How to understand bash vulnerability?

[Peng Yu]

Hi,

https://securityintelligence.com/articles/shellshock-vulnerability-in-depth/

The above link shows the following three commands to test whether Bash
has a vulnerability.

env X="() { :;} ; echo Bash is Infected" /bin/sh -c "echo completed"
env X="() { :;} ; echo Bash is Infected" `which bash` -c "echo completed"
env VAR='() { :;}; echo Bash is Infected' bash -c "echo completed"

But I don't have an old version of bash installed that has this
vulnerability. Could anybody let me know what the problem was and how
to understand the above code? Thanks.

-- 
Regards,
Peng