Third-Party License

You do not need permission to publish adventures, supplements, hacks, or other material for this game. You may freely reference the rules, mechanics, and terminology of this game when creating compatible material. No approval, endorsement, or notification is required.

The game is released under Creative Commons Attribution-ShareAlike 4.0 (CC-BY-SA 4.0). The source files are available here.

FAQ

Do I need permission to publish an adventure or supplement

No.