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, 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. 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. However, in 2010, Jailbreaking was declared officially legal in the United States by the DMCA. Jailbroken iPhones may be susceptible to computer viruses, but few such incidents have been reported.