Very secure. Simply put; We need to stick to the highest standards possible because we're located in the E.U.
Our privacy and security standards are compliant with the high level of personal data protection required by the Dutch Personal Data Protection Act (Wet bescherming persoonsgegevens), based on the E.U. Privacy Directive (95/46/EC).
Your files are encrypted when they are being transferred (TLS) and when they are stored (AES-256). Once your files are safely stored, they can only be accessed using the unique links sent to the sender and recipient.
On top of all of that, we are compliant with EU privacy law, which makes us abide by stricter rules than US regulations would.