CVE-2026-13539: Wavlink WL-NU516U1-A Stack Buffer Overflow in Wireless Configuration

Technical summary

The vulnerability resides in function sub_407504 within /cgi-bin/wireless.cgi, which handles POST parameters for wireless settings. The Guest_ssid parameter is not properly validated for length before being copied into a stack-allocated buffer. An attacker with valid credentials can supply a string exceeding the buffer's capacity, overwriting adjacent stack memory including return addresses and local variables. The vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and CWE-121 (Stack-based Buffer Overflow). CVSS v3.1 score of 8.8 (HIGH severity) reflects the combination of network accessibility, low complexity, and high impact on confidentiality, integrity, and availability.

Business impact

Compromised Wavlink routers can serve as a foothold for lateral network movement, man-in-the-middle attacks on client traffic, or staging points for broader infrastructure compromise. For organizations relying on these devices as edge network gateways or guest network controllers, exploitation could lead to data exfiltration, unauthorized network access, and disruption of wireless services. The requirement for authenticated access reduces but does not eliminate risk, as default or weak credentials are common in deployments.

Affected systems

Wavlink WL-NU516U1-A routers with firmware M16U1_V240425 are confirmed affected. The specific firmware version should be checked on deployed devices; users should verify their current firmware build against official Wavlink advisories to confirm whether they are running the vulnerable release. Other Wavlink models or firmware versions may be affected but have not been explicitly confirmed in available disclosures.

Exploitability

This vulnerability is remotely exploitable but requires valid authentication credentials to access the /cgi-bin/wireless.cgi endpoint. Public exploits are known to exist, lowering the barrier to weaponization by threat actors with network access and valid user credentials. The straightforward nature of the overflow (parameter parsing without bounds checking) and the availability of public proof-of-concept code suggest that exploitation is straightforward for any attacker with basic technical capability.

Remediation

Organizations should immediately identify and inventory Wavlink WL-NU516U1-A devices and verify their current firmware version. Wavlink has released a patched firmware version to address this vulnerability. Upgrade to the fixed firmware as soon as feasible. As an interim control, restrict administrative and wireless configuration access to trusted IP ranges or VLANs, enforce strong credentials on all router accounts, and disable remote management interfaces if not required. Monitor network traffic to these devices for suspicious POST requests to the wireless.cgi endpoint.

Patch guidance

Verify the current firmware version on each affected Wavlink device (typically found in administration settings or via SSH/telnet console). Contact Wavlink support or visit their product support portal to obtain the patched firmware version that addresses this overflow. Follow the vendor's documented firmware update procedure carefully, ensuring devices remain powered and network-connected throughout the upgrade process. Consider staging the update in a test environment first if feasible. After patching, validate device operation and document the new firmware version for compliance tracking.

Detection guidance

Monitor HTTP POST requests to /cgi-bin/wireless.cgi for unusually large or malformed Guest_ssid parameters. Intrusion detection systems should alert on payloads exceeding typical wireless SSID lengths (max 32 characters). Review web server logs on affected Wavlink devices for POST requests with parameter values that appear to contain shellcode patterns or excessive data. Monitor the device for unexpected process crashes, segmentation faults, or service restarts that could indicate exploitation attempts. Endpoint detection and response (EDR) tools on systems that manage these routers can detect abnormal process behavior following failed exploitation attempts.

Why prioritize this

Although authentication is required, the combination of public exploit availability, straightforward overflow mechanics, and the potential for complete device compromise warrants prompt patching. Wireless infrastructure devices are frequently targeted in attacks aimed at intercepting or manipulating network traffic. The HIGH CVSS score and confirmed fix availability justify prioritizing this for near-term remediation cycles.

Risk score, explained

The CVSS 8.8 score reflects a remotely exploitable vulnerability (AV:N) with low attack complexity (AC:L) that requires valid credentials (PR:L) but no user interaction (UI:N). The impact scope is unchanged (single system affected), but the attacker gains high-level access to confidentiality, integrity, and availability. The requirement for authentication prevents a higher score, but the severity remains HIGH given the criticality of router security and the existence of public exploits.

Frequently asked questions