On September 24, 2014, a GNU Bash vulnerability, referred to as Shellshock or the "Bash Bug", was disclosed. In short, the vulnerability allows remote attackers to execute arbitrary code given certain conditions, by passing strings of code following environment variable assignments. Because of Bash's ubiquitous status amongst Linux, BSD, and Mac OS X distributions, many computers are vulnerable to Shellshock; all unpatched Bash versions between 1.14 through 4.3 (i.e. all releases until now) are at risk.
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
andmod_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