Linux appliances, printers and embedded devices - a security disaster coming to a network near you.


Kurt Seifried, [email protected],for

December 12, 2001 - Dear god, is it December already? I haven't written my weekly column in quite some time, been busy and all. So we start off with an article on just what a disaster Linux based devices are going to be, security wise. I'm picking on Linux for one simple reason, because it is OpenSource and GPL licensed, and because of the structure of most embedded Linux companies it is possible to create reasonably secure Linux systems. Other proprietary systems such as Microsoft WinCE do not offer this possibility, and building insecure devices with this OS is pretty much a foregone conclusion. On the other hand secure Linux can be done, either with NSA SELinux and things like MAC/Type enforcement and so on, or hardened Linux distributions such as ImmunixOS from WireX (which is as far as I am concerned the only real mainstream Linux distribution focused on security).

Linux based devices such as appliances, printers and other embedded devices are becoming increasingly popular. Everything from firewalls, network data appliances to CD jukeboxes and mp3 players are now running Linux, with network connections. With the coming of a BlueTooth, a severely flawed wireless protocol (the only real way to secure it being at the application level, ensuring virtually no-one will do it correctly) the ability to talk over devices remotely and use them to attack others becomes trivial. Linux based devices offer a wonderful platform to base attacks from, precompiled tools are available, and precompiling tools is trivial with the majority of these systems being based on easily accessible hardware platforms such as Intel or StrongArm. Developer kits for these systems cost at most a few thousands dollars (often this is primarily hardware cost), or for free in many cases.

Lack of updates

The majority of embedded devices traditionally were not network aware. Indeed many were custom built chips designed specifically for a single purpose. These chips are rather expensive however, and with the prices on general purpose chips having fallen through the floor it is now cheaper in some cases to simply deploy a 486 or faster CPU with ram, storage and interfaces such as PCI, ISA, IDE and simply write your software in C or Java running on a commodity operating system such as Linux then to build a special purpose chip or OS for the task. Simply put security updates are not a serious concern for such devices, as exploiting them requires tearing the device apart. However newer devices, such as GSM phones that do have connectivity can be attacked over networks, in one case it was found possible to render some popular brands of GSM phones completely inoperable by sending a malformed SMS message. So it is inevitable that software updates for these products will become inevitable.

Companies are in the business of making money. As we learned from the movie Fight Club:

X is the chance of a problem occurring, Y is the number of units out there, and Z is the cost of each out of court settlement. If X times Y times Z costs less then a recall we don't do it.

With software products if a vulnerability is found it is often present in 100% of the units. The cost of settling with consumer for flawed products can be expensive, but often companies can get away with settlements that do not directly penalize them. Offering exchanges to customers who send the unit in significantly reduces the number of updates needed since many will not bother. In other cases companies have given voucher for discounts away, you get money back by buying more of their products, which is rather ironic. Unless problems are widely exploited, and result in a completely non-functional product, there is no real incentive for companies to provide updates in most cases, especially if a product is no longer being produced. While it certainly is possible to download a new copy of the kernel source this is useless for most devices as they require other software components such as drivers, utilities and so forth to make the device useful.

Interface issues

Assuming a vendor bothers to create updates and distribute them freely getting the client to install the updates is a non trivial task. Most of these devices have very limited interfaces, for example the HP print server based on Linux uses a web based interface, and a harddrive, making it possible to upload software updates and install them with relatively few problems (assuming the updates are created correctly). Other embedded devices however have much more limited interfaces, if any. Without proper care for design, implementation and configuration something designed to facilitate security upgrades can actually become a security problem itself. Cisco routers for example use TFTP to store and retrieve configuration data as well as system software. If the TFTP server is not properly secured then an attacker can potentially gain access to the images, and crack the passwords out, or simply replace the image with one that has known passwords.

Even when something is made easy to upgrade very few people will actually bother to upgrade. Most Linux distributions now include automated tools to assist users in upgrading, in some cases like Red Hat's up2date or custom scripts the process can be almost completely automated, and simply left to run. Some users choose not to install these software components, some disable them, or some lack proper connectivity to access the updated software. In some cases these components do not upgrade systems properly, breaking the system and generally making life miserable for users (this has happened several times with critical components such as the Kernel, libraries and so on). It isn't possible for vendors to force an upgrade, would you purchase a product that will stop working in 12 months unless you apply mandatory vendor updates?


With new products like HP's Linux based printer (which features a 266mhz CPU, 64 megs ram and a 5.2gig harddrive), Oracle database servers and so on it becomes an attackers dream. Unmaintained, fully functional, network aware systems with the capacity for compilers, and virtually any software you can imagine. The ease of installation, simply plug your new print server/database server/file server in, access a web based interface and 5 minutes later it is finished and working. You have nothing to worry about. Until an attacker breaks in that is.



Last updated 12/12/2001

Copyright Kurt Seifried 2001