CVE-2026-76866 Overview
CVE-2026-76866 is an OS command argument injection vulnerability in the Netcore NR255-V router running firmware version 1.5.130703. The flaw resides in DDNSset_cgi.c and related ddns_Proc.c components, which build root-run command lines from unquoted, user-supplied Dynamic DNS (DDNS) input. Attackers with sufficient access to the router's management interface can inject additional command arguments that execute with root privileges. The weakness is classified under CWE-88: Improper Neutralization of Argument Delimiters in a Command.
Critical Impact
Successful exploitation grants root-level command argument injection on the router, enabling full compromise of confidentiality, integrity, and availability of the device.
Affected Products
- Netcore NR255-V router
- Firmware version 1.5.130703
- DDNS handler components DDNSset_cgi.c and ddns_Proc.c
Discovery Timeline
- 2026-09-15 - CVE-2026-76866 published to the National Vulnerability Database (NVD)
- 2026-09-16 - Last updated in NVD database
Technical Details for CVE-2026-76866
Vulnerability Analysis
The Netcore NR255-V firmware processes DDNS configuration input through CGI handlers that assemble shell command strings without quoting or sanitizing the user-controlled parameters. When the router constructs the DDNS update command line, attacker-supplied fields are concatenated directly into the argument list passed to a root-privileged process. Because the input is unquoted, additional command-line arguments can be smuggled in and interpreted by the downstream binary. This allows an authenticated attacker to alter program behavior, override options, or trigger command execution with root privileges. Full technical exploitation notes are documented in the GitHub PoC writeup and the VulnCheck advisory.
Root Cause
The root cause is improper neutralization of argument delimiters ([CWE-88]) in the DDNS request handling code path. DDNSset_cgi.c and ddns_Proc.c accept DDNS parameters from HTTP requests and pass them into a command line without escaping whitespace or leading dashes. The unquoted concatenation allows attackers to break out of the intended single-argument context and add controllable arguments.
Attack Vector
Exploitation requires network access to the router's web management interface and high privileges to reach the DDNS configuration handler. An attacker submits crafted DDNS field values whose contents introduce additional command-line arguments. The resulting root-run process interprets these injected arguments, giving the attacker control over the executed operation on the embedded Linux system.
No verified proof-of-concept code is reproduced here. See the GitHub PoC documentation for exploitation details.
Stop Remote Code Execution Before It Runs
Singularity™ Endpoint detects and blocks exploit attempts at machine speed, including zero-days and novel attack techniques.
Stop Exploit AttemptsDetection Methods for CVE-2026-76866
Indicators of Compromise
- HTTP POST requests to the DDNS configuration CGI endpoint containing shell metacharacters, dashes, or whitespace inside DDNS username, password, hostname, or service parameters.
- Unexpected child processes spawned by the router's HTTP daemon that reference DDNS update binaries with anomalous argument counts.
- Modifications to router configuration or persistent files outside the normal DDNS update workflow.
Detection Strategies
- Inspect router HTTP request logs for DDNS parameter values containing characters such as -, spaces, or backticks that do not match expected domain or credential formats.
- Compare active firmware version against 1.5.130703 on all deployed NR255-V devices and flag matches as at-risk assets.
- Monitor egress from router management interfaces for outbound connections initiated shortly after DDNS configuration changes.
Monitoring Recommendations
- Forward router syslog and CGI access logs to a centralized logging platform for retention and query.
- Alert on any configuration write to DDNS settings originating from IP addresses outside authorized administrative ranges.
- Baseline normal DDNS parameter lengths and character sets, then alert on deviations that indicate injection attempts.
How to Mitigate CVE-2026-76866
Immediate Actions Required
- Restrict access to the router's web management interface to trusted management VLANs or IP allowlists.
- Disable the DDNS feature on affected NR255-V devices if it is not required for operations.
- Rotate administrative credentials on the router to reduce the risk of an attacker reaching the DDNS handler with valid high-privilege access.
Patch Information
No vendor advisory or firmware patch is referenced in the NVD entry for CVE-2026-76866 at the time of publication. Track the VulnCheck advisory for updates from Netcore and apply any released firmware update for the NR255-V as soon as it becomes available.
Workarounds
- Block WAN-side access to the router administrative interface at an upstream firewall.
- Disable DDNS functionality in the router configuration where the feature is unused.
- Replace vulnerable NR255-V devices with supported hardware if a firmware fix is not released in a timeframe acceptable to the organization's risk posture.
# Example upstream firewall rule to restrict router management access
# Replace 203.0.113.10 with the authorized management host
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -s 203.0.113.10 -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
