The Shadowserver Foundation is a registered 501(c)3 tax-exempt non-profit organization in the US (EIN: 26-2267933) and Stichting The Shadowserver Foundation Europe is a registered tax-exempt non-profit organization with public benefit status in The Netherlands (KvK Number 61199613).
You can verify an organization’s current 501(c)3 status on the IRS website.
Note that during the 2020 COVID-19 pandemic, the data returned by the IRS web search may not be up to date. If you receive an auto-revocation warning such as “Important note: Just because an organization appears on this list, it does not mean the organization is currently revoked, as they may have been reinstated” then you can check the current underlying source data in the IRS’s Exempt Organizations Business Master File Extract (EO BMF). Download the current state of California data and confirm the active 501(c)3 tax-exempt status there, since that master data is updated daily.