CUPS Filters is developed by OpenPrinting and distributed as open source software under the Apache License, Version 2.0 with exceptions to allow linking to GPL2/LGPL2 code (same license as for CUPS).
Contributions should be submitted either as pull requests or as attachments (unified diffs) to bug reports on the OpenPrinting CUPS GitHub project at https://github.com/openprinting/cups-filters.