CVE-2026-76861: Netcore NR255-V Buffer Overflow Vulnerability

CVE-2026-76861 Overview

CVE-2026-76861 is a stack-based buffer overflow [CWE-121] in the Netcore NR255-V router, firmware version 1.5.130703. The flaw resides in the ntools_tcpdump_start_set.cgi endpoint, which uses an unsized sprintf call to process form values submitted by an authenticated user. An attacker with low-level privileges can send crafted input to this CGI endpoint, overflow the fixed-size stack buffer, and potentially execute arbitrary code on the device. Because the endpoint is exposed over the network, exploitation does not require physical or local access. Successful exploitation compromises the router's confidentiality, integrity, and availability.

Critical Impact
Attackers who reach the router's management interface can overflow the stack in ntools_tcpdump_start_set.cgi and gain arbitrary code execution on the device.

Affected Products

Netcore NR255-V router
Firmware version 1.5.130703
ntools_tcpdump_start_set.cgi handler

Discovery Timeline

2026-09-15 - CVE-2026-76861 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-76861

Vulnerability Analysis

The vulnerability affects the router's diagnostic tooling exposed through the web management interface. The ntools_tcpdump_start_set.cgi handler accepts form values that are intended to configure a tcpdump capture session. Instead of validating input length or using a bounded string function, the handler passes attacker-controlled form data directly into an sprintf call that writes to a fixed-size stack buffer. Because sprintf performs no length checking, oversized input walks past the buffer boundary and overwrites adjacent stack memory, including saved return addresses. An authenticated attacker on the local network can weaponize this behavior to hijack control flow and execute arbitrary code with the privileges of the CGI process, which typically runs as root on embedded router platforms.

Root Cause

The root cause is unsafe use of sprintf without input length validation, classified as [CWE-121] Stack-Based Buffer Overflow. The handler trusts user-supplied form data and formats it into a fixed-size stack buffer with no boundary checks.

Attack Vector

Exploitation requires network access to the router's management interface and low-privilege authentication. An attacker submits an HTTP request to ntools_tcpdump_start_set.cgi with a form value long enough to overflow the destination buffer. The overflow can overwrite the saved return address and redirect execution to attacker-controlled shellcode or a return-oriented programming (ROP) chain.

Detailed exploitation notes and a proof of concept are published in the GitHub TCP Dump Overflow PoC and the VulnCheck Advisory Buffer Overflow.

Detection Methods for CVE-2026-76861

Indicators of Compromise

HTTP POST requests to /ntools_tcpdump_start_set.cgi containing form parameters with abnormally long values.
Router web service crashes, restarts, or watchdog resets shortly after CGI activity.
Unexpected outbound connections from the router following diagnostic-page access.
New or modified processes on the router that were not present before management activity.

Detection Strategies

Inspect web server and CGI logs for requests to ntools_tcpdump_start_set.cgi with parameter values exceeding expected sizes.
Alert on repeated authentication followed by requests to diagnostic CGI endpoints from a single source.
Correlate router crash telemetry with recent management-plane HTTP activity.

Monitoring Recommendations

Forward router management interface logs to a centralized logging platform for retention and analysis.
Baseline normal use of diagnostic CGI endpoints and alert on deviations in request size or frequency.
Monitor administrative account usage and flag logins from unusual source addresses.

How to Mitigate CVE-2026-76861

Immediate Actions Required

Restrict access to the router's web management interface to trusted management VLANs or hosts only.
Disable remote WAN-side administration if it is enabled.
Rotate administrative credentials and enforce strong, unique passwords on the router.
Audit recent access to ntools_tcpdump_start_set.cgi and investigate any anomalous requests.

Patch Information

No vendor patch was referenced in the NVD entry at time of publication. Consult the VulnCheck Advisory Buffer Overflow and the Netcore vendor support channel for firmware updates addressing ntools_tcpdump_start_set.cgi in versions later than 1.5.130703.

Workarounds

Block access to /ntools_tcpdump_start_set.cgi at an upstream firewall or reverse proxy where feasible.
Place vulnerable NR255-V devices behind a segmented management network with strict ACLs.
Remove or disable low-privilege accounts that do not require access to diagnostic tooling.
Consider replacing end-of-support hardware if the vendor does not release a firmware fix.

bash

# Example: restrict router management access to a trusted subnet using iptables on an upstream gateway
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -s 10.0.10.0/24 -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -s 10.0.10.0/24 -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -j DROP