The freedom to audit. In addition to being free and forkable, open-source systems also have to be accountable:
“The open-source movement champions an approach to product development in which there is universal access to a blueprint, as well as universal ability to modify and redistribute the blueprint. Wikipedia is perhaps the best-known example of a product inspired by the movement. Open-source advocates typically emphasize two kinds of freedom that their products afford: they are available free of charge, and they can be used and manipulated free of restrictions.
“But there is a third kind of freedom inherent in open-source systems: the freedom to audit. With open-source software, independent security experts can scrutinize the code for vulnerabilities — whether accidentally or intentionally introduced. The more auditing by the programming masses, the better the security. As the open-source software advocate Eric S. Raymond has put it, ‘given enough eyeballs, all bugs are shallow.'”