Under US copyright law the "preparation of a derivative work" (in this context more or less making a change to the code base) is one of the six exclusive rights

Under US copyright law the "preparation of a derivative work" (in this context more or less making a change to the code base) is one of the six exclusive rights under the control of the copyright holder [1]. 1. https://www.law.cornell.edu/uscode/text/17/106 https://www.law.cornell.edu/uscode/text/17/106