Do you perform hacker tests or security audits, and do you close security vulnerabilities?

Yes. At POLYAS, penetration tests are performed every year by independent partners (Physikalisch-Technische Bundesanstalt, ISICO GmbH and Micromata GmbH). Any security loopholes identified are closed. Reports can be viewed on request, but some parts of it need to be blacked out to avoid disclosing product secrets.