Shellshock vulnerability: should we be concerned?

We are continually hearing about bugs and vulnerabilities that could potentially be serious. The latest one named ShellShock can potentially be used to remotely take control of almost any system that is using a software component called Bash. This sounds devastating and it course of could be, but don’t start running for the hills or deciding to unplug from the Internet quite yet though.

Bash is a software component that exists on many Linux systems including Apple’s Mac OSX. As Linux is the operating system used on a large number of the web servers, a bug like this could mean cybercriminals have the potential to exploit the vulnerability and cause harm to users of the web server or indeed to the company whose web server it is. They do this by inserting malware on the server that could potentially collect data, cracks passwords or do something particularly malicious.

At the time of writing this blog there is already a large number of patches available that address this vulnerability for servers and reputable companies have teams in place that watch for these alerts and update their servers to protect them and the users of the services they offer. A good example is our own security team here at AVG who immediately ran an audit to see if we had any servers that may have this vulnerability, and they have already confirmed that our servers are safe.

 

If you are a Mac user should you be concerned and what do you need to do?

Apple has, as expected, reacted quickly and is releasing an automatic update to OSX that users will be prompted to install. They have also made it clear that the issue does not affect the majority and is an issue for power users that take advantage of the advanced UNIX services within OSX. If the previous sentence has baffled you then you are in the group that Apple say are not at risk.

Even as a power user at home you are likely to be sitting behind a firewall that would detect someone trying to execute commands on your machine and they would be blocked. However bad guys may well try and trick users to into installing files that could leave them more vulnerable to attack, a good rule is to not click something that you don’t recognize and remember the update will only come directly from Apple. When you see the update appear through on your Mac, install it immediately so that you stay safe.

There are also other devices in our homes that run Linux. Many of the routers and broadband modems we use to connect to the Internet also utilize Linux as an operating system and because of this we recommend you watch for updates from those vendors and take the action to install them. If your router is provided by your ISP then they should push the update to the router automatically.

It is good practice to allow the automatic updates on your devices so that they are maintained by the manufacturer of the device to protect you from issues like this. Having up to date anti-virus software installed and active is also of paramount importance in today’s environment where more of our data than ever before is held by us on our devices. The protection provided will detect and block an exploit such as this where cybercriminals attempt to install malware on your machine. AVG’s Free Antivirus is available for Mac and PC users and can be downloaded from www.avg.com

Tor-ramdisk i686 UClibc-based Linux Distribution x86 20140925

Tor-ramdisk is an i686 uClibc-based micro Linux distribution whose only purpose is to host a Tor server in an environment that maximizes security and privacy. Tor is a network of virtual tunnels that allows people and groups to improve their privacy and security on the Internet. Security is enhanced by employing a monolithically compiled GRSEC/PAX patched kernel and hardened system tools. Privacy is enhanced by turning off logging at all levels so that even the Tor operator only has access to minimal information. Finally, since everything runs in ephemeral memory, no information survives a reboot, except for the Tor configuration file and the private RSA key which may be exported/imported by FTP. x86_64 version.

Dhclient Bash Environment Variable Injection

When bash is started with an environment variable that begins with the string “() {“, that variable is treated as a function definition and parsed as code. If extra commands are added after the function definition, they will be executed immediately. When dhclient receives an ACK that contains a domain name or hostname, they are passed to configuration scripts as environment variables, allowing us to trigger the bash bug. Because of the length restrictions and unusual networking scenario at time of exploitation, this Metasploit module achieves code execution by echoing our payload into /etc/crontab and cleans it up when we get a shell.

Ubuntu Security Notice USN-2364-1

Ubuntu Security Notice 2364-1 – Florian Weimer and Todd Sabin discovered that the Bash parser incorrectly handled memory. An attacker could possibly use this issue to bypass certain environment restrictions and execute arbitrary code. In addition, this update introduces a hardening measure which adds prefixes and suffixes around environment variable names which contain shell functions. Various other issues were also addressed.

Red Hat Security Advisory 2014-1311-01

Red Hat Security Advisory 2014-1311-01 – The GNU Bourne Again shell is a shell and command language interpreter compatible with the Bourne shell. Bash is the default shell for Red Hat Enterprise Linux. It was found that the fix for CVE-2014-6271 was incomplete, and Bash still allowed certain characters to be injected into other environments via specially crafted environment variables. An attacker could potentially use this flaw to override or bypass environment restrictions to execute shell commands. Certain services and applications allow remote unauthenticated attackers to provide environment variables, allowing them to exploit this issue.

Red Hat Security Advisory 2014-1312-01

Red Hat Security Advisory 2014-1312-01 – The GNU Bourne Again shell is a shell and command language interpreter compatible with the Bourne shell. Bash is the default shell for Red Hat Enterprise Linux. Shift_JIS, also known as “SJIS”, is a character encoding for the Japanese language. This package provides bash support for the Shift_JIS encoding. It was found that the fix for CVE-2014-6271 was incomplete, and Bash still allowed certain characters to be injected into other environments via specially crafted environment variables. An attacker could potentially use this flaw to override or bypass environment restrictions to execute shell commands. Certain services and applications allow remote unauthenticated attackers to provide environment variables, allowing them to exploit this issue.