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'''Shellshock''', also known as '''Bashdoor''',<ref name="NYT-20140925-NP">{{cite news |last=Perlroth |first=Nicole |title=Security Experts Expect 'Shellshock' Software Bug in Bash to Be Significant |url=https://www.nytimes.com/2014/09/26/technology/security-experts-expect-shellshock-software-bug-to-be-significant.html |date=25 September 2014 |work=[[New York Times]] |access-date=25 September 2014 }}</ref> is a family of [[security bug]]s<ref name="TSM-20140927">Although described in some sources as a "virus," Shellshock is instead a design flaw in a program that comes with some operating systems. See => {{cite web |author=Staff |title=What does the "Shellshock" bug affect? |url=
On 12 September 2014, Stéphane Chazelas informed Bash's maintainer Chet Ramey<ref name="NYT-20140925-NP" /> of his discovery of the original bug, which he called "Bashdoor". Working with security experts, Mr. Chazelas developed a [[Patch (computing)|patch]]<ref name="NYT-20140925-NP" /> (fix) for the issue, which by then had been assigned the vulnerability identifier ''{{CVE|2014-6271}}''.<ref name="seclist-q3-650">{{cite mailing list|url=http://seclists.org/oss-sec/2014/q3/650 |mailing-list=oss-sec |title=Re: CVE-2014-6271: remote code execution through bash|author=Florian Weimer|date=24 September 2014|access-date=1 November 2014}}</ref> The existence of the bug was announced to the public on 2014-09-24, when Bash updates with the fix were ready for distribution.<ref name="seclist-q3-666">{{cite mailing list|url=http://seclists.org/oss-sec/2014/q3/666|mailing-list=oss-sec |title=Re: CVE-2014-6271: remote code execution through bash|author=Florian Weimer|date=24 September 2014|access-date=1 November 2014}}</ref>
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==Background==
The Shellshock bug affects [[Bash (Unix shell)|Bash]], a program that various [[Unix]]-based systems use to execute command lines and command scripts. It is often installed as the system's default [[command-line interface]]. Analysis of the [[source code]] history of Bash shows the bug was introduced on 5 August 1989, and released in Bash version 1.03 on 1 September 1989.<ref name="BASH105_CHANGELOG">{{cite web |last=Fox |first=Brian |title=Bash 1.05 ChangeLog |url=http://www.oldlinux.org/Linux.old/bin/old/bash-1.05/ChangeLog |date=21 March 1990 |access-date=14 October 2014 |archive-date=6 December 2023 |archive-url=https://web.archive.org/web/20231206061143/http://www.oldlinux.org/Linux.old/bin/old/bash-1.05/ChangeLog |url-status=dead }}</ref><ref name="BASHBUG-20141010-SC">{{cite web |last=Chazelas |first=Stéphane |work=Stéphane Chazelas and Chet Ramey confirm the vulnerability introduction date on Bash official communication channel |title=when was shellshock introduced |url=http://thread.gmane.org/gmane.comp.shells.bash.bugs/22418 |date=10 October 2014 |access-date=14 October 2014 |archive-url=https://web.archive.org/web/20161220033324/http://thread.gmane.org/gmane.comp.shells.bash.bugs/22418 |archive-date=20 December 2016 |url-status=dead }}</ref><ref name="Stack Exchange Thread">{{cite web |last=Chazelas |first=Stéphane |url=https://unix.stackexchange.com/questions/157381/when-was-the-shellshock-cve-2014-6271-7169-bug-introduced-and-what-is-the-pat/157495#157495 |title=When was the shellshock (CVE-2014-6271/7169) bug introduced, and what is the patch that fully fixes it? |date=25 September 2014}}</ref>
Shellshock is an [[arbitrary code execution]] vulnerability that offers a way for users of a system to execute commands that should be unavailable to them. This happens through Bash's "function export" feature, whereby one Bash [[process (computing)|process]] can share command scripts with other Bash processes that it executes.<ref>{{cite web|url=https://www.gnu.org/software/bash/manual/bash.html#Shell-Functions|title= Bash Reference Manual: Shell Functions |access-date= 2 October 2014}}</ref> This feature is implemented by encoding the scripts in a table that is shared between the processes, known as the [[environment variable]] list. Each new Bash process scans this table for encoded scripts, assembles each one into a command that defines that script in the new process, and executes that command.<ref name="exported-function">{{cite web|url= http://git.savannah.gnu.org/cgit/bash.git/tree/variables.c?id=ac50fbac377e32b98d2de396f016ea81e8ee9961#n315 |title=Bash 4.3 source code, file variables.c, lines 315-388 |access-date= 2 October 2014}}</ref> The new process assumes that the scripts found in the list come from another Bash process, but it cannot verify this, nor can it verify that the command that it has built is a properly formed script definition. Therefore, an attacker can execute arbitrary commands on the system or exploit other bugs that may exist in Bash's command interpreter, if the attacker has a way to manipulate the environment variable list and then cause Bash to run. At the time the bug was discovered, Bash was installed on [[macOS]] and many [[Linux]] operating systems as the main command interpreter, so that any program that used the <code>system</code> function to run any other program would use Bash to do so.
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