We support the last 3 minor app versions. For example, if 3.15.5 is the latest version, the supported versions are:

• 3.15.5, 3.15.4, 3.15.3, 3.15.2, 3.15.1, 3.15.0
• 3.14.2, 3.14.1, 3.14.0
• 3.13.0

All supported versions can be found in official deb and rpm repositories. Once a new minor version comes out, the oldest one is removed from the repositories.

See https://hackerone.com/nordsecurity?type=team for how to report a vulnerability.