Bash Exploit

Bash Code Injection Vulnerability via Specially Crafted Environment Variables (CVE-2014-6271, CVE-2014-7169)

Well I read this article whilst I was at work. Upon getting home I started up my Fedora machine and ran the following check to see if I was vulnerable.

To test if your version of Bash is vulnerable to this issue, run the following command:

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

If the output of the above command looks as follows:

this is a test

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. Thus, if you run the above example with the patched version of Bash, you should get an output similar to:

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

I was vulnerable. But that is what I expected. Knowing that the fix might take a little time I added the suggested workaround to my firewall script.

Workaround: Using IPTables:

A note on using IPTables string matching:

iptables using -m string --hex-string '|28 29 20 7B|'

Is not a good option because the attacker can easily send one or two characters per packet and avoid this signature easily. However, it may provide an overview of automated attempts at exploiting this vulnerability.

In full knowledge that this vulnerability was/is going to get exploited I was monitoring my firewall logs. I had set the rules to log and drop the packets. Note that the work around says it can be bypassed. A couple of hours with the rule in place and nothing unusuall was being logged and I was carrying on as normal. All of a sudden 4-5 packets were caught by the rule. My system was shutdown seconds later. I susspect that this is some opportunistic scanning taking place, however I am not taking any chances until a patch is ready.

On the plus side it has given me the opportunity to boot and update my Windows machine.

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