BashFAQ/111

What is the Shellshock vulnerability in Bash?

"Shellshock" refers to two remotely-exploitable vulnerabilities in Bash, discovered in September 2014. The first vulnerability exploits the mechanism that Bash used to export and import functions, and allowed arbitrary command execution. The second vulnerability exploits a parser bug and allowed local files to be created.

What these vulnerabilities have in common is that they are triggered by Bash scanning the environment, finding malicious data, and stumbling on it. Malicious data can be inserted into the environment by remote attackers in some system configurations, the most common of which is a web server with CGI capability. Most CGI setups pass along user-supplied data (e.g. HTTP user agent) through the environment.

There are official (i.e. issued by Chet Ramey) patches for Bash which fix the Shellshock vulnerabilities. These patches are available for all Bash versions from 2.05b through 4.3. There is also a third official patch which changes how Bash exports and imports functions through the environment. This third patch is believed to close any and all "tainted environment variable" attacks.

Many systems were never vulnerable to a remote attack, but it's safer to patch all systems anyway.

Other potential problems (parser bugs) were identified during the investigation, but are considered separate from the Shellshock bug. These bugs have no remote exploits (so far as we know). These bugs are currently being patched as well, albeit with less urgency.

Vulnerability Patches
bash43-025	CVE-2014-6271	2014-09-24
bash43-026	CVE-2014-7169	2014-09-26
bash43-027	CVE-2014-6278	2014-09-27
bash43-028	CVE-2014-7186 and CVE-2014-7187	2014-10-01
bash43-029	CVE-2014-6277	2014-10-02

Are my bash binaries fixed?

Your OS should have patched bash by now, but maybe you have multiple binaries installed (some by the OS, some by other means) and want to check that all of them are safe. You can copy/paste the following snippet into a terminal emulator running bash or some other posix compliant shell.

Replace /bin/bash and /usr/local/bin/bash with the paths to the bash binaries you have installed/want to test in the for-loop below.

for s in /bin/bash /usr/local/bin/bash ; do
  VAR='() { :;};x=FAIL' "$s" -c 'printf "%-20s CVE-2014-6271 %-4s (%s)\n" "$BASH_VERSION" "${x-OK}" "$0"'
  VAR='() {}>\' "$s" -c '/dev/null x=FAIL;printf "%-20s CVE-2014-7169 %-4s (%s)\n" "$BASH_VERSION" "${x-OK}" "$0"'
done 2>/dev/null

The output will look something like this:

3.2.48(1)-release    CVE-2014-6271 FAIL (/bin/bash)
3.2.48(1)-release    CVE-2014-7169 FAIL (/bin/bash)
4.3.27(1)-release    CVE-2014-6271 OK   (/usr/local/bin/bash)
4.3.27(1)-release    CVE-2014-7169 OK   (/usr/local/bin/bash)

This shows that /usr/local/bin/bash is at version 4.3.27 and patched for both of the issues, while /bin/bash is at version 3.2.48 and fails both (meaning it is vulnerable).

A more comprehensive script called bashcheck is available. In addition to the two above mentioned vulnerabilities, it is able to test for a few others that have since been discovered. Note, however, that these additional vulnerabilities are distinct from Shellshock and are not currently known to be exploitable.

What is the Shellshock vulnerability in Bash?

Are my bash binaries fixed?

Further reading