Safe from Shellshock: How to protect your home computer from the Bash shell bug

Brad Chacos
29 September, 2014
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Shellshock-malware-macworld-australiaOn the surface, the critical ‘Shellshock’ bug revealed last week sounds devastating. By exploiting a bug in the Bash shell command line tool found in Unix-based systems, attackers can run code on your system – essentially giving them access to your system. Bad guys are already developing exploits that use Shellshock to crack your passwords and install DDoS bots on computers. And since Bash shell is borderline ubiquitous, a vast swath of devices are vulnerable to Shellshock: Macs, Linux systems, routers, web servers, ‘Internet of Things’ gizmos, you name it.

Yeah, it sounds bad.

But really, the impact on you at home should be minimal, especially if you take some basic precautions. Windows systems aren’t vulnerable whatsoever – though your router may very well be – unless you’re running a program like Cygwin.

How to determine if your computer is vulnerable to Shellshock

Before we dive in, let’s quickly talk about determining whether or not your system is running a vulnerable version of Bash shell. (If you’re running a modern version of OS X or Linux, it probably is.)

Simply open the Terminal on your computer and type in the following:

env x=’() { :;}; echo vulnerable’ bash -c “echo this is a test”

If your system is vulnerable to the Bash bug, you’ll see the following:

vulnerable

this is a test

If your system has already been patched to protect against the bug, on the other hand, you’ll see something similar to this:

$ env x=’() { :;}; echo vulnerable’ bash -c “echo this is a test” bash: warning: x: ignoring function definition attempt bash: error importing function definition for `x’ this is a test

How to keep your computer safe from the Shellshock bug

Oh no! Your system is still vulnerable to Shellshock! What should you do now?

Nothing drastic, if you’re an average computer user. If your computer is tucked safely behind a firewall – as it should be – the impact on you should be minimal, since attackers won’t have any way to execute malicious code through the Bash shell on your system unless they trick you into running the command locally somehow. Shellshock is more dangerous for web servers and devices that ‘listen’ for internet commands than home PCs.

Apple drove that point home in its response to the Shellshock bug, which was provided to iMore:

“The vast majority of OS X users are not at risk to recently reported bash vulnerabilities… With OS X, systems are safe by default and not exposed to remote exploits of bash unless users configure advanced UNIX services. We are working to quickly provide a software update for our advanced UNIX users.”

If you are one of those advanced Unix users on a Mac, this StackExchange thread can show you how recompile Bash with Xcode to plug the bug immediately.

shellshock command diagram 600px v2 symantec

Symantec. Symantec's visual breakdown of how the Shellshock Bash bug works.

 

If you’re running Linux, most of the big-name distributions have already released updates that patch Shellshock, including Red HatUbuntuDebianFedoraCentOS and more. Be warned, however, that while this critical update mostly plugs the Shellshock, it is still considered incomplete, as Red Hat explains:

“Red Hat is aware that the patch for CVE-2014-6271 is incomplete. An attacker can provide specially-crafted environment variables containing arbitrary commands that will be executed on vulnerable systems under certain conditions… We are working on patches in conjunction with the upstream developers as a critical priority… Red Hat advises customers to upgrade to the version of Bash which contains the fix for CVE-2014-6271 and not wait for the [additional] patch.”

Beyond your computer’s operating system, many internet-enabled devices are vulnerable to Shellshock – including network gear. Check your router manufacturer’s website and make sure your firmware is up to date.

The bottom line

Don’t panic! Shellshock isn’t the end of the world.

But if you’re running Linux or OS X, install the newest security updates as soon as possible. Make sure your networking gear is running the latest available firmware as well. (Check back on your router manufacturer’s website over the coming days if there’s nothing available now). And definitely be on the lookout for malicious emails that try to convince you to run software locally, or attempt to play off Shellshock fears to phish your personal information or login credentials to services. Big scares like this always bring the creeps out of the woodwork.

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