MailEnable < 10.54 Reflected XSS in FieldBcc Parameter of AddressBook.aspx

MailEnable versions prior to 10.54 contain a reflected cross-site scripting (XSS) vulnerability in the FieldBcc parameter of /Mondo/lang/sys/Forms/AddressBook.aspx. The FieldBcc value is not properly sanitized when processed via a GET request and is reflected inside a block in the JavaScript variable var BCCFieldProvided. By supplying a crafted payload that terminates the existing LoadCurAddresses() function, inserts attacker-controlled script, and comments out remaining code, a remote attacker can execute arbitrary JavaScript in a victim’s browser during normal email composition. Successful exploitation can redirect victims to malicious sites, steal non-HttpOnly cookies, inject arbitrary HTML or CSS, and perform actions as the authenticated user.