TL;DR: run apt-get update; apt-get upgrade
Then keep doing often over the next few days...
If you're on MacOS, here's a good guide on How to patch OSX in the wake of shellshock
"Remember Heartbleed? If you believe the hype today, Shellshock is in that league and with an equally awesome name albeit bereft of a cool logo (someone in the marketing department of these vulns needs to get on that). But in all seriousness, it does have the potential to be a biggie and as I did with Heartbleed, I wanted to put together something definitive both for me to get to grips with the situation and for others to dissect the hype from the true underlying risk."
"Which versions of Bash are affected?
The headlines state everything through 4.3 or in other words, about 25 years’ worth of Bash versions. Given everyone keeps comparing this to Heartbleed, consider that the impacted versions of OpenSSL spanned a mere two years which is a drop in the ocean compared to Shellshock. Yes people upgrade their versions, but no they don’t do it consistently and whichever way you cut it, the breadth of at-risk machines is going to be significantly higher with Shellshock than what it was with Heartbleed.
But the risk may well extend beyond 4.3 as well. Already we’re seeing reports of patches not being entirely effective and given the speed with which they’re being rolled out, that’s not all that surprising. This is the sort of thing those impacted by it want to keep a very close eye on, not just “patch and forget”."
heroku are (at this time of writing) working to resolve it on their servers: heroku shellshocked status page