The Open Web Application Security Project (OWASP) is a community, founded in 2001, which produces and makes available for free articles, methodologies, tools...
Every year, it publishes the Top 10 Web security vulnerabilities. It publishes the OWASP Testing Guide: a guide to best practices in depentesting.
It publishes the OWASP Development Guide: a guide to writing code without security holes. Official website: https://www.owasp.org

Mitre is the organization, funded by the United States Defense Department, which has implemented and maintains the CVE referencing (Common Vulnerabilities and Exposures). https://en.wikipedia.org/wiki/Mitre_Corporation

A CVE, for Common Vulnerabilities and Exposures, is a reference for a security flaw.
https://en.wikipedia.org/wiki/Common_Vulnerabilities_and_Exposures