How would I go about gaining access to a locked-down Linux device I own.
How would I go about gaining access to a locked-down Linux device I own.
Five years ago, I bought a Supernote A5. It was (and mostly still is) a great device for reading and writing on an eInk display, and it runs plain old linux.
The deciding reason I went for this device instead of the competition is that I was "under the impression" that they were about to enable full SSH access to the device! Awesome!
"Why were you under that impression?", I hear the skeptics ask. Well, their spokesperson has stated that they would do so. Via mail, and on reddit, publicly, multiple times. I was still torn, so sent them a DM, asking if this was ineed factual. "Yes", they said, "the next quarterly update will enable SSH access!".
Great!
Well, it's been 5 years. They did not follow through. A couple updates were published, none contained the promised functionality, the spokesperson stopped answering questions about SSH. The last software update I received is from 2.5yrs ago. Mentions of the original Supernote A5 have largely been scrubbed from their website.
Let me be clear, the device still functions perfectly. But it is in danger of becoming e-waste because it is so needlessly complicated to get stuff on the device. I'm currently in need of an ebook reader with (ideally) OPDS capability, and I am pretty confident I'd be able to get something like koreader running on this, or at least just run a script to sync files over SSH. Also, I frankly feel wounded in my pride having a Linux device in my possession which refuses to do my bidding (I'm joking of course, but also I am 100% serious).
Here's all I know:
- plugging it in via USB, the device reads as an MTP device, with access only to the documents/books/... stored on it
- you can place an
update.zipfile (obtained from the SN website) into the root of that MTP directory, and upon reboot, the device will update. To me, this appears to be the most promising route of gaining access. - unfortunately, the zip file is encrypted. The decryption key clearly has to be known to the device, but since I have no access to it,...
I'm a software engineer, but I have zero knowledge of the "dark arts", so to speak. If anyone could help me (or point me into the right direction!), I would really be grateful. I don't want this (generally nice) product to turn into a paperweight instead of a paper replacement :(
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The entries in
update.zipare encrypted using the weak ZipCrypto scheme, which is known to be seriously flawed. If you feel motivated, and can guess at least 12 bytes of plaintext for an entry, it is possible to recover the internal state of the generator, which is enough to decipher the data entirely, as well as other entries which were encrypted with the same password. The bkcrack project implements this attack.Since some of the entries are zip files themselves, it is within the realm of possibility to guess 12 bytes of plaintext. Parts of the zip local file header are pretty static, and you can use some of the values from the local file header of
update.zipitself. Still, this would require a bit of luck / inspired guesswork.Oh, wow. I am so giving this a try. Huge kudos for checking the zip itself, btw! Thank you :D
Just for clarification though, do I need 12 bytes of the original content or of the compressed (but unencrypted) byte-representation of the zip file?
Edit: Ah, the repo links the paper. Reading now :)
The attack worked, the password is
cmF0dGEK.This was obtained by generating 32 possible plaintexts for the first 10 bytes of system.zip (based on the different values in the headers of ~300 zip files on my system), plus three null bytes for the high bytes of compressed size, file name length and extra field length.
No way!! You're the goat. I spent the day trying to get behind how the cracking worked by making simple examples, and you just... Solve the puzzle :D
Awesoms, thank you so much!! I'll appreciate update this thread if this leads to something :D
I did quickly check the files on update.zip and it looks like they're tarballs embedded in a shell script and image files including pretty much the whole operating system on the thing.
You can extract those even without a VM and do whatever you want with the files and package them back up, so, you can override version checks and you can inject init.d scripts, binaries and pretty much everything to the device, including changing passwords to /etc/shadow and so on.
I don't know how the thing actually operates, but if it isn't absolutely necessary I'd leave bootloader (appears to be uboot) and kernel untouched as messing up those might end up with a bricked device and then easy options are broken and you'll need to try to gain access via other means, like interfacing directly with the storage on the device (which most likely includes opening the thing up and wiring something like arduino or an serial cable to it).
But beyond that, once you override version checks, it should be possible to upload the same version number over and over again until you have what you need. After that you just need suitable binaries for the hardware/kernel, likely some libraries from the same package and a init-script and you should be good to go.
The other way you can approach this is to look for web server configurations from the image and see if there's any vulnerabilities (like apache running as root and insecure script on top of that to inject system files via http), which might be the safest route at least for a start.
I'm not really experienced on a things like this, but I know a thing or two about linux, so do your homework before attempting anything, have a good luck and have fun while tinkering!
Thanks! Yep, same thought about the version checks. I'll spin up a VM for now and see if that allows for suitable experimentation, otherwise fingers crossed I don't brick the device.
The web-server thing is probably safer, agreed, but packaging my own update is just so much more tempting... :D
Check out the file update.zip > system.zip > zImage
It's the image for the device probably, check this guide out
https://jamchamb.net/2022/01/02/modify-vmlinuz-arm.html
You can probably get some sort of boot script implanted in there, or even just load the image in a vm, modify it, and recreate it.
You might also need to modify the install script there since it seems to check if the update already exists and it might not run thinking you are up to date.
Fantastic.
Since the zip also includes a bunch of shell scripts, I think it's possible I could also just install ssh directly - but the image will certainly make experimenting in a VM the safer option until something works out... ^^
Oh man, I can't wait to get home from work on Friday (currently stuck on the other side of the country 🫠)
Edit: also, can I somehow buy you a beer/coffee somewhere digitally?
Nah, I'm good, 0v0 did the work. this was fun!
Keep us posted :)
Oh whoops I thought you were 0v0 🤦🏼♀️ Thanks anyways though :D
The inner zip files are just stored, uncompressed:
Archive: update.zip Index Encryption Compression CRC32 Uncompressed Packed size Name ----- ---------- ----------- -------- ------------ ------------ ---------------- 0 ZipCrypto Store d1bca061 65761967 65761979 system_lib.zip 1 ZipCrypto Deflate 64a3f383 2183 741 config.json 2 ZipCrypto Store 3731280f 89300292 89300304 app.zip 3 ZipCrypto Store a2bd64f5 135518964 135518976 app_lib.zip 4 ZipCrypto Store 700eb186 5996410 5996422 system.zipSo 12 bytes from the original content.
I had the same idea, and I'm trying it right now... Not something I've ever done before though.