The change has occurred thanks to the latest review of the Digital Millennium Copyright Act (DMCA), which is conducted every three years by the Librarian of Congress.
This week, acting Librarian David Mao released his office's latest list of exemptions to the act, two of which apply to car drivers.
Firstly, the US government will now allow owners to modify their car's software, so long as these modifications don't end up causing the car to violate any applicable laws, such as the Department of Transport's regulations or the Environmental Protection Agency's rules on emissions.
While this change will now allow owners to legally modify how, say, a car's engine runs, the Library of Congress still prohibits the modification of the software in charge of a car's "telematics or entertainment systems".
These latest changes to the application of the DMCA are said to be relevant to personal cars, commercial vehicles and agricultural machinery.
Another exemption issued by the library relates to "good-faith testing for and the identification, disclosure and correction of malfunctions, security flaws and vulnerabilities in computer programs", which clears up a legal grey area that's long concerned security researchers.
This good news for car modifiers and security personnel is tempered by the fact that these exemptions will not come into effect for a year.
According to The Verge, these latest changes to the DMCA were opposed to varying to degrees by GM, the Association of Global Automakers, the Motor and Equipment Manufacturers Association, the Department of Transport, the Environmental Protection Agency, and the California Air Resources Board.
You can read the full set of changes to the DMCA here.