Thursday, September 25

ShellShock Basics - Updated Oct 1st

Update 2 - October 1st: As expected still lots going on;

As the mainstream media attempts to make sense of all the hype surrounding the latest security vulnerability, IT support staff are left to try to make sense of it all and determine what if anything needs to be done.

Background: Bash (aka Bourne-again Shell) is used as an interactive shell on most Unix-like operating systems.  It comes by default on many popular distributions such as Ubuntu, OSX, and other Linux platforms.

The bug:  One feature of the shell is to allow a user to set environment variables, unfortunately the bash shell does a poor job of interpreting these values and if the right sequence of characters is used, extra commands can be executed.
Exploitation:  By simply injecting extra code that will get passed to bash it will get executed in the context of the process reading it.  This happens as soon as the environment variables are read by bash.
Am I vulnerable:  Any software that you use that reads environment variables from untrusted, unauthenticated inputs should be examined.  Example if a CGI script parses HTTP headers.  It is prudent to review all of your public interfaces for potential exposure.  Use the Cert list to see if your vendors are listed and get a link to the specific advisory.
  • CERT List of Vendors Affected - http://www.kb.cert.org/vuls/byvendor?searchview&Query=FIELD+Reference=252743&SearchOrder=4
  • Nmap tests
  • Masscan tests
Is it patched yet:  There are numerous vendors affected.  Many of the major vendors were informed about the bug prior to release to prepare patches, some have patches that work, others do not.  Basic patches have been released.
What else should I do:  Monitor requests (in the past if you have the capability) this will tell you if people are attempting to exploit you.  Look for signatures that have been released by Sourcefire, BroIDS and other IDS vendors.  If you can look at past traffic captures then you might be able to determine if you had been a target prior to the bug's disclosure.
Monitor the situation closely, it is likely that there will be details of the specific applications and software affected as well as other mitigations that can be taken until robust patches are released.