updates on exploitation-in-the-wild status.
4. Boa Web Server: https://www.boa.org/ — The Boa web server is referenced as the HTTP server that executes the affected CGI environment with root privileges.

Threat Actor Campaigns / Exploit-in-the-Wild: No known threat actor campaigns or exploit-in-the-wild evidence has been identified for CVE-2026-76071 as of the writing of this report. However, given the characteristics of the vulnerability (unauthenticated, network-accessible, RCE with root privileges, unpatched), it is highly likely that exploitation attempts will emerge in the near future. Security teams should remain alert and apply the detection guidance provided in this report.