The Hedera services codebase is officially open source and can be found in the official GitHub repository: https://github.com/hashgraph/hedera-services You can read more about the open sourcing of Hedera services in this blog posting: https://www.hedera.com/blog/all-our-services-are-belong-to-you 

The hashgraph platform is available as open review and has been audited by multiple code audit firms. The results of these audits are expected to be made available in late Q4 2020.

Open review ensures the source code is made available for anyone to read, recompile, and verify that it is correct. For more information regarding open review of the hashgraph source code, please read Dr. Leemon Baird’s blog posting on Hedera.com