Prática de Shadowing: Watch this before you boot up another device - Aprenda a falar inglês com vídeo

Ladies and gentlemen, boys and girls, lend me, lend me, lend me your ears.

I want to show you this really neat vulnerability in U-Boot that everyone's talking about.

Not everyone, but I am.

So if you run the script here, QEMU drive, QMU, bada bing, bada boom, you will see the magical values.

The 41414141, the 41414141, and the 4242423E.

What the hell is even going on?

What are we talking about?

Guys, we're talking about a vulnerability in U-Boot,

the bootloader that allows you to bypass signature verification by exploiting a vulnerability in the bootloader itself.

Bugs like this are really, really neat because they actually invalidate an entire trust chain in the way that weird devices like this, like embedded ones and even ones that you sit in your pocket,

validate if they use U-Boot.

Obviously, this one does not.

This one might.

Anyway, I'm going to break down the bug, what went wrong, what you can do about it, and then also what the hell all this stuff means.

The finding today was found by a company named Binarly.

I've done videos on their stuff before.

They do a really good job on finding vulnerabilities in like lower level.

That's the name of the channel, by the way.

Low level.

Low level systems like bootloaders or BMC, board management controllers, stuff like that.

Again, not an ad for this company, just they do a lot of good work.

So the vulnerability today is a stack buffer underflow, kind of hard to say, in uBoot during the FIT image signature verification,

specifically in the FDT find regions functions.

A lot of words said there.

I'm gonna go into what all that means here in a little bit.

Okay, so enter uBoot, right?

What is uBoot?

Well, uBoot, kind of like the name of this folder that I had, I called it Das Boot.

uBoot is a system that allows you to, in an embedded world, boot other OSes, right?

So typically, the way these weird devices work is you have to bring them up.

You gotta get the board to be alive, you have to bring the CPU to live.

And then the first thing that you run on a router like this is this thing called a bootloader.

A bootloader is a piece of code that allows you to run other code.

The idea being, you don't have to make that piece of code know how to boot against every other CPU.

You just bring the CPU up to the bootloader, and the bootloader does the rest of the work, right?

You can use that bootloader to run your OS, like Linux or Windows or whatever.

So if the job of the bootloader is to run code that we want it to run, why does RCE in a bootloader matter?

Why does the vulnerability in U-Boot matter to anybody?

Great question.

What actually is interesting about U-Boot is its ability to facilitate this thing called verified boot, okay?

There's this whole world of boot processes

that the entire job is to make sure that code

that is not meant to be ran does not run on the device.

And we do this thing where we use a bunch of cryptographical algorithms using asymmetric encryption with like RSA keys

and public keys and stuff to sign the software image before it tries to get loaded.

And then using a public key on the device, we verify if that manufacturer signed image is actually what we want.

Is it the proper thing that is allowed to run on this device?

because maybe we just don't want anyone to be able to

run any software they want on random piece of Chinese router, right?

Maybe that's a thing that they don't want.

Okay.

So the issue that happens here is when you go to boot with a U-boot image in this fit format, this flattened image tree format,

it is signed with a signature.

It is basically taking a private key that the manufacturer owns.

We use that to embed a signature cryptographically that says, yup, it was signed by this manufacturer.

And then we use the public key on the device to verify the signature

that it was manufactured and it is the proper image.

But we can invalidate this entire signature verification process.

We can upload and execute a piece of code that is not intended by the manufacturer

if we can find and exploit a vulnerability inside of the bootloader.

And this is exactly what Binarily did.

And I'm sorry if you're worth with Binarily and I'm mispronouncing the company's name.

I'm not very good at English.

Okay, so stack buffer underflow in uBoot during the FIT image signature verification and FDT find regions.

Basically, there's code in the bootloader that is finding different regions of the actual image itself.

And the vulnerability is based on the same issue as one above, where we rely on the fact that FTT getName, which is a call that happens above, can fail.

But this time we're interested in what it writes to the length argument upon failure.

The error code.

Very interesting.

So there's this function called FTT getName, that if for any reason in this searching of the image during this FTT getName process,

where it's getting the name of a leaf of this image format, right?

If for some reason it's searching and it finds that one of the leaves does not have a slash, right?

So strrcare is a thread-safe version of strcare, which basically looks for a token in a string.

If that fails, we set error equal to some value and we go to fail.

And then in the fail label, we say if the length pointer exists, we are going to write out that length pointer, which should be the output of the actual length of the name.

Instead, we're going to output an error value, and that error value is FTT error bad structure.

And then notice that later on in the code we're going to do is we're going to say start copy onto end, the name that we found from that function, oh, and also end plus equals length.

Well, the issue here is that length holds a negative value.

It is a large negative error code.

So instead of actually moving the end of that cursor of parsing forward, the pointer moves backwards.

It'll go backwards by the size or the value of error bad structure, which is negative 11th.

And then it will increment that and add a slash.

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Let's get back to it.

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So the end result is like actually beautiful, right?

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So what you have here is a stack frame for the function in question, right?

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You have the stack frame for this function and you have this end pointer, right?

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So it's end is this and it's saying end plus equals length.

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So the pointer for end should go up and up and up the stack grows more positively down.

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That is important for you to know.

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If you don't know that, you should know that watching this channel.

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Damn it.

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But what's actually happening is we have another stack frame above us, and every time we exploit this vulnerability with these sections that have bad names, that don't have a slash in them,

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we're actually subtracting the end pointer back and back and back, 11 bytes at a time,

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until eventually the end pointer points to the return address of the previous stack frame.

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And I know you're thinking, if you're like a binary exploiter person, and you're like, wait, if this is its own stack frame here, isn't there nothing above it?

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Well, no, the actual, the contents of this buffer lives in another stack frame below it.

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So just trust me, this is how it works.

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So N goes up, up, up.

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And then using a new malformed section name that does have a slash,

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we can write arbitrary data we want to the program counter and get it to return to whatever address we want.

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Again, if this is, you know, 32-bit, like ARM, we're going to have like A, A, A, B, right?

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And if you're new to the world of exploitation, right, what's happening here, by overriding the program counter value,

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we now have the ability to arbitrarily run or at least point the CPU to a place to run other code, right?

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Like we could have put code in like another part of the image

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and we can like wrap around to like mProtect or whatever, like set the page table entry to make it executable.

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We do a bunch of evil stuff, right?

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The end goal here is

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that we're able to run code in the context of a bootloader that is doing signature verification without a valid signature.

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We are exploiting the code that is doing signature verification, and this could invalidate an entire secure boot-like process.

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I want to highlight that this in and of itself is not secure boot.

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This image would have to be signed by a secure boot key, like the manufacturer key that the actual motherboard has.

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But that being said, right, it's a really interesting primitive in the world of U-Boot, of bootloader vulnerabilities.

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Not going to dive super deep into a ton of these, but you have another bunch of vulnerabilities, like, you know, the same kind of bug, denial of service in U-Boot using an FIT image signature because of an unchecked size value.

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There are other parts of the code that use a name pointer where that name pointer can be null pointer.

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And because of that, it's going to do a bunch of other stuff.

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There's a whole bunch of interesting bugs on this write-up.

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I'll put it in the description below.

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But it's an interesting case study in the world of U-Boot, of bootloader vulnerabilities.

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And the question for the class, the question that everyone probably wants to know the answer to, would Rust have fixed this?

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Sort of.

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So again, U-Boot, the whole point of this is to have the CPU be brought up, the CPU runs a bootloader, the bootloader then enables the OS, right?

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We are at the super, super low level, name of the channel, by the way, in the boot process, okay?

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Now, doing this in Rust is actually like there are these things called peripheral access crates

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or these things called hardware abstraction layer crates

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that allow you to write code that is not unsafe for the board in question.

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That being said, doing things like raw MMIO

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that maybe there isn't a peripheral extraction crate written for is very hard to do.

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And it always does depend on unsafe code, even inside of the peripheral abstraction crates or access crates.

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There is there has to be by the nature of how embedded software works, right?

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It's global mutability, which is what Rust is designed to get rid of.

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Inside of even this code, there has to be a piece of unsafe code, unfortunately.

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But that being said, it's possible, just not easy.

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And yes, Rust would have fixed bugs like this, right?

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You would, well, actually, kind of think about it.

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I don't think you're able to fix this bug

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because all you're doing here is writing out to a pointer and a value, right?

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And then from there, it's causing the underflow.

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So I'm not necessarily sure if that's the case.

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You can't raw pointers in rust

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so there you go that's that's the answer actually you know the rust um error

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and not error the the okay and error type the result type would fix this

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because what this is actually the classical issue in c where

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you're using the error code in the error state as the

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actual value slot right this is like the entire problem

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that the rust t e or rust result type fixes

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and before we leave by the way in the code here what we're showing you

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so what i did here is i i wrote um i compiled uboot the vulnerable version

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which there is no patch for by the way yet.

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I compiled UBoot to run in QMU.

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QMU is like a system emulator, emulates an arm chip.

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And then I ran UBoot and I tried to signature validate with the FIT image that has the vulnerable structures in it.

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And then by using this, I was able to take control of the registers that are put on the stack, right?

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So R10, the frame pointer, and ultimately PC, where PC is the return address that we're going to execute next, right?

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And by doing that, this proves

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that I have at least the the ability to point the CPU to other places in the code

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that I want to run my evil stuff right now.

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Obviously, the question of like NX and ASLR comes up, but that is a different question for a different video.

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Anyway, thank you for watching, guys.

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I appreciate it.

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If you like this stuff, check out my other stuff, and then give me a little kiss on the cheek.

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Okay, we'll see you later.

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Goodbye.

O que é a Técnica de Shadowing?

Shadowing é uma técnica de aprendizado de idiomas com base científica, originalmente desenvolvida para o treinamento de intérpretes profissionais. O método é simples, mas poderoso: você ouve áudio em inglês nativo e repete imediatamente em voz alta — como uma sombra seguindo o falante com 1-2 segundos de atraso. Pesquisas mostram melhora significativa na precisão da pronúncia, entonação, ritmo, sons conectados, compreensão auditiva e fluência na fala.

Prática de Shadowing: Watch this before you boot up another device - Aprenda a falar inglês com vídeo

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