CVE-2026-3613: Wavlink WL-NU516U1 Buffer Overflow Flaw

CVE-2026-3613 Overview

A stack-based buffer overflow vulnerability has been identified in Wavlink WL-NU516U1 firmware version V240425. This vulnerability affects the function sub_401A0C within the file /cgi-bin/login.cgi, where improper handling of the ipaddr argument allows an attacker to trigger a stack-based buffer overflow condition. The vulnerability can be exploited remotely, and proof-of-concept exploit code has been publicly disclosed, increasing the risk of exploitation in the wild.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially execute arbitrary code, compromise device integrity, and gain unauthorized access to network resources managed by the affected Wavlink USB print server.

Affected Products

• Wavlink WL-NU516U1 Firmware (version M16U1_V240425)
• Wavlink WL-NU516U1 Hardware Device
• Network environments utilizing the affected USB print server

Discovery Timeline

• 2026-03-06 - CVE-2026-3613 published to NVD
• 2026-03-10 - Last updated in NVD database

Technical Details for CVE-2026-3613

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer), which encompasses memory corruption issues where software performs operations on a memory buffer without properly validating that the operations stay within the intended boundaries.

The vulnerability exists within the CGI-based login interface of the Wavlink WL-NU516U1 USB network print server. When the login.cgi script processes the ipaddr parameter, it passes user-supplied input to the sub_401A0C function without adequate bounds checking. This allows an attacker to supply an oversized input value that exceeds the allocated stack buffer, potentially overwriting adjacent memory including the return address.

The network-accessible nature of this vulnerability, combined with the high-privilege context in which embedded device CGI handlers typically execute, makes successful exploitation particularly severe. Compromising this device could provide attackers with a foothold for lateral movement within the target network.

Root Cause

The root cause of this vulnerability is insufficient input validation in the sub_401A0C function when processing the ipaddr argument. The function allocates a fixed-size buffer on the stack to store the IP address parameter but fails to verify that the incoming data does not exceed this buffer's capacity. When an attacker supplies an ipaddr value larger than the allocated buffer, the excess data overwrites adjacent stack memory, leading to memory corruption.

This is a classic buffer overflow pattern common in embedded systems and IoT devices, where resource constraints often lead developers to use fixed-size buffers without implementing proper boundary checks.

Attack Vector

The attack can be executed remotely over the network by sending a crafted HTTP request to the /cgi-bin/login.cgi endpoint. An attacker would construct a request with a maliciously long ipaddr parameter designed to overflow the stack buffer. The exploitation requires high privileges, suggesting that some level of authentication may be required before reaching the vulnerable code path.

A successful attack could allow the attacker to corrupt stack memory, potentially hijacking the program's control flow to execute arbitrary code with the privileges of the CGI process. For technical details on the exploitation mechanism, refer to the GitHub PoC for Buffer Overflow.

Detection Methods for CVE-2026-3613

Indicators of Compromise

• Unusual or malformed HTTP requests to /cgi-bin/login.cgi with abnormally long ipaddr parameter values
• Device crashes, reboots, or unresponsive behavior following network activity
• Unexpected outbound connections from the Wavlink device to unknown external IP addresses
• Modified device configuration or firmware without administrator intervention

Detection Strategies

• Deploy network intrusion detection signatures to identify HTTP requests to /cgi-bin/login.cgi containing oversized ipaddr parameters (e.g., values exceeding 64 characters)
• Monitor web application firewall logs for blocked requests targeting Wavlink device CGI endpoints
• Implement anomaly detection for devices exhibiting unusual network behavior patterns
• Review device access logs for suspicious authentication attempts or unusual client IP addresses

Monitoring Recommendations

• Enable verbose logging on network security appliances monitoring traffic to and from Wavlink devices
• Configure alerts for device availability issues that could indicate exploitation attempts causing crashes
• Establish baseline network behavior for print server devices and alert on deviations
• Regularly audit device firmware versions against known vulnerable releases

How to Mitigate CVE-2026-3613

Immediate Actions Required

• Isolate affected Wavlink WL-NU516U1 devices from untrusted network segments immediately
• Restrict network access to the device's web management interface using firewall rules or ACLs
• Place vulnerable devices behind a VPN or require authentication at the network level before access
• Monitor the vendor's support channels for firmware update announcements

Patch Information

At the time of publication, no official patch has been released by Wavlink. The vendor was contacted during responsible disclosure. Organizations should monitor VulDB #349221 and Wavlink's official support resources for patch availability. Given the public availability of exploit information, prioritizing mitigation through network controls is essential until a patch is released.

Workarounds

• Implement firewall rules to block external access to port 80/443 on affected Wavlink devices
• Restrict management interface access to trusted internal IP addresses only
• Deploy a web application firewall (WAF) with rules to filter requests containing oversized parameters
• Consider replacing vulnerable devices with alternatives that receive regular security updates if no patch is forthcoming

# Example iptables rule to restrict access to Wavlink device (replace IP addresses)
# Block all external access to device web interface
iptables -A FORWARD -d 192.168.1.100 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.100 -p tcp --dport 443 -j DROP

# Allow only trusted management subnet
iptables -I FORWARD -s 10.0.0.0/24 -d 192.168.1.100 -p tcp --dport 80 -j ACCEPT