Security experts discovered a bug in Bash — which has been named “Shellshock” — that could be used to compromise systems that utilize the Bash shell. The bug dates back to Version 1.13. Bourne-Again Shell, or Bash, is software that is built into nearly 70 percent of machines connected to the Internet. Bash has been widely used for the GNU operating system, as well as a default shell on Linux since 1989. It is also extensively used as the GNU shell for Mac OS X. Learn more.