The iPhone's operating system is designed to only run software that has an Apple-approved cryptographic signature. This restriction can be overcome by "jailbreaking" the phone,[187] which involves replacing the iPhone's firmware with a slightly modified version that does not enforce the signature check. Doing so may be a circumvention of Apple's technical protection measures.[188] Apple, in a statement to the United States Copyright Office in response to Electronic Frontier Foundation (EFF) lobbying for a DMCA exception for this kind of hacking, claimed that jailbreaking the iPhone would be copyright infringement due to the necessary modification of system software.[189] However, in 2010, Jailbreaking was declared officially legal in the United States by the DMCA.[190] Jailbroken iPhones may be susceptible to computer viruses, but few such incidents have been reported.[191][192]