CVE-2026-76071 in NC63
Summary
by MITRE • 08/24/2026
Netis NC63 firmware through V3.0.0.3327 contains a stack-based buffer overflow vulnerability that allows unauthenticated remote attackers to overwrite saved stack state by supplying an oversized destHost parameter to the ipFilterList=mod action in netis.cgi. Attackers can exploit widthless sscanf conversions that copy user-supplied input into fixed-size stack buffers before authentication is verified, achieving remote code execution as root due to the Boa web server executing the CGI environment with root privileges.
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Analysis
by VulDB Data Team • 08/24/2026
The vulnerability identified in Netis NC63 firmware versions up to V3.0.0.3327 represents a critical security flaw rooted in improper input validation within the device's embedded web management interface. The core of this issue lies in the handling of the ipFilterList=mod action executed by netis.cgi, specifically regarding the destHost parameter. This component is responsible for processing network filtering configurations but fails to adequately restrict the length or format of user-supplied data before it is processed. Because the vulnerability exists within a CGI script that operates under the context of the Boa web server, which typically runs with elevated privileges on embedded Linux devices, any successful exploitation results in immediate and complete compromise of the target system. The lack of authentication checks prior to this buffer manipulation further exacerbates the severity, allowing unauthenticated remote attackers to trigger the flaw from anywhere on the network or potentially over the internet if port forwarding is configured.
From a technical perspective, the vulnerability manifests as a stack-based buffer overflow caused by widthless sscanf conversions. In C programming, when using functions like sscanf without specifying field widths for string conversion specifiers such as %s, the function will continue reading input until it encounters whitespace or end-of-string markers, regardless of the destination buffer's allocated size. The netis.cgi script copies user-supplied input from the destHost parameter into fixed-size stack buffers without verifying that the input length fits within those bounds. This architectural oversight allows an attacker to supply a payload significantly larger than the buffer capacity. When this oversized data is written to the stack, it overwrites adjacent memory locations, including critical saved state information such as the return address and frame pointer. By carefully crafting the overflowed data, attackers can manipulate these overwritten values to redirect program execution flow toward malicious shellcode injected into the same buffer or nearby memory regions.
The operational impact of this vulnerability is severe, leading directly to remote code execution with root privileges. Since the Boa web server executes CGI scripts as the root user on many embedded router implementations, gaining control over the instruction pointer via stack smashing grants the attacker full administrative access to the device. This level of compromise allows for a wide range of malicious activities beyond simple denial of service or information disclosure. An adversary can install persistent backdoors, modify firmware settings to bypass security controls, redirect traffic through man-in-the-middle attacks, or use the compromised router as a pivot point for lateral movement within the local network. The ability to execute arbitrary code means that sensitive data such as Wi-Fi passwords, DNS configurations, and connected device information can be exfiltrated at will. Furthermore, because this is an unauthenticated vulnerability, it requires no prior access credentials, making it highly attractive for automated scanning tools and worm-like propagation mechanisms targeting IoT devices globally.
This flaw aligns with several established industry standards that categorize the nature of the defect. It is primarily classified under CWE-120: Buffer Copy without Checking Size of Input, which describes situations where data is copied into a buffer without verifying if it fits within the allocated space. Additionally, because the overflow allows for arbitrary code execution by overwriting stack metadata, it falls squarely under CWE-121: Stack-based Buffer Overflow. The exploitation technique leverages memory corruption to hijack control flow, which corresponds to ATT&CK tactic Tactic 1204: User Execution or more specifically techniques related to command and script interpretation within the context of compromised services. In terms of attack vectors, this represents a classic example of CWE-78: Improper Neutralization of Special Elements used in an OS Command, although the primary mechanism is memory corruption rather than direct shell injection, it often leads to similar outcomes where system commands are executed under elevated privileges due to the nature of the CGI environment.
Mitigation strategies for this vulnerability must address both immediate remediation and long-term security posture improvements. The most effective solution is to apply the firmware update provided by Netis that patches the netis.cgi script, ensuring that input validation checks are implemented before data is passed to sscanf or similar functions. Developers should enforce strict length limits on all user-supplied parameters and utilize safe string handling libraries that prevent buffer overflows. For organizations unable to immediately patch their devices, network-level controls can provide temporary relief. Implementing firewall rules to restrict access to the router's management interface (typically port 80 or 443) to only trusted IP addresses reduces the attack surface significantly. Additionally, disabling remote administration features if they are not strictly necessary prevents external actors from reaching the vulnerable CGI endpoint altogether. Regular vulnerability scanning and monitoring for unusual outbound traffic can also help detect exploitation attempts early, although prevention through patching remains the definitive countermeasure against this class of stack-based buffer overflow vulnerabilities in embedded web servers.