Remote exploit vulnerability in bash CVE-2014-6271

September 24, 2014

Dave Lewis

A remotely exploitable vulnerability has been discovered by Stephane Chazelas in bash on Linux and it is unpleasant. The vulnerability has the CVE identifier CVE-2014-6271 and has been given the name Shellshock by some. This affects Debian as well as other Linux distributions. You will need to patch ASAP.

Bash supports exporting shell variables as well as shell functions to other bash instances. This is accomplished through the process environment to a child process.

The major attack vectors that have been identified in this case are HTTP requests and CGI scripts.

From Akamai:

Akamai has validated the existence of the vulnerability in bash, and confirmed its presence in bash for an extended period of time. We have also verified that this vulnerability is exposed in ssh---but only to authenticated sessions. Web applications like cgi-scripts may be vulnerable based on a number of factors; including calling other applications through a shell, or evaluating sections of code through a shell.

There are several functional mitigations for this vulnerability: upgrading to a new version of bash, replacing bash with an alternate shell, limiting access to vulnerable services, or filtering inputs to vulnerable services. Akamai has created a WAF rule to filter this exploit; see "For Web Applications" below for details.

If you have a username in your authorization header this could also be an attack vector.

Another attack surface is OpenSSH through the use of AcceptEnv variables. As well through TERM and SSH_ORIGINAL_COMMAND. An environmental variable with an arbitrary name can carry a nefarious function which can enable network exploitation. This is fire bad.

The race is on. Will you be able to patch before Metasploit has a working exploit?

Tod Beardsley, engineering manager from Rapid7, had this to say,

"As you might have guessed, we're busy at work putting together a Metasploit module that demonstrates the bash bug (CVE-2014-6271), as is the rest of the world of open source security contributors. I expect to see a first version today.

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