I’m a big fan of GraphQL but as with every technology it comes with it's own set of (security) challenges.
To properly secure your GraphQL API, I can recommend to check the GraphQL Cheat Sheet. It handles common attack vectors and best practices to avoid them.
If you have never heard about the OWASP Cheat Sheet Series, also have a look at all the other sheets in the series.
A must read for every developer!