As per CVE-2014-6271, a flaw in bash that permits malicious shell code injection was found.

Here is a code snippet to check if your linux server is vulnerable:

env x='() { :;}; echo *VULNERABLE*' bash -c "echo this is a test"

Example output if system is vulnerable:

*VULNERABLE*
this is a test

Output with patched bash will instead look as follows:

this is a test

source: https://access.redhat.com/articles/1200223