Default or blank username/password configurations are a common vulnerability in data storage systems. They can be exploited by attackers who attempt to access systems using commonly known default credentials or no credentials at all when the username and password fields are left blank. This issue often arises from improper initial setup and configuration, leaving the system open to unauthorized access. Incorrect answers like 'Data sanitation' or 'Two-factor authentication' do not directly relate to this type of vulnerability. 'Data sanitation' is a technique to prevent injection attacks by cleaning the data inputs, not a vulnerability itself, while 'Two-factor authentication' is an additional security layer, not typically a vulnerability when correctly implemented.