Stack-based Buffer Overflow in Wavlink WL-NU516U1 nas.cgi

A critical stack-based buffer overflow vulnerability has been identified in the Wavlink WL-NU516U1 router (firmware version 708c073-mt7628). The vulnerability exists within the 'nas.cgi' binary, specifically in how it processes the 'CONTENT_LENGTH' HTTP header parameter using the 'fgets' function. By sending a crafted HTTP request with an excessively large value for 'CONTENT_LENGTH', an unauthenticated remote attacker can trigger a memory corruption condition. This flaw enables remote code execution (RCE) or denial-of-service (DoS) conditions on the affected network device. The manufacturer has provided a firmware update to address this memory safety issue. Organizations using these devices should prioritize applying the provided patch to prevent unauthorized access and potential persistent compromise of the networking infrastructure.

Attack Chain

1. Attacker performs network reconnaissance to identify accessible Wavlink WL-NU516U1 routers via HTTP.
2. Attacker crafts an HTTP request targeting the vulnerable 'nas.cgi' endpoint.
3. Attacker injects a malicious, oversized value into the 'CONTENT_LENGTH' header.
4. The 'nas.cgi' binary processes the input using the vulnerable 'fgets' function.
5. The oversized input exceeds the allocated stack buffer, resulting in a buffer overflow.
6. Attacker overwrites the stack return pointer with a payload address.
7. The application executes the attacker's shellcode or return-oriented programming (ROP) chain.
8. Attacker gains arbitrary code execution with the privileges of the web service process.

Impact

Successful exploitation of CVE-2026-18588 allows for full remote compromise of the affected Wavlink routers. As these devices are typically placed at the perimeter of the network, impact includes interception of internal traffic, unauthorized network access, and the potential for persistent backdoors. Given the CVSS 3.1 base score of 9.8, exploitation is trivial and does not require authentication or user interaction.

Recommendation

Prioritize the immediate application of the vendor-supplied firmware update to address CVE-2026-18588 on all affected Wavlink WL-NU516U1 units. Implement network segmentation for all edge networking equipment to restrict access to management interfaces, including CGI-based endpoints, to authorized internal administrative hosts only. Monitor web server logs for anomalies in 'CONTENT_LENGTH' headers or unusual request patterns targeting 'nas.cgi' as an indicator of attempted exploitation.