OWASP Open Web Application Security Project
A non-profit that maintains "OWASP Top 10" — a regularly updated list of the most common web vulnerabilities.
Latest list (2021): broken access control, cryptographic failures, injection (incl. SQL injection), insecure design, security misconfiguration, vulnerable dependencies, authentication failures, data integrity failures, logging/monitoring failures, SSRF.
If you build web applications, you should know them. Many security scans (including the one included with Thern AI Solutions design services) specifically test against OWASP Top 10.