The Shellshock vulnerability can be exploited on systems that are running Services or applications that allow unauthorized remote users to assign Bash environment variables. Examples of exploitable systems include the following:

• Apache HTTP Servers that use CGI scripts (via mod_cgi and mod_cgid) that are written in Bash or launch to Bash subshells
• Certain DHCP clients
• OpenSSH servers that use the ForceCommand capability
• Various network-exposed services that use Bash

A detailed description of the bug can be found at CVE-2014-6271 and CVE-2014-7169.

Because the Shellshock vulnerability is very widespread–even more so than the OpenSSL Heartbleed bug–and particularly easy to exploit, it is highly recommended that affected systems are properly updated to fix or mitigate the vulnerability as soon as possible. We will show you how to test if your machines are vulnerable and, if they are, how to update Bash to remove the vulnerability.

Note: (Sept. 25, 2014 – 6:00pm EST) At the time of writing, only an “incomplete fix” for the vulnerability has been released. As such, it is recommended to update your machines that run Bash immediately, and check back for updates and a complete fix.

Check System Vulnerability

On each of your systems that run Bash, you may check for Shellshock vulnerability by running the following command at the bash prompt:

env VAR='() { :;}; echo Bash is vulnerable!' bash -c "echo Bash Test"

The highlighted echo Bash is vulnerable! portion of the command represents where a remote attacker could inject malicious code; arbitrary code following a function definition within an environment variable assignment. Therefore, if you see the following output, your version of Bash is vulnerable and should be updated:

Bash is vulnerable!
Bash Test

Otherwise, if your output does not include the simulated attacker’s payload, i.e. “Bash is vulnerable” is not printed as output, your version of bash is not vulnerable. It may look something like this:

bash: warning: VAR: ignoring function definition attempt
bash: error importing function definition for `VAR'
Bash Test

If your version of Bash is vulnerable, read on to learn how to update Bash and fix the vulnerability.

Test Remote Sites

If you simply want to test if websites or specific CGI scripts are vulnerable, use this link:‘ShellShock’ Bash Vulnerability CVE-2014-6271 Test Tool.

Simply enter the URL of the website or CGI script you want to test in the appropriate form and submit.

Fix Vulnerability: Update Bash

The easiest way to fix the vulnerability is to use your default package manager to update the version of Bash. The following subsections cover updating Bash on various Linux distributions, including Ubuntu, Debian, CentOS, Red Hat, and Fedora.

Note: (Sept. 25, 2014 – 6:00pm EST) At the time of writing, only an “incomplete fix” for the vulnerability has been released. As such, it is recommended to update your machines that run Bash immediately, and check back for updates and a complete fix.

APT-GET: Ubuntu / Debian

Update Bash to the latest version available via apt-get:

sudo apt-get update && sudo apt-get install --only-upgrade bash

Now check your system vulnerability again by running the command in the previous section (Check System Vulnerability).

YUM: CentOS / Red Hat / Fedora

Update Bash to the latest version available via the yum:

sudo yum update bash

Now check your system vulnerability again by running the command in the previous section (Check System Vulnerability).

Conclusion

Be sure to update all of your affected servers to the latest version of Bash!