CVE-2026-2566: Wavlink WL-NU516U1 Buffer Overflow Flaw

CVE-2026-2566 Overview

CVE-2026-2566 is a stack-based buffer overflow vulnerability affecting Wavlink WL-NU516U1 firmware versions up to 130/260. The flaw resides in the sub_406194 function within /cgi-bin/adm.cgi, where improper handling of the firmware_url argument allows attackers to overflow a fixed-size stack buffer. Attackers can trigger the vulnerability remotely over the network. The exploit has been disclosed publicly, increasing the likelihood of opportunistic abuse. Wavlink was contacted prior to disclosure but did not respond, leaving affected devices without an official vendor patch at the time of publication.

Critical Impact

Remote attackers with valid high-privilege credentials can corrupt stack memory in the router's administrative CGI handler, enabling arbitrary code execution or denial of service on the affected device.

Affected Products

• Wavlink WL-NU516U1 firmware version 130
• Wavlink WL-NU516U1 firmware version 260
• Wavlink WL-NU516U1 /cgi-bin/adm.cgi administrative interface

Discovery Timeline

• 2026-02-16 - CVE-2026-2566 published to the National Vulnerability Database (NVD)
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2026-2566

Vulnerability Analysis

The vulnerability is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). The defect exists in the sub_406194 function inside the /cgi-bin/adm.cgi binary, which handles administrative requests on the Wavlink WL-NU516U1 router. The function copies the user-controlled firmware_url parameter into a fixed-size stack buffer without validating its length. When an attacker supplies an oversized value, adjacent stack memory, including saved return addresses, becomes overwritten. Successful exploitation can hijack control flow on the embedded MIPS or ARM target. The vendor did not respond to disclosure attempts, and no firmware fix is currently available.

Root Cause

The root cause is missing bounds checking on the firmware_url HTTP request parameter before it is copied into a local stack buffer in sub_406194. Embedded CGI handlers in consumer router firmware commonly use unsafe string operations such as strcpy or sprintf, which do not enforce destination buffer limits. The unchecked copy enables the overflow.

Attack Vector

The attack vector is network-based and requires authenticated access with high privileges to the device's administrative interface. An attacker sends a crafted HTTP request to /cgi-bin/adm.cgi with an oversized firmware_url parameter. No user interaction is needed beyond submitting the malicious request. Devices that expose the management interface to untrusted networks face the greatest exposure. Detailed technical analysis of the parameter handling is documented in the GitHub Firmware Documentation and the VulDB Entry #346173.

Detection Methods for CVE-2026-2566

Indicators of Compromise

• HTTP POST or GET requests to /cgi-bin/adm.cgi containing abnormally long firmware_url parameter values
• Unexpected reboots or crashes of the Wavlink WL-NU516U1 router immediately following administrative requests
• Outbound connections from the router to unfamiliar hosts after firmware-related administrative activity
• Authentication log entries showing administrator logins from unfamiliar external IP addresses

Detection Strategies

• Inspect network captures for requests targeting adm.cgi where the firmware_url parameter exceeds typical URL length, indicative of an overflow payload
• Correlate router syslog or remote logging output with parameter-based anomalies observed at the network perimeter
• Apply intrusion detection signatures that flag oversized parameter values directed at embedded device management endpoints

Monitoring Recommendations

• Forward router administrative logs to a centralized SIEM and alert on repeated authentication attempts or crashes
• Monitor administrative interfaces for traffic originating from outside trusted management networks
• Track firmware version inventory to identify Wavlink WL-NU516U1 devices running affected versions 130 or 260

How to Mitigate CVE-2026-2566

Immediate Actions Required

• Restrict access to /cgi-bin/adm.cgi so that only trusted management networks or VPN clients can reach the administrative interface
• Disable remote management on WAN interfaces if it is currently enabled on the Wavlink WL-NU516U1
• Rotate administrator credentials and enforce strong, unique passwords to reduce the chance of a privileged attacker reaching the vulnerable code path
• Place affected devices behind a network firewall and segment them from sensitive internal hosts

Patch Information

Wavlink has not issued an official patch for CVE-2026-2566. The vendor did not respond to disclosure outreach. Administrators should monitor the Wavlink support site for future firmware updates and apply any release that addresses the sub_406194 handler in /cgi-bin/adm.cgi. Until a fix is published, treat affected devices as unpatched and apply compensating controls.

Workarounds

• Block external access to the router's HTTP and HTTPS management ports at the network perimeter
• Deploy an upstream web application firewall or reverse proxy that drops requests where firmware_url exceeds a safe length
• Replace end-of-life or unsupported Wavlink WL-NU516U1 units with a currently supported device if no firmware fix becomes available

# Example iptables rule restricting management access to a trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP