Am sure everyone’s heard of the Bash Shellshock bug by now. Basically, Bash (like most other shells) has environment variables. When launching Bash you can set an environment variable too, and the newly launched Bash process will inherit that variable. So far so good, but what researchers discovered is that due to the bug you can also set environment variables such that you sneak in some commands into them; and the resulting Bash process will execute these commands for you. That’s wicked because you (the admin) might not want users to run such commands, or worse the Bash process might be running with additional rights in which case the sneaked in commands too will run with these additional rights.
There’s a lot of hue and cry over this and blame assigned to the GNU project and its creator Richard Stallman, but I liked this post by Andrew Auernheimer. The whole post is a must read but here’s the bit I would like to highlight (emphasis mine):
Shellshock is not a critical failure in bash. It is a critical failure in thousands of people who knew a tool so useful that they decided to deploy it far beyond its scope. A tool so resilient that it it did not fall over when everyone deployed against best practices. Everyone knew in the nineties that when you execute a UNIX command with untrusted input, you clear away the environment variables first. Anyone that has untrusted input embedded within a shell script does not know what they are doing. The fact that there is a way to get bash to execute untrusted code is unsurprising. The thing that surprises me is the sheer number of developers who thought it would be otherwise in complete contrast to UNIX parables and common sense.
Here’s a blog post on clearing the environmental variables. More techniques are in this StackOverflow post. (I haven’t made Bash scripts for a long time now, so I just got these via a quick DuckDuckGo search. There might be other/ better techniques around!)
ps. Since posting this I came across a post on the bad code state of Bash. It doesn’t take away the fact that Bash isn’t solely to be blamed for Shellshock but I thought to mention it because I found it an interesting read.