Hidden backdoor authentication mechanism in multiple versions of Tenda firmware allows admin access to web management interface

The web server binary /bin/httpd contains a hidden backdoor authentication mechanism in the login() function at 004c88b8.

- The function contains a normal authentication path using MD5/hash-based password verification (prod_encode64/PasswordToMd5/check_rand_key).

- After normal authentication fails, it calls GetValue("sys.rzadmin.password") to read a backdoor password from the device configuration.

- It performs a direct strcmp() comparison (plaintext, not hashed) between the config value and the user-supplied password.

A successful match grants role=2 (admin-level access) and creates a valid session. The rzadmin username is never checked — any username works with the backdoor