A breach is an incident where data is inadvertently exposed in a vulnerable system, usually due to insufficient access controls or security weaknesses in the software. HIBP aggregates breaches and enables people to assess where their personal data has been exposed.

The public search facility cannot return anything other than the results for a single user-provided email address or username at a time. Multiple breached accounts can be retrieved by the domain search feature but only after successfully verifying that the person performing the search is authorised to access assets on the domain.

Nothing is explicitly logged by the website. The only logging of any kind is via Google Analytics, Application Insights performance monitoring and any diagnostic data implicitly collected if an exception occurs in the system.