Apple is pushing ahead with a lawsuit against Corellium, in what is being labelled as a move to end jailbreaking.
The tech giant began its legal battle in August, claiming the firm profits from "perfect replicas" of iOS.
Apple has now amended its lawsuit, alleging copyright infringement.
Corellium's chief executive Amanda Gorton responded in an open letter, in which she said developers and jailbreakers should be "concerned" by the move.
Jailbreaking is a process by which Apple's operating systems are modified to remove restrictions and give greater control to the user.
It also allows users to install apps that haven't been approved by Apple, and gives the option to customise the interface in various ways.
Corellium specialises in virtualising iOS software, which it says allows security researchers to discover bugs within the platform.
However, Apple has accused the firm of encouraging users to sell any flaws they discover to the open market for the highest price.
Corellium says Apple's allegations are "deeply disappointing" and a misrepresentation of the jailbreaking community's goals.
"Corellium's conduct plainly infringes Apple's copyrights," the tech giant said in a statement. "It simply copies everything: the code, the graphical user interface, the icons - all of it, in exacting detail.
"This is not a case in which it is questionable or unclear whether the defendant reproduced the rights-owner's works, or more subtly, whether particular portions of the works that the defendant took are ultimately protected by federal copyright law."
Corellium's chief executive issued a statement on Sunday in response to Apple's latest claims of copyright infringement.
She criticised Apple's targeting of the jailbreaking community, and suggested the company has failed to acknowledge that it has directly benefited from users of its platform in the past.
"Apple is asserting that anyone who provides a tool that allows other people to jailbreak, and anyone who assists in creating such a tool, is violating the DMCA (Digital Millennium Copyright Act)," said Ms Gorton.
"They have directly benefited from the jailbreak community in a number of ways. Many of the features of iOS originally appeared as jailbreak tweaks and were copied by Apple."
Apple's latest move comes ahead of the company's surprise return to the Las Vegas CES technology conference next month for the first time in decades.
Senior director of privacy Jane Horvath will discuss privacy issues, regulation and consumer demands, marking Apple's first appearance at CES since 1992.