GNU Bash has been widely adopted because it is a free (as in freedom), reliable, and featureful shell. This popularity means the serious bug that was published yesterday is just as widespread. Fortunately, GNU Bash's license, the GNU General Public License version 3, has facilitated a rapid response. It allowed Red Hat to develop and share patches in conjunction with Bash upstream developers efforts to fix the bug, which anyone can download and apply themselves. Everyone using Bash has the freedom to download, inspect, and modify the code -- unlike with Microsoft, Apple, or other proprietary software.
Software freedom is a precondition for secure computing; it guarantees everyone the ability to examine the code to detect vulnerabilities, and to create new and safe versions if a vulnerability is discovered. Your software freedom does not guarantee bug-free code, and neither does proprietary software: bugs happen no matter how the software is licensed. But when a bug is discovered in free software, everyone has the permission, rights, and source code to expose and fix the problem. That fix can then be immediately freely distributed to everyone who needs it. Thus, these freedomsare crucial for ethical, secure computing.
Read more at the Free Software Foundation.