CVE-2026-76869: Netcore NR255-V Buffer Overflow Vulnerability

CVE-2026-76869 Overview

CVE-2026-76869 is a stack-based buffer overflow [CWE-121] in the Netcore NR255-V router, firmware version 1.5.130703. The flaw resides in the reboot_timer_set.cgi endpoint, which improperly parses input tokens using sscanf. Attackers with authenticated access can submit crafted requests to corrupt stack memory on the device. Successful exploitation compromises the confidentiality, integrity, and availability of the router.

Critical Impact
Authenticated attackers can trigger stack corruption in the router's web management interface, enabling potential code execution and full device compromise.

Affected Products

Netcore NR255-V router
Firmware version 1.5.130703
reboot_timer_set.cgi web management endpoint

Discovery Timeline

2026-09-15 - CVE-2026-76869 published to NVD
2026-09-17 - Last updated in NVD database

Technical Details for CVE-2026-76869

Vulnerability Analysis

The vulnerability exists in the reboot_timer_set.cgi handler within the router's web administration interface. The handler uses sscanf to parse tokens from user-supplied input without enforcing destination buffer bounds. When an attacker submits input exceeding the expected token length, the parser writes past the stack-allocated buffer.

Stack-based buffer overflows in embedded router firmware typically enable overwriting saved return addresses or adjacent stack variables. On MIPS or ARM-based routers running without stack canaries or address space layout randomization (ASLR), this class of flaw commonly leads to arbitrary code execution. The reboot_timer_set.cgi binary runs with elevated privileges to manage device state, amplifying the impact.

Root Cause

The root cause is the unsafe use of sscanf format specifiers without width limits when parsing HTTP request parameters. The function copies tokens into fixed-size stack buffers with no length validation. This pattern maps directly to CWE-121 (Stack-Based Buffer Overflow) and reflects a broader failure to validate input boundaries before memory operations.

Attack Vector

Exploitation requires network access to the router's management interface and valid administrative credentials. An attacker submits a crafted HTTP request to the reboot_timer_set.cgi endpoint with an oversized parameter value. The malformed input passes through sscanf, overflows the destination buffer, and corrupts the stack frame. Reliable exploitation on embedded targets often yields shell access or persistence on the device.

No public proof-of-concept exploit is currently available. See the VulnCheck Security Advisory and GitHub Vulnerability Documentation for technical details.

Detection Methods for CVE-2026-76869

Indicators of Compromise

Unexpected HTTP POST or GET requests to /reboot_timer_set.cgi containing unusually long parameter values.
Router crashes, unexpected reboots, or httpd process restarts correlated with inbound management traffic.
Outbound connections from the router to unfamiliar hosts following administrative interface access.
New or modified startup scripts and cron entries on the device filesystem.

Detection Strategies

Inspect web server and management interface logs for oversized query strings or POST bodies targeting reboot_timer_set.cgi.
Deploy network intrusion detection signatures that flag requests to router CGI endpoints exceeding expected parameter length thresholds.
Baseline normal administrative traffic patterns and alert on deviations in request size, frequency, or source IP.

Monitoring Recommendations

Forward router syslog and HTTP access logs to a centralized SIEM for correlation with authentication events.
Monitor for repeated authentication attempts against the router web interface preceding suspicious CGI requests.
Track device availability metrics to identify crashes indicative of failed exploitation attempts.

How to Mitigate CVE-2026-76869

Immediate Actions Required

Restrict access to the router's web management interface to trusted internal networks only, blocking WAN-side administration.
Rotate administrative credentials and enforce strong, unique passwords to limit authenticated attack paths.
Audit recent administrative sessions on affected devices for signs of anomalous CGI requests.

Patch Information

No vendor patch information is currently listed in the enriched CVE data. Consult the VulnCheck Security Advisory for the latest remediation guidance and monitor Netcore vendor channels for firmware updates addressing reboot_timer_set.cgi.

Workarounds

Disable remote management on the WAN interface and restrict LAN-side access to a dedicated management VLAN.
Place vulnerable devices behind a reverse proxy or firewall that enforces request size limits on CGI endpoints.
Replace end-of-life or unpatched NR255-V devices with supported hardware where firmware fixes are unavailable.

bash

# Configuration example: block external access to router management interface
iptables -A INPUT -i wan0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i wan0 -p tcp --dport 443 -j DROP

# Restrict LAN-side management to a specific admin subnet
iptables -A INPUT -i br-lan -p tcp --dport 80 ! -s 192.168.10.0/24 -j DROP