Stack-Based Buffer Overflow in Wavlink WL-NU516U1-A Firmware

Executive Summary

CVE-2026-13539 is a stack-based buffer overflow vulnerability found in the Wavlink WL-NU516U1 router firmware version M16U1_V240425. It occurs in the function sub_407504 within the /cgi-bin/wireless.cgi file, specifically in the handling of the POST parameter Guest_ssid.

The vulnerability arises because the code uses the sprintf function without proper bounds checking to construct a command string, where the buffer size is limited to 1028 bytes. If the combined length of parameters, including Guest_ssid, exceeds this buffer size, it causes a stack overflow.

An attacker can exploit this by sending a crafted HTTP POST request with a large Guest_ssid value (around 1000+ bytes), which overwrites the stack return address. Due to the MIPS little-endian architecture, this can potentially lead to remote code execution by manipulating the return address register.

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Impact Analysis

This vulnerability can have severe impacts including remote code execution on the affected device. An attacker can exploit the buffer overflow to execute arbitrary code, potentially gaining control over the router.

Such control could allow the attacker to alter device configurations, intercept or redirect network traffic, disrupt network services, or use the compromised device as a foothold for further attacks within the network.

Because the exploit is remotely executable and publicly available, the risk of exploitation is high if the device is exposed to untrusted networks.

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Detection Guidance

This vulnerability can be detected by monitoring for suspicious HTTP POST requests sent to the endpoint /cgi-bin/wireless.cgi that include an unusually large Guest_ssid parameter (around 1000+ bytes). Such requests may indicate attempts to exploit the stack-based buffer overflow.

A practical detection method is to capture and analyze network traffic targeting the router's web interface, specifically looking for POST requests with oversized Guest_ssid parameters.

Example command using tcpdump to capture such traffic:

• tcpdump -i <interface> -A 'tcp dst port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) > 1000)'

Alternatively, using curl to test the vulnerability by sending a crafted POST request with a large Guest_ssid parameter:

• curl -X POST http://<target-ip>/cgi-bin/wireless.cgi -d 'Guest_ssid=$(python3 -c "print('A'*1500)")'

If the device crashes, behaves unexpectedly, or returns errors, it may be vulnerable.

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Mitigation Strategies

The immediate recommended step is to upgrade the affected Wavlink WL-NU516U1 router firmware to the fixed version released by the vendor.

Until the upgrade can be applied, it is advisable to restrict access to the router's web interface, especially blocking remote access to /cgi-bin/wireless.cgi to prevent exploitation.

Additional mitigation steps include monitoring network traffic for suspicious POST requests targeting the Guest_ssid parameter and disabling guest Wi-Fi features if not needed.

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