Avenger is a computer program, named after a comic book character, developed by Jay Felger, a scientist based at the SGC.
The Avenger program was developed as a virus, designed to affect a DHD. The idea behind it was that the virus, uploaded into the target Stargate, would make the symbols on the DHD no longer correspond with other Stargate’s coordinates, rendering the DHD and Stargate useless. The virus was sent to P5S-117, a planet occupied by the System Lord Ba’al. When the entire gate network was affected, it was thought that the virus had moved through the gates through automatic correlative updates designed to compensate for stellar drift. When Felger and Carter went to the planet to fix the problem, Felger realised that the virus wasn’t the virus he had written, and Carter suggested that, because he had the most to gain, Ba’al had modified the virus to affect every Stargate. An anti-virus was uploaded, and a patch added, to make sure that no one could modify the virus in the future.