Matthew Garrett ([personal profile] mjg59) wrote2013-04-05 10:33 am
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Leaked UEFI signing keys

(See here for an update to this)

A hardware vendor apparently had a copy of an AMI private key on a public FTP site. This is concerning, but it's not immediately obvious how dangerous this is for a few reasons. The first is that this is apparently the firmware signing key, not any of the Secure Boot keys. That means it can't be used to sign a UEFI executable or bootloader, so can't be used to sidestep Secure Boot directly. The second is that it's AMI's key, not a board vendor - we don't (yet) know if this key is used to sign any actual shipping firmware images, or whether it's effectively a reference key. And, thirdly, the code apparently dates from early 2012 - even if it was an actual signing key, it may have been replaced before any firmware based on this code shipped.

But there's still the worst case scenario that this key is used to sign most (or all) AMI-based vendor firmware. Can this be used to subvert Secure Boot? Plausibly. The attack would involve producing a new, signed firmware image with Secure Boot either disabled or with an additional key installed, and then to reflash that firmware. Firmware images are very board-specific, so unless you're engaging in a very targeted attack you either need a large repository of firmware for every board you want to attack, or you need to perform in-place modification.

Taking a look at the firmware update tool used for AMI systems, the latter might be possible. It seems that the AMI firmware driver allows you to dump the existing ROM to a file. It'd then be a matter of pulling apart the firmware image, modifying the key database, putting it back together, signing it and flashing it. It looks like doing this does require that the user enter the firmware password if one's set, so the simplest mitigation strategy would be to do that.

So. If this key is used by most vendors shipping AMI-based firmware, and if it's a current (rather than test) key, then it may well be possible for it to be deployed in an automated malware attack that subverts the Secure Boot trust model on systems running AMI-based firmware. The obvious lesson here is that handing out your private keys to third parties that you don't trust is a pretty bad idea, as is including them in source repositories.

(Wow, was this really as long ago as 2004? How little things change)

Still has results

(Anonymous) 2013-04-05 07:33 pm (UTC)(link)
Still look to be 'valid' results for that Google search. Eeep.

Re: Still has results

(Anonymous) 2013-04-06 12:00 am (UTC)(link)
More recently, when github started offering filename search, quite a few repositories matched searches for ".ssh/id_rsa", ".ssh/known_hosts", ".gnupg/secring.gpg", the Firefox passwords database, GNOME keyrings, and so on.