The Phoenix Shield was a security firewall used to protect computers and/or networks. It was described as a poison pill firewall that performed a cold start erase of the hard drive it was protecting if anyone attempted to break into it, resulting in the loss of all data.
Day 5 Edit
During Day 5, Christopher Henderson informed a group of CTU agents in briefing in the Situation Room that Joseph Malina's computer system was protected by a Phoenix Shield. Jack Bauer was confident CTU could break in, but Chloe informed him it would be impossible.
When Henderson went to Malina's apartment, he immediately revealed that he was working with CTU, in an attempt to get Malina to lift the Phoenix Shield and dump his data onto a flash drive. However, CTU raided the apartment before that happened, so Chloe had to decrypt the data from the mainframe. ("Day 5: 4:00am-5:00am")