From Halopedia, the Halo wiki
Secure Network, more commonly referred to as SecNet, was a UNSC communications network. It could relay text messages, similar to an instant messaging program. SecNet users were identified by their username while using the network.
Sometime after the Fall of Reach, the Office of Naval Intelligence conducted an investigation to determine whether a SecNet message exchange between SGT Alasdair R. Schicker and SGT Joseph M. Poteet could have alerted the Covenant to the location of Reach.