Nicholas Lee Jacobsen was arrested in October on suspicion of accessing the personal information of potentially hundreds of T-Mobile customers.
Under the initial charges Jacobsen faced two felony charges. That will now be reduced to a single charge of intentional access to a protected computer and recklessly causing damage as part of his guilty plea. The hacker could face up to five years in prison and a $250,000 fine.
The US Government said Jacobsen was able to read email and personal files of hundreds of customers on the wireless network. At one stage he even stumbled upon files of Peter Cavicchia, one of the Secret Service agents investigating him. He also read a Secret Service memo and other sensitive information regarding the US Government.
Jacobsen's arrest came as a result of US secret Service investigation into three alleged hackers. His sentence will be passed on May 16.
T-Mobile is one of the major players in the world of mobile working and serves over 16 million customers in the US alone.