CVE-2026-13539: Wavlink WL-NU516U1-A Buffer Overflow Flaw

CVE-2026-13539 Overview

CVE-2026-13539 is a stack-based buffer overflow in the Wavlink WL-NU516U1-A router running firmware M16U1_V240425. The flaw resides in the sub_407504 function within /cgi-bin/wireless.cgi, part of the POST parameter handler. Attackers manipulate the Guest_ssid argument to trigger memory corruption on the stack [CWE-119]. The vulnerability is remotely exploitable over the network and a public proof-of-concept exists on GitHub. Wavlink was contacted and released a fixed firmware image. Successful exploitation can lead to arbitrary code execution on the affected device with router-level privileges.

Critical Impact
Remote attackers with low privileges can corrupt stack memory through the Guest_ssid POST parameter, potentially achieving arbitrary code execution on the router.

Affected Products

Wavlink WL-NU516U1-A router
Firmware version M16U1_V240425
Component: /cgi-bin/wireless.cgi POST parameter handler

Discovery Timeline

2026-06-29 - CVE-2026-13539 published to NVD
2026-06-29 - Last updated in NVD database

Technical Details for CVE-2026-13539

Vulnerability Analysis

The vulnerability is a stack-based buffer overflow classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). It resides inside the internal function sub_407504, invoked by /cgi-bin/wireless.cgi when processing POST requests related to guest Wi-Fi configuration. The Guest_ssid parameter supplied by the client is copied into a fixed-size stack buffer without adequate length validation. Sending a request with an overlong Guest_ssid value overwrites adjacent stack memory, including saved return addresses. On MIPS-based Wavlink devices, this class of flaw commonly enables control of the program counter and subsequent code execution. Because the CGI binary runs with elevated privileges, exploitation gives an attacker full control of network services on the router.

Root Cause

The root cause is the use of an unbounded string copy operation in sub_407504 that trusts attacker-controlled input from the Guest_ssid POST field. No length check enforces the destination buffer size before the copy.

Attack Vector

The attack is executed over the network against the router's HTTP management interface. An authenticated user with low privileges submits a crafted POST request to /cgi-bin/wireless.cgi containing an oversized Guest_ssid value. No user interaction from an administrator is required. A public exploit is available in the GitHub Buffer Overflow Exploit repository. Technical details are also documented at VulDB CVE-2026-13539.

// Conceptual illustration - see referenced PoC for actual exploitation details
POST /cgi-bin/wireless.cgi HTTP/1.1
Host: <router-ip>
Content-Type: application/x-www-form-urlencoded

Guest_ssid=<oversized payload overflowing the stack buffer>&...

Detection Methods for CVE-2026-13539

Indicators of Compromise

POST requests to /cgi-bin/wireless.cgi containing abnormally long Guest_ssid parameter values
Unexpected reboots or crashes of the wireless.cgi process on the router
Unauthorized changes to guest Wi-Fi configuration or new administrative sessions from unknown IP addresses
Outbound connections from the router to unknown hosts following configuration changes

Detection Strategies

Inspect HTTP request logs for Guest_ssid values exceeding typical SSID lengths (32 bytes per IEEE 802.11)
Deploy network intrusion detection signatures matching oversized POST bodies to /cgi-bin/wireless.cgi
Monitor router syslog output for CGI process termination or watchdog-triggered restarts

Monitoring Recommendations

Forward router management interface logs to a centralized logging platform for continuous analysis
Alert on repeated POST requests to wireless.cgi from a single source in a short window
Baseline normal firmware behavior and flag deviations in service uptime or listening ports

How to Mitigate CVE-2026-13539

Immediate Actions Required

Upgrade Wavlink WL-NU516U1-A firmware to the vendor-supplied fixed version referenced in the Wavlink Firmware File
Restrict access to the router's HTTP management interface to trusted management VLANs only
Rotate administrative credentials on affected devices after patching

Patch Information

Wavlink released a fixed firmware build dated 2026-06-22 (WINSTAR_NU516U1-WO-A-2026-06-22-5ccde97). Administrators should download the sysupgrade image from the official Wavlink distribution URL and apply it through the router's firmware update interface. Verify the running firmware version after upgrade to confirm remediation.

Workarounds

Disable the guest Wi-Fi feature until firmware can be applied, reducing exposure of the affected code path
Block access to /cgi-bin/wireless.cgi from untrusted networks using upstream firewall or ACL rules
Enforce strong, unique administrative credentials to raise the bar for the low-privilege precondition

bash

# Example: restrict router management interface to a trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP