The GlassFish Admin Console is protected with weak or default credentials. Acunetix was able to guess the username and password required to access this page. A weak password is short, common, a system default, or something that could be rapidly guessed by executing a brute force attack using a subset of all possible passwords, such as words in the dictionary, proper names, words based on the user name or common variations on these themes.
Enforce a strong password policy. Don't permit weak passwords or passwords based on dictionary words.