On Thursday, security experts warned that Bash contained a particularly alarming software bug that could be used to take control of hundreds of millions of machines around the world, potentially including Macintosh computers and smartphones that use the Android operating system.

The bug, named “Shellshock,” drew comparisons to the Heartbleed bug that was discovered in a crucial piece of software last spring.

But Shellshock could be a bigger threat. While Heartbleed could be used to do things like steal passwords from a server, Shellshock can be used to take over the entire machine. And Heartbleed went unnoticed for two years and affected an estimated 500,000 machines, but Shellshock was not discovered for 22 years.
— http://www.nytimes.com/2014/09/26/technology/security-experts-expect-shellshock-software-bug-to-be-significant.html
"When half the net is running Apache (which is typically found on Linux), that’s a significant size of a very, very large pie. That same Netcraft article is reporting that we’ve just passed the one billion websites mark too and whilst a heap of those are sharing the same hosts, that’s still a whole lot of Bash installations. Oh – that’s just web servers too, don’t forget there are a heap of other servers running Linux and we’ll come back to other devices with Bash a bit later too."

"When half the net is running Apache (which is typically found on Linux), that’s a significant size of a very, very large pie. That same Netcraft article is reporting that we’ve just passed the one billion websites mark too and whilst a heap of those are sharing the same hosts, that’s still a whole lot of Bash installations. Oh – that’s just web servers too, don’t forget there are a heap of other servers running Linux and we’ll come back to other devices with Bash a bit later too."