Tag Archives: CVE-2014-6278

Bash specially-crafted environment variables code injection attack

Update 2014-09-30 19:30 UTC

Questions have arisen around whether Red Hat products are vulnerable to CVE-2014-6277 and CVE-2014-6278.  We have determined that RHSA-2014:1306, RHSA-2014:1311, and RHSA-2014:1312 successfully mitigate the vulnerability and no additional actions need to be taken.


Update 2014-09-26 12:00 UTC

We have written a FAQ to address some of the more common questions seen regarding the recent bash issues.

Frequently Asked Questions about the Shellshock Bash flaws

Update 2014-09-26 02:20 UTC

Red Hat has released patched versions of Bash that fix CVE-2014-7169.  Information regarding these updates can be found in the errata.  All customers are strongly encouraged to apply the update as this flaw is being actively attacked in the wild.
Fedora has also released a patched version of Bash that fixes CVE-2014-7169.  Additional information can be found on Fedora Magazine.

Update 2014-09-25 16:00 UTC

Red Hat is aware that the patch for CVE-2014-6271 is incomplete. An attacker can provide specially-crafted environment variables containing arbitrary commands that will be executed on vulnerable systems under certain conditions. The new issue has been  assigned CVE-2014-7169.
We are working on patches in conjunction with the upstream developers as a critical priority. For details on a workaround, please see the knowledgebase article.
Red Hat advises customers to upgrade to the version of Bash which contains the fix for CVE-2014-6271 and not wait for the patch which fixes CVE-2014-7169. CVE-2014-7169 is a less severe issue and patches for it are being worked on.

Bash or the Bourne again shell, is a UNIX like shell, which is perhaps one of the most installed utilities on any Linux system. From its creation in 1980, Bash has evolved from a simple terminal based command interpreter to many other fancy uses.

In Linux, environment variables provide a way to influence the behavior of software on the system. They typically consists of a name which has a value assigned to it. The same is true of the Bash shell. It is common for a lot of programs to run Bash shell in the background. It is often used to provide a shell to a remote user (via ssh, telnet, for example), provide a parser for CGI scripts (Apache, etc) or even provide limited command execution support (git, etc)

Coming back to the topic, the vulnerability arises from the fact that you can create environment variables with specially-crafted values before calling the Bash shell. These variables can contain code, which gets executed as soon as the shell is invoked. The name of these crafted variables does not matter, only their contents. As a result, this vulnerability is exposed in many contexts, for example:

  • ForceCommand is used in sshd configs to provide limited command execution capabilities for remote users. This flaw can be used to bypass that and provide arbitrary command execution. Some Git and Subversion deployments use such restricted shells. Regular use of OpenSSH is not affected because users already have shell access.
  • Apache server using mod_cgi or mod_cgid are affected if CGI scripts are either written in Bash, or spawn subshells. Such subshells are implicitly used by system/popen in C, by os.system/os.popen in Python, system/exec in PHP (when run in CGI mode), and open/system in Perl if a shell is used (which depends on the command string).
  • PHP scripts executed with mod_php are not affected even if they spawn subshells.
  • DHCP clients invoke shell scripts to configure the system, with values taken from a potentially malicious server. This would allow arbitrary commands to be run, typically as root, on the DHCP client machine.
  • Various daemons and SUID/privileged programs may execute shell scripts with environment variable values set / influenced by the user, which would allow for arbitrary commands to be run.
  • Any other application which is hooked onto a shell or runs a shell script as using Bash as the interpreter. Shell scripts which do not export variables are not vulnerable to this issue, even if they process untrusted content and store it in (unexported) shell variables and open subshells.

Like “real” programming languages, Bash has functions, though in a somewhat limited implementation, and it is possible to put these Bash functions into environment variables. This flaw is triggered when extra code is added to the end of these function definitions (inside the enivronment variable). Something like:

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

The patch used to fix this flaw, ensures that no code is allowed after the end of a Bash function. So 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

We believe this should not affect any backward compatibility. This would, of course, affect any scripts which try to use environment variables created in the way as described above, but doing so should be considered a bad programming practice.

Red Hat has issued security advisories that fixes this issue for Red Hat Enterprise Linux. Fedora has also shipped packages that fixes this issue.

We have additional information regarding specific Red Hat products affected by this issue that can be found at https://access.redhat.com/site/solutions/1207723

Information on CentOS can be found at http://lists.centos.org/pipermail/centos/2014-September/146099.html.

Frequently Asked Questions about the Shellshock Bash flaws

The recent few days have been hectic for everyone who works in the Linux/Unix world. Bash security flaws have rocked the globe leaving people confused, worried, or just frustrated. Now that the storm is over and patches are available for most operating systems, here are the answers to some of the common questions we’ve been asked:

Why are there four CVE assignments?

The original flaw in Bash was assigned CVE-2014-6271. Shortly after that issue went public a researcher found a similar flaw that wasn’t blocked by the first fix and this was assigned CVE-2014-7169. Later, Red Hat Product Security researcher Florian Weimer found additional problems and they were assigned CVE-2014-7186 and CVE-2014-7187. It’s possible that other issues will be found in the future and assigned a CVE designator even if they are blocked by the existing patches.

Is CVE-2014-7169 the same severity as the original flaw?

Our research, and that of others, shows that it would not have been possible to exploit the CVE-2014-7169 flaw remotely in the same way that it was for the previous flaw. So, even though there were security consequences of the CVE-2014-7169 flaw, it was certainly not as severe as the original flaw.

Why did Red Hat delay in providing a patch for CVE-2014-7169?

When a second issue with Bash was found a few minutes after the first one went public, we knew there was something wrong. We could have followed a duct-tape approach and issued patches to our customers quickly or we could have done this correctly. Applying multiple security updates is extremely difficult!

When CVE-2014-7169 went public, there was a lot of visible confusion around how to address this issue. This was fueled by the media and by the fact that exploits were immediately available on the Internet.

Red Hat carefully analyzed the root cause of the issue and wrote and tested patches. We posted these patches to the community for review and allowing everyone to freely use them if they wanted to. Doing things correctly takes time!

Why is Red Hat using a different patch then others?

Our patch addresses the CVE-2014-7169 issue in a much better way than the upstream patch, we wanted to make sure the issue was properly dealt with.

I have deployed web application filters to block CVE-2014-6271. Are these filters also effective against the subsequent flaws?

If configured properly and applied to all relevant places, the “() {” signature will work against these additional flaws.

Does SELinux help protect against this flaw?

SELinux can help reduce the impact of some of the exploits for this issue. SELinux guru Dan Walsh has written about this in depth in his blog.

Are you aware of any new ways to exploit this issue?

Within a few hours of the first issue being public (CVE-2014-6271), various exploits were seen live, they attacked the services we identified at risk in our first post: from dhclient, CGI serving web servers, sshd+ForceCommand configuration, git repositories. We did not see any exploits which were targeted at servers which had the first issue fixed, but were affected by the second issue. We are currently not aware of any exploits which target bash packages which have both CVE patches applied.

Why wasn’t this flaw noticed sooner?

The flaws in Bash were in a quite obscure feature that was rarely used; it is not surprising that this code had not been given much attention. When the first flaw was discovered it was reported responsibly to vendors who worked over a period of under 2 weeks to address the issue.

Did you have an outage?

Our security blog article was widely regarded as the definitive source of information about the flaw, being referenced in news articles, on Wikipedia, and from organizations such as US-CERT. This caused more demand than we expected so we did have some periods on Thursday where the blog was unavailable. Our customer portal also had some problems keeping up with demand at times. Many of the issues we saw have already been corrected.

Is my lightbulb really affected by these flaws?

Only if your lightbulb runs Bash! Lots of press have latched onto the fact that this flaw could affect the Internet Of Things, allowing attackers to take control of your systems via home appliances. In reality, embedded devices rarely use Bash, going for more lightweight solutions such as BusyBox, which includes the ash shell that was not vulnerable to these issues. So while it’s certainly plausible that some devices may be affected by this flaw, it won’t be very common.