Contributor(s): Madelyn Bacon

Shellshock is the common name for a coding vulnerability found in the Bash shell user interface that affects Unix-based operating systems, including Linux and Mac OS X, and allows attackers to remotely gain complete control of a system.

Discovered by Stéphane Chazelas in September 2014, the vulnerability, also known as CVE-2014-6271 and CVE-2014-7169, had existed for more than 20 years. Shellshock is present in every version of shell up to 4.3.

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The Shellshock flaw may be exploited without any authentication by adding arbitrary malicious code at the end of a specifically crafted Bash function. This technique could enable an attacker to gain command-line access to a system, which often results unrestricted access to run programs, filter through memory for sensitive data, or facilitate a self-propagating worm.

Most affected server and operating system providers have released software updates that correct the Shellshock vulnerability. A variety of tools exist to check whether a system is affected by Shellshock or whether a patch has successfully resolved the problem. Organizations should use log monitoring techniques to detect evidence of attempted Shellshock exploitation; such a payload is delivered through a URL or HTTP header, hence it would leave evidence.

US-CERT's National Vulnerability Database rated the flaw's severity as a 10.0. It has been compared to the Heartbleed vulnerability largely because of its severity rating.

This was last updated in December 2014

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