Physical memory attacks via Firewire/DMA - Part 1: Overview and Mitigation (Update)

Firewire cables

This is part 1 of a series on articles about the Firewire security issues mentioned below.

For many years now, attacks via Firewire / i.LINK / IEEE 1394 have been a known security issue. Basically, if you gain physical access to a PC or laptop which has Firewire ports (or PCMCIA/Cardbus/ExpressCard, more on that later) you can

  • read arbitrary RAM contents from the victim's system,
  • overwrite arbitrary RAM contents with whatever you want,
  • and perform many, many severe attacks based on the two issues above. Examples include grabbing a full RAM dump via Firewire (takes only a few minutes), grabbing ssh-agent keys, grabbing screen contents, modifying screen contents, bypassing login/password screens, and many, many more...

All of this is done by exploiting a "feature" of the Firewire spec (OHCI-1394) (PDF), namely that it allows read/write access to physical memory (via DMA) for external Firewire devices. Worse, as this is DMA, the CPU/OS will not even know what's going on. Even worse, this works regardless of whether you have locked your screen with a password-protected screensaver, or xlock, or vlock, or whatever. As long as the system is running, you're vulnerable.

In this article, I intend to give a fairly complete overview of the available papers published on this issue, tools for testing the attacks, as well as mitigation techniques for various OSes. If I'm missing some important papers or tools, please post a comment!

Papers, Attacks, and Tools

Over the years a number of presentations and papers have been released with information about these Firewire issues.

Maximilian Dornseif et. al.

The first publication that I know of was done by Maximilian Dornseif, Michael Becher, and Christian Klein. They gave a number of talks on various security conferences on this topic:

They also released a number of tools, Firewire libraries for Mac OS X and Linux, as well as small demo scripts which use those libs:

Adam Boileau

In 2006 Adam Boileau (a.k.a. Metlstorm) gave a talk called Hit by a Bus: Physical Access Attacks with Firewire (PDF) at Ruxcon 2006. In 2008 he then released a set of tools:

  • pythonraw1394-1.0.tar.gz: Python bindings for libraw1394 (Linux). Tools: businfo, romtool, 1394memimage
  • winlockpwn: Python script which can circumvent a locked Windows XP screen (an arbitrary password will log you in)
  • bioskbsnarf: Grabs/shows the BIOS keyboard buffer via Firewire (which often contains your BIOS password)

Peter Panholzer

As of early 2008 Peter Panholzer from sec-consult.com published a two-page whitepaper which says they were able to run a winlockpwn-like attack on Windows Vista via Firewire. There's not much information in the PDF unfortunately, and no tools were released, as far as I know.

David R. Piegdon

The most recent toolset and papers I know of are from David R. Piegdon (a.k.a. IosTrace), who gave a number of talks in 2007/2008 about the issue, and also released a toolset called SEAT1394.

I'll go into much more detail on how the tools are used and what they can do in another follow-up article.


There are ways to eliminate or at least mitigate these attack vectors. The simplest and most secure way is to not have any Firewire ports installed (don't put Firewire PCI/PCIe cards in your PC, don't use Firewire PCMCIA/Cardbus/ExpressCard cards). Now, if you have a laptop with built-in Firewire ports, you have a problem, of course. In that case you could still physically destroy the port (by opening the laptop and cutting/desoldering stuff, or by putting glue/epoxy in the port in order to prevent any Firewire cables being attached). These are slightly drastic (but effective!) measures.

Note: Even if you don't have any Firewire ports, you're not automatically safe and secure. If your laptop has a PCMCIA/Cardbus/ExpressCard slot, an attacker can simply insert a PCMCIA Firewire card (for instance) in that slot. Chances are, that your OS will automatically load the driver for that card and also the Firewire drivers you'll need if you want to use the card for attaching Firewire devices. Game over. Your "secure" laptop is now vulnerable...

If you cannot (or don't want to) remove/destroy/disable your Firewire ports, the next best thing is to ensure that nobody except yourself ever gets physical access to your PC/laptop. This is hard to do for a PC, and almost impossible for a laptop, mind you.

Finally, there are some software measures you can use to prevent at least physical DMA access for Firewire devices:

Mitigation: Linux

Pretty much every Linux system with the "old" Firewire drivers loaded (kernel module ohci1394 et. al.) is vulnerable to these issues. Newer kernels now also ship with a new Firewire stack called "juju" (kernel module firewire_ohci et. al.) which may or may not have the same issues (not fully tested by me so far, will report back later).

Per default, all recent kernels, e.g. 2.6.26, are vulnerable, but see below.

Under Linux, simply using a kernel which doesn't have any Firewire support (neither built-in, nor as a module) is the most secure option. If you must have Firewire support you can load the ohci1394 module with the phys_dma=0 parameter to at least disable physical DMA support:

  $ rmmod ohci1394
  $ modprobe ohci1394 phys_dma=0

I have personally tested this on some boxes and I can confirm that it renders the currently published tools useless.

If you don't use Firewire at all, you can simply rmmod ohci1394, and (for a permanent fix) add the following lines in /etc/modprobe.d/blacklist and then (important!) run update-initramfs -u afterwards!

  # Prevent automatic loading of the ohci1394 module.
  blacklist ohci1394
  # Prevent manual loading of the ohci1394 module.
  install ohci1394 false
  # Iff we should ever load the ohci1394 module, force the use of the 'phys_dma=0' option.
  options ohci1394 phys_dma=0

As for the new "juju" Firewire stack, I'm not so sure. A few quick tests showed that the currently available tools don't work with the new stack, but you shouldn't feel too secure! AFAIK the new stack does support (or will support soon) physical DMA for Firewire, so it's probably just a matter of adapting the tools a bit (I'll do some testing/research on this later, as time permits).

Mitigation: Mac OS X

On Mac OS you might also be able to completely remove Firewire support from the kernel (but I don't know if/how that can be done, not sure if you can easily recompile Mac OS kernels, and/or if you even have buildable source code and toolchains for that). However, you can at least remove the Firewire support in the default Mac OS installation by unloading AppleFWOHCI.kext:

  $ sudo kextunload /System/Library/Extensions/IOFireWireFamily.kext/Contents/PlugIns/AppleFWOHCI.kext

Thanks to a Daniel Reutter for letting me abuse his MacBook via Firewire and for finding the above kextunload command line. We have successfully tested that after unloading AppleFWOHCI.kext the current tools won't work anymore.

The tests were done on a Mac OS X 10.5 (Leopard) with all recent security updates applied. Please leave a comment if you can test other versions of Mac OS X...

Mitigation: Windows

As for Windows, well, I guess you're screwed. While Windows XP does implement sort of "protection" in that it only allows physical DMA access via Firewire to devices which "deserve it", e.g. iPods (or any other Firewire mass storage device, I guess) this can be easily defeated by having your attack PC/laptop pretend to be an iPod (see the romtool Python script by Adam Boileau).

The only remaining option I know of (short of removing/destroying Firewire ports or preventing physical access alltogether) is to disable the Firewire ports/drivers in the device manager (untested by me so far). If you do that, remember to also disable all PCMCIA/Cardbus/ExpressCard controllers, of course (see above).

So far I've tested Windows XP SP2 successfully with Adam Boileau's winlockpwn. Windows XP SP3 doesn't seem to work, though (winlockpwn likely needs tweaking). I haven't yet been able to test Windows 95/98/Vista, if you can verify one of them, please leave a comment.

Mitigation: OpenBSD/FreeBSD/NetBSD/OpenSolaris/...

On OpenBSD you're likely not vulnerable as OpenBSD doesn't have any Firewire drivers at all, as far as I know ;-)

As for FreeBSD, NetBSD, OpenSolaris, and other OSes I don't have any information. I might be able to test one or two of them in the nearer future, but please leave a comment if you have some information about whether they are vulnerable and/or how you can secure your system...

Further Resources


That's it for now. I hope you now have a good overview of these issues and how to protect. I can only urge you to take this problem seriously! Three or four minutes of leaving your laptop unattended are fully sufficient for an attacker to get a full forensic image of all your RAM contents for later analysis. This is at least as critical as the Cold Boot attacks, if not worse.

I will follow-up with more articles about some more interesting details on these Firewire issues, how to use the above tools, and I'll report on some of the stuff I was able to find in RAM dumps gathered via Firewire...

Update 2008-08-15: Added information on how to blacklist the Firewire modules on Linux (for permanent mitigation).
Update 2008-08-16: Added links to further articles. Windows XP SP3 doesn't seem to work with winlockpwn.


The RedTeam, a penetration testing group, has released two security advisories which explain security holes in two podcast clients (podcatchers).

Both exploits are possible because the input of the programs is not properly (or at all) sanitized. Basically, they call system($wget_cmd) where $wget_cmd is shell (/bin/sh) code which shall download a file via wget. As the $wget_cmd string contains contents from an untrusted source (HTML/XML on some random server), this results in an "arbitrary code execution" vulnerability, the worst-case scenario you could imagine.

If someone is naive enough to even run such a podcatcher as root, this means a remote root exploit!

Anyways, the RedTeam is definately correct in saying that more and more people start listening to podcasts, and more and more podcatchers are written. But few of them are written with security in mind, which leaves many listeners at risk... I wonder how popular closed-source podcatchers such as iTunes are affected here. Are there any published audits/audit-results (black-box auditing, obviously, as you don't have the source code) for iTunes?

As for Free Software implementations... consider this a call for reviews and audits! If you know/use one of the many podcatchers (or an RSS feed aggregator, which are affected by similar issues), and have some knowledge on secure programming, get the source and review the application. Make the software you use, and the world at large, a little safer.

I'll definately have a look at the programs I'm using soonish...

22C3: Impressions, lectures, hacking, bb ported to iPod

Train Station

It's day 2 of the 22C3 congress — time to post some stuff.

I've been travelling to Berlin by train with Daniel Reutter, taking a bunch of photos with my new PowerShot A610 (see photo on the right, for an example). You can browse the 22c3 flickr group for more photos.

We've listened to quite a lot of lectures already, and many more will follow. Especially the keynote by Joi Ito was really great, the network here works most of the time (wired net, as well as WLAN), so everything's fine for now...

As far as hacking is concerned, we played around with iPodLinux a bit. We managed to port aalib to the iPod an then (more interesting) we also ported bb, the portable text-mode demo. There are some issues on both the 4g iPod and the 5g iPod we tried this on, but we're working on it... We have documented the ports in the form of HOWTOs (currently Debian-specific) in the iPodLinux wiki (aalib, bb).

bb on a 4g iPod
bb on a 5g iPod photo 1
bb on a 5g iPod photo 2

Up next: porting Ruby to the iPod.

Update 2006-01-05: This post made it to Symlink a few days ago.

How to watch videos from your 5g video iPod on TV without buying the Apple video cable [Update]

I stumbled over a very nice article over at MacDevCenter which explains how to use a cheap, standard A/V-to-RCA cable to direct the video output of a 5g video iPod to your TV.

That's sure something I need to try soonish! I was a bit reluctant to buy the not-so-cheap Apple cable. This guide not only saves me some money, but it's also nicer being able to use standard components instead of proprietary stuff...

Update 2006-01-07: I have tested video playback using such a standard cable today and it works fine! The video quality is quite good, too, even at 320x240 (but you can probably also have bigger videos, I guess). Using a cable which only has two connectors (white and red probably) works too, but you only get audio and no video then.

iPod as RSS reader?

I noticed iFeedPod, which seems to be an RSS Reader for the iPod. It sounds like it syncs the feed items from your computer to the iPod for "offline" reading. I really like the idea, but the implementation is Mac-only and only free as in beer (not free as in speech) as far as I can tell.

Does anybody know of a similar software which runs on Linux and is Free Software? Do I have to implement it from scratch?

Syndicate content