A former Massachusetts prison inmate has been given an 18-month prison sentence for hacking prison computers while he was incarcerated.Francis "Frank" Janosko, 44, was sentenced Tuesday in Federal court in Boston for abusing a computer provided by the Plymouth County Correctional Facility. The computer had been set up to help inmates with their legal research.In 2006, Janosko managed to circumvent computer controls and use the machine to send e-mail and cull data on more than 1,100 Plymouth County prison employees. He gained access to sensitive information such as their dates of birth, Social Security Numbers, telephone numbers, home addresses and employment records.The computer he used was a so-called thin client computer that simply connected to another machine on the network and did not store any data itself, prosecutors said in Janosko's indictment. The only program it was supposed to run was the prison's legal research application.However, Janosko found a way of "exploiting an idiosyncrasy in the legal research software" so he could access other programs via the terminal. He even found a way of downloading Internet video, prosecutors said.Following his latest prison sentence, Janosko will serve three years of supervised release, during which he is prohibited from using any Internet-connected devices without the approval of his probation officer. Whether he will face any such restrictions in prison is unclear. The U.S. Department of Justice could not be reached for comment late Tuesday.Janosko pleaded guilty to one count of damaging a protected computer on Sept. 15.