Apple released an update for OS X Lion, OS X Mountain Lion, and OS X Mavericks that fixes a security flaw in the bash UNIX shell exploited by a hack called Shellshock. While Shellshock only affects Mac users who manually set up advanced UNIX services, it’s still a serious enough flaw for Apple to address.
Available for: OS X Lion v10.7.5, OS X Lion Server v10.7.5, OS X Mountain Lion v10.8.5, OS X Mavericks v10.9.5
Impact: In certain configurations, a remote attacker may be able to execute arbitrary shell commands
Description: An issue existed in Bash’s parsing of environment variables. This issue was addressed through improved environment variable parsing by better detecting the end of the function statement.
This update also incorporated the suggested CVE-2014-7169 change, which resets the parser state. In addition, this update added a new namespace for exported functions by creating a function decorator to prevent unintended header passthrough to Bash. The names of all environment variables that introduce function definitions are required to have a prefix “__BASH_FUNC< " and suffix ">()” to prevent unintended function passing via HTTP headers.
Source: About OS X bash Update 1.0