The US Copyright Office has given the green light to the practice, saying jailbreaking any smartphone to make the operating system on that phone interoperable with an independently created application is fair use.
The Copyright Office also refuted Apple's contention that iPhone buyers are licensees of the technology, not owners and thereby bound by the company's licensing agreement.
In the European Union the distinction between whether a purchaser "buys" or "licenses" the software in an Apple iPhone is a lot less relevant, said Dai Davis, partner and IT legal expert at law firm Brooke North.
But even if the software is supplied as a licence, EU law clearly provides strong rights to a buyer to require Apple to supply interoperability information, he said.
This is provided by the EU Directive 2009/24/EC of 23 April 2009 on the legal protection of computer programs.
Merely changing a minimal number of lines of code does not necessarily mean that a breach of copyright has occurred, said Davis.
"In this case, altering 50 bytes of code out of more than eight billion bytes in an iPhone is most unlikely to be regarded as a breach of copyright," he said.
This is particularly the case since no copy of the software is being made, merely a change in the iPhone firmware.
"By trying to preclude unauthorised third-party apps, Apple may be in breach of both EU and UK competition laws," said Davis.
This story was first published by Computer Weekly