Patching Your Redhat Server for the Shellshock Vulnerability

Introduction

Alright guys, this is a biggie. Shellshock allows remote code execution and file creation for any server relying on bash v3.4 through v1.1. If you are using Redhat or CentOS and the default shell, your server is vulnerable.

The patching history was sketchy, as well. If you patched immediately when the bug came out using CVE-2014-6271, you are still likely vulnerable (as of right now, 9/26/2013 12:50PM EST). Run the following to apply the patch:

#yum update bash

You need CVE-2014-7169 if you are using Red Hat Enterprise Linux 5, 6, and 7. Note that 2014-7169 DOES NOT address the following operating systems, which as of right now are still not fully patched: Shift_JIS, Red Hat Enterprise Linux 4 Extended Life Cycle Support, Red Hat Enterprise Linux 5.6 Long Life, Red Hat Enterprise Linux 5.9 Extended Update Support, Red Hat Enterprise Linux 6.2 Advanced Update Support, and Red Hat Enterprise Linux 6.4 Extended Update Support

If you applied CVE-2014-6271 and need the rest of the patch, reference RHSA-2014-1306

Diagnosis / Am I Vulnerable?

Copy, paste and run the following command from your shell prompt:

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

If the output of the above command contains a line with only the word "vulnerable" you are still vulnerable. Depending on what version you are using and what patches you have applied, the command output will be different.

A completely vulnerable system will do this:

$ 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

Systems patched with CVE-2014-6271 but not CVE-2014-7169 will do this:

$ 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

Systems that used the RHSA-2014-1306  patch do this:

$ 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

Next we have to test the file creation aspect of the Shellshock vulnerability. Execute the following command, in its entirety, from your shell:

cd /tmp; rm -f /tmp/echo; env 'x=() { (a)=>\' bash -c "echo date"; cat /tmp/echo

This is what a non-vulnerable system will provide:

$ 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 you're extra paranoid like me you may just want to double check that there is no file "echo" in your /tmp directory. A system that is still vulnerable will respond to the command by providing the data and time according to your system clock and creating the file. The initial output will look similar to this:

$ 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

Please guys, check your servers and get this wrapped up as quickly as possible. I can't stress enough how dangerous this vulnerability is, particularly given how many administrators allow direct access to their servers through one port or another. Feel free to contact me if you have any additional questions or concerns. I am happy to help.