From doing some cursory research, it appears the software in question is called MOSS (Measure of Software Similarity) and is currently being provided as a service [0].

Since it is intended to be used by instructors and staff, the source is restricted (though "anyone may create a MOSS account"). According to the paper describing how it's used [1], "False positives have never been reported, and all false negatives were quickly traced back to the source, which was either an implementation or a user misunderstanding."

Sources:

[0]: https://theory.stanford.edu/~aiken/moss

[1]: http://theory.stanford.edu/~aiken/publications/papers/sigmod...