Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-3613 is a stack-based buffer overflow vulnerability found in the Wavlink WL-NU516U1 router firmware version V240425, specifically in the function sub_401A0C of the file /cgi-bin/login.cgi.'}, {'type': 'paragraph', 'content': 'The vulnerability occurs because the function uses a fixed-size 128-byte stack buffer to build a system command string using sprintf with the user-supplied "ipaddr" parameter, but it does not validate the length of this input.'}, {'type': 'paragraph', 'content': 'If the "ipaddr" parameter exceeds about 105 bytes, sprintf writes beyond the buffer boundary, overwriting adjacent stack variables, saved registers, and the return address, which can cause a segmentation fault and crash the CGI process.'}, {'type': 'paragraph', 'content': "This leads to a Denial of Service (HTTP 500 Internal Server Error). Under certain conditions, the overflow could be exploited for Remote Code Execution (RCE). Exploitation requires knowledge of the administrator password's MD5 hash and sending a valid POST request with a session cookie."}] [1, 2, 3]

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Impact Analysis

[{'type': 'paragraph', 'content': "This vulnerability can impact you by allowing an attacker to remotely crash the router's login CGI process, causing a Denial of Service (DoS) and making the device unavailable."}, {'type': 'paragraph', 'content': 'Additionally, under certain conditions, the vulnerability might be exploited for Remote Code Execution (RCE), potentially allowing an attacker to execute arbitrary code on the device.'}, {'type': 'paragraph', 'content': "Exploitation requires authentication knowledge (the administrator password's MD5 hash) and the ability to send crafted POST requests, but the exploit is publicly available and considered easy to perform."}, {'type': 'paragraph', 'content': 'Because of the severity and lack of disclosed mitigations, the affected device may be at significant security risk, potentially compromising confidentiality, integrity, and availability.'}] [1, 2, 3]

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by monitoring for Denial of Service (DoS) symptoms such as HTTP 500 Internal Server Errors on the /cgi-bin/login.cgi endpoint, which occur when a crafted POST request with an excessively long ipaddr parameter (over approximately 105 bytes) is sent.'}, {'type': 'paragraph', 'content': 'Detection can also involve sending a controlled POST request to /cgi-bin/login.cgi with a long ipaddr parameter while authenticating with the administrator password (default "admin" with MD5 hash 21232f297a57a5a743894a0e4a801fc3) to see if the CGI process crashes.'}, {'type': 'paragraph', 'content': 'A suggested command to test for the vulnerability is to craft a POST request with a long ipaddr parameter exceeding 105 bytes, for example using curl:'}, {'type': 'list_item', 'content': "curl -X POST -b 'session_cookie=valid_session' -d 'password=admin&ipaddr=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA' http://<target-ip>/cgi-bin/login.cgi"}, {'type': 'paragraph', 'content': 'If the server responds with HTTP 500 Internal Server Error or the CGI process crashes, it indicates the vulnerability is present.'}] [1, 3]

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Mitigation Strategies

[{'type': 'paragraph', 'content': 'Immediate mitigation steps include restricting access to the /cgi-bin/login.cgi interface to trusted networks or IP addresses to prevent remote exploitation.'}, {'type': 'paragraph', 'content': 'Ensure strong administrator passwords are used instead of the default "admin" password to reduce the risk of successful authentication required for exploitation.'}, {'type': 'paragraph', 'content': 'Monitor the device for unusual crashes or HTTP 500 errors indicating attempted exploitation.'}, {'type': 'paragraph', 'content': 'Since no official patches or countermeasures have been disclosed by the vendor, consider replacing the affected device with a non-vulnerable model if possible.'}] [2]

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