Find Shellshock Exploit Attempts & Probes From the Command Line and Papertrail

Never hurts to make sure

I’ve written about Papertrail a few times before, I love the service and it’s just too valuable to not use.

Papertrail makes it super easy to find Shellshock exploit attempts and probes. Probes are just checking a machine to see if it’s vulnerable to Shellshock. If you’re using CloudFlare, you’ll never see any Shellshock attempts show up in your logs, CloudFlare doesn’t even let them through.

See If Shellshock Affects You

Checking to see if your system is vulnerable to Shellshock is quite easy. It takes a relatively simple bash command:

env x='() { :;}; echo vulnerable to shellshock' bash -c "echo All good"

Run that code in a terminal. If you see All good, you’re not vulnerable. However, if you see vulnerable to shellshock, you are potentially vulnerable.

Yahoo-WinZip-Servers-Shellshock-Bug

Shellshocker.net provides a script that will download, compile, and install the newest version of bash for you. You should only use it though if your Linux distribution hasn’t already provided updated security release packages. If you’re interested, the code that runs Shellshocker.net is available on GitHub.

Find Shellshock Attemps and Probes Via The Command Line

This is very easy as long as you know the location of your Apache access log file. It’s typically something like /var/log/apache2/access.log. Assuming that’s the location of your Apache access log file, this command will pull out all the Shellshock probes and attempts:

grep '() {' /var/log/apache2/access.log

If nothing was returned, that means nobody has been trying to exploit Shellshock on your system, or even checking to see if your system is susceptible to Shellshock. If results are returned, look them over carefully to examine where the attempts are coming from, an IP address will be associated with every attempt.

Shellshocker.net Checker

Shellshocker.net also provides a bash script to check your machines for the Shellshock vulnerability. You can download the script and run it manually from your terminal, or, if you have cURL installed, run the following command:

curl https://shellshocker.net/shellshock_test.sh | bash

Running that command will produce results similar to the screenshot seen below. It checks for a number of Shellshock related vulnerabilities.
shellshocker

Find Shellshock Attemps and Probes With Papertrail

Go to your Papertrail events tab and search for the following:
"() {"

If anything is returned, those are Shellshock probes. Some example probes are listed in the gist that’s embedded below. None of the offending IP addresses have been redacted.

These actually made it through to Papertrail, which shouldn’t happen since longren.io sits behind Cloudflare. I’ll open a support ticket with them about it and update this post later.

Send Apache Logs to Papertrail With Rsyslog

Over the last few days, I’ve been looking at Apache web server logs, a lot, mostly quick checks for Shellshock probes and exploit attempts. All on client servers, thankfully. All of the servers I operate through DigitalOcean are patched up. It just so happens that all the sites I host have their DNS hosted by Cloudflare, which has been blocking all Shellshock attempts.

A majority of my sites send their Apache logs to Papertrail. Having all my apache logs easily accessible and searchable is extremely nice. It’d make sniffing out Shellshock attempts quite simple. You can check for Shellshock attempts relatively easily from the command line, as well, something like the command below would work:

grep '() {' /var/log/apache2/access.log

1. Setup Rsyslog to Send to Papertrail

Anyway, sending Apache logs to Papertrail is pretty easy. I’m going to assume you’ve already setup rsyslog to send logs to Papertrail. If not, this post should help.

2. Add CustomLog Directive To Your VirtualHost

You just need to modify your virtualhost configuration and add a CustomLog directive. Here’s what I do to send longren.io logs to Papertrail:

CustomLog "| /usr/bin/logger -t httpd -p local1.info" "%{%b %d %X}t longren.io apache %h %l %u %t "%r"%>s %b "%{Referer}i" "%{User-agent}i""

The -t httpd piece sets the service name for Papertrail. The -p local1.info flag sets the priority. You’ll want to change the longren.io piece in the above code to whatever site you’re capturing logs for. You can also change or remove apache that immediately follows longren.io.

3. Reload Apache

After you’ve added the CustomLog directive to your virtualhost, you’ll want to reload Apache:

sudo service apache reload

That’s all there is to it. You should start seeing your Apache logs in Papertrail shortly after reloading Apache.