Well, this isn't good. Akamai security researcher Stephane Chazelas has discovered a devastating flaw in the Unix Bash shell, leaving Linux machines, OS X machines, routers, older IoT devices, and more vulnerable to attack. "Shellshock," as it's been dubbed, allows attackers to run deep-level shell commands on your machine after exploiting the flaw, but the true danger here lies in just how old Shell Shock is—this vulnerability has apparently been lurking in the Bash shell for years.

Why this matters: A large swath of the web-connected devices, web servers, and web-powered services run on Linux distributions equipped with the Bash shell, and Mac OS X Mavericks is also affected. The fact that Shellshock's roots are so deep likely means that the vulnerability will still be found in unpatched systems for the foreseeable future—though the odds of it directly impacting you appear somewhat slim if you use standard security precautions.

Update: Security researchers are already finding evidence of the Shellshock Bash bug being exploited in the wild, according to ZDNet. One exploit attempts to install a denial-of-service attack bot and guess the login information for affected servers using a list of commonly used passwords.

Heartbleed redux

The news comes as the security community is just shaking off the effects of Heartbleed, a critical vulnerability in the widely used OpenSSL security protocol. "Today's bash bug is as big a deal as Heartbleed," says Errata Security's Robert Graham, a respected researcher.

Hold your horses, Robert. Before we dive into dire warnings, let's focus on the positive side of this story. Numerous Linux variants have already pushed out patches that plug Shellshock, including Red Hat, Fedora, CentOS, Ubuntu, and Debian, and big Internet services like Akamai are already on the case.

For the record, cygwin is vulnerable, as well as bash-3.1 from 2005 pic.twitter.com/yYtVnPlTAJ — Robert Graham (@ErrataRob) September 25, 2014

But Graham says Shellshock's danger will nevertheless linger for years, partly because "an enormous percentage of software interacts with the shell in some fashion"—essentially making it impossible to know exactly how much software is vulnerable—and partly because of the vulnerability's age.

"Unlike Heartbleed, which only affected a specific version of OpenSSL, this bash bug has been around for a long, long time. That means there are lots of old devices on the network vulnerable to this bug. The number of systems needing to be patched, but which won't be, is much larger than Heartbleed."

Now consider that more than two months after Heartbleed was disclosed, hundreds of thousands of systems remained vulnerable to the exploit.

Maybe not Heartbleed redux?

But don't panic! (Or at least not yet.) While Heartbleed had the potential to be widely exploited, Jen Ellis of security firm Rapid7 says the Shellshock bug's outlook isn't quite as grim, even if it is rampant.

"The vulnerability looks pretty awful at first glance, but most systems with Bash installed will NOT be remotely exploitable as a result of this issue," Ellis writes. "In order to exploit this flaw, an attacker would need the ability to send a malicious environment variable to a program interacting with the network and this program would have to be implemented in Bash, or spawn a sub-command using Bash."

As a result, Ellis and Rapid7 urge keeping a level head about the bug.

"We’re not keen to jump on the 'Heartbleed 2.0' bandwagon. The conclusion we reached is that some factors are worse, but the overall picture is less dire… there are a number of factors that need to be in play for a target to be susceptible to attack. Every affected application may be exploitable through a slightly different vector or have different requirements to reach the vulnerable code. This may significantly limit how widespread attacks will be in the wild. Heartbleed was much easier to conclusively test and the impact way more widespread."

While older Internet-connected devices (like, say, security cameras) seem to be likely victims of Shellshock, respected security researchers Michal Zalewski and Paul McMillan note that many embedded devices don't actually use the Bash shell at all.

The #shellshock bash bug is not the end of the embedded world. Most embedded devices use busybox, which is not vulnerable. — Paul McMillan (@PaulM) September 25, 2014

How to tell if you're vulnerable

Beyond Linux-based systems, Graham and Ars Technica report that Mac OS X Mavericks contains a vulnerable version of Bash.

To test if your version of Bash is vulnerable to this issue, Red Hat says to run this command:

$ env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

If the system responds with the following, then you're running a vulnerable version of Bash and you should apply any available updates immediately:

vulnerable this is a test

"The patch used to fix this issue ensures that no code is allowed after the end of a Bash function," Red Hat reports. So rather than spitting out "Vulnerable," a protected version of Bash will spit out the following when you run the aforementioned command:

$ 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

What does this mean?

When it gets down to brass tacks, most major websites and modern gadgets you own likely won't be affected by this Bash vulnerability, and Apple will no doubt patch the OS X implementation quickly. (Here's a highly technical DIY fix for now.)

It's impossible to know just how far this flaw reaches, and it's likely to linger on in neglected websites, older routers, and some legacy Internet of Things devices—many of which are impossible to patch—providing an opening for determined hackers to sneak into those systems.

So what should you do? Here's some actionable advice from security researcher Troy Hunt's tremendous in-depth primer on Shellshock:

"In short, the advice to consumers is this: watch for security updates, particularly on OS X. Also keep an eye on any advice you may get from your ISP or other providers of devices you have that run embedded software. Do be cautious of emails requesting information or instructing you to run software – events like this are often followed by phishing attacks that capitalize on consumers’ fears."

PCWorld's guide to protecting your PC against devious security traps can help you I.D. bad actors, while Ian Paul has three tips for spotting malicious emails over at his Hassle-Free PC column.

This article was originally published at 9:21 AM and was updated later to include information about the bug being exploited in the wild.