Today, June 1st 2017, WikiLeaks publishes documents from the "Pandemic"
project of the CIA, a persistent implant for Microsoft Windows machines
that share files (programs) with remote users in a local network.
"Pandemic" targets remote users by replacing application code on-the-fly
with a trojaned version if the program is retrieved from the infected
machine. To obfuscate its activity, the
original file on the file server remains unchanged; it is only
modified/replaced while in transit from the pandemic file server before
being executed on the computer of the remote user. The implant allows
the replacement of up to 20 programs with a maximum size of 800 MB for a
selected list of remote users (targets).
As the name suggests, a single computer on a local network with shared
drives that is infected with the "Pandemic" implant will act like a
"Patient Zero" in the spread of a disease. It will infect remote
computers if the user executes programs stored on the pandemic file
server. Although not explicitly stated in the documents, it seems
technically feasible that remote computers that provide file shares
themselves become new pandemic file servers on the local network to
reach new targets.