Shellshock (software bug): Difference between revisions

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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}}</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 a [[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 commandone scriptsBash created[[process in(computing)|process]] onecan runningshare instancecommand ofscripts Bashwith canother beBash sharedprocesses withthat subordinateit instancesexecutes.<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 withinin a table that is shared between the instances, known as the [[environment variable]] list. Each new instanceBash of Bashprocess scans this table for encoded scripts, assembles each one into a command that defines that script in the new instanceprocess, 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 instanceprocess assumes that the scripts found in the list come from another instanceBash 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.

The presence 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 web|url=http://seclists.org/oss-sec/2014/q3/666|title=oss-sec: Re: CVE-2014-6271: remote code execution through bash|author=Florian Weimer|work=[[Seclists.org]]|date=24 September 2014|access-date=1 November 2014}}</ref> though it took some time for computers to be updated to close the potential security issue.