Millet performs I/O to read arbitrary user input and, in the case of formatting only, also overwrite user files.
When using smlfmt as the formatting engine, it searches for a binary called smlfmt in your PATH and executes it.
Millet should never access the network.
A user could probably "DOS" themselves by asking Millet to analyze a massive or complicated set of files, but we don't really consider this to be a security threat.
That being said, if there is a security issue with Millet, let us know at:
ariel DOT z DOT davis AT icloud DOT com