Shellshock vulnerability (CVE-2014-6271, CVE-2014-7169)

This vulnerabilityCVE-2014-6271 could allow for arbitrary code execution. Certain services and applications allow remote unauthenticated attackers to provide environment variables, allowing them to exploit this issue.

You can also manually test your version of Bash by running the following command:

$ env 'x=() { :;}; echo vulnerable' 'BASH_FUNC_x()=() { :;}; echo vulnerable' bash -c "echo test"

If the output of the above command contains a line containing only the word vulnerable you are using a vulnerable version of Bash. The patch used to fix this issue ensures that no code is allowed after the end of a Bash function.

Note that different Bash versions will also print different warnings while executing the above command. The Bash versions without any fix produce the following output:

$ env 'x=() { :;}; echo vulnerable' 'BASH_FUNC_x()=() { :;}; echo vulnerable' bash -c "echo test"
vulnerable
bash: BASH_FUNC_x(): line 0: syntax error near unexpected token `)'
bash: BASH_FUNC_x(): line 0: `BASH_FUNC_x() () { :;}; echo vulnerable'
bash: error importing function definition for `BASH_FUNC_x'
test

The versions with only the original CVE-2014-6271 fix applied produce the following output:

$ env 'x=() { :;}; echo vulnerable' 'BASH_FUNC_x()=() { :;}; echo vulnerable' bash -c "echo test"
bash: warning: x: ignoring function definition attempt
bash: error importing function definition for `x'
bash: error importing function definition for `BASH_FUNC_x()'
test

The versions with additional fixes from RHSA-2014:1306, RHSA-2014:1311 and RHSA-2014:1312 produce the following output:

$ env 'x=() { :;}; echo vulnerable' 'BASH_FUNC_x()=() { :;}; echo vulnerable' bash -c "echo test"
bash: warning: x: ignoring function definition attempt
bash: error importing function definition for `BASH_FUNC_x'
test

The difference in the output is caused by additional function processing changes explained in the “How does this impact systems” section below.

The fix for CVE-2014-7169 ensures that the system is protected from the file creation issue. To test if your version of Bash is vulnerable to CVE-2014-7169, run the following command:

$ cd /tmp; rm -f /tmp/echo; env 'x=() { (a)=>\' bash -c "echo date"; cat /tmp/echo
bash: x: line 1: syntax error near unexpected token `='
bash: x: line 1: `'
bash: error importing function definition for `x'
Fri Sep 26 11:49:58 GMT 2014

If your system is vulnerable, the time and date information will be output on the screen and a file called /tmp/echo will be created.

If your system is not vulnerable, you will see output similar to:

$ cd /tmp; rm -f /tmp/echo; env 'x=() { (a)=>\' bash -c "echo date"; cat /tmp/echo
date
cat: /tmp/echo: No such file or directory

If your system is vulnerable, you can fix these issues by updating to the most recent version of the Bash package by running the following command:

# yum update bash