Computer virus designed by Dr. Jay Felger, with the aid of his lab assistant Chloe and Samantha Carter. Avenger was designed to infiltrate the Ancients' Stargate technology, specifically the dial-home device, and essentially scramble the gate symbols, thereby making all Stargate addresses invalid and rendering the target gate useless. Felger launched the program at Baal's Stargate on P5S-117, where Baal discovered the virus and modified it -- into Avenger "2.0."
Baal's remade virus caused it to be transmitted to other Stargates during the system's periodic correlative updates, eventually immobilizing the entire gate network. The System Lord did this to gain a tactical advantage over his enemy Goa'uld.
Felger and Carter discovered this when they visited 117 to correct the problem. They reformatted the virus, adding a patch (to prevent tampering) and retransmitting it to the rest of the Stargate network via the correlative update process.
Avenger 2.0 - In a last-ditch effort to save his job, Jay Felger creates Avenger and sends it off to a Goa'uld mining world. Baal discovers the virus and modifies it to give him an advantage over the rest of the Goa'uld.