Anatomy of a Wii U: hexFW

As I mentioned in my last post, in parallel to researching the Vita's kernel I decided to research the Wii U and see what I could come up with.

For all this to make any sense, we must go back to the beginning and waste a couple minutes looking into the Wii U hacking scene's history.
It all began back in 2013 when team fail0verflow made a presentation at 30C3 describing their journey in hacking the Wii U.
For quite some time they became the reference in terms of research and documentation for the Wii U's internals and even set up a wiki for this purpose: http://wiiubrew.org/wiki/Main_Page
However, they had no intention to release their exploits or talk about any vulnerabilities. This led to a community-driven effort to recreate their steps and hack this system.

Attempts to exploit the vWii date from late 2013, but it wasn't until mid 2014 that Wii U userspace code execution was achieved. This was done originally by what would become the libwiiu team.
One year later, the same team achieved the first PowerPC kernel exploit (a TOCTOU involving the OSDriver structures) and opened the doors for homebrew (with some restrictions).
The libwiiu project became a very valuable framework to write, compile and execute code from within the Wii U's Web Browser and it kept being used across firmware changes.
Other projects were developed during this time frame and eventually branched away from libwiiu (e.g.: Loadiine, homebrew_launcher).

Later that year, the first reports of someone being able to break into the Wii U's secure processor (IOP, running a custom operating system called IOS and dubbed IOSU by the community) showed up. The hacker and developer hykem claimed that had defeated IOSU around December of 2015.
This was shortly followed by another team claiming the same feat. This time it came from smealum, plutoo, naehrwert, derrek and yellows8, all very well known video game console hackers from the 3DS (and PS3 in naehrwert's case) scene.
The 3DS group wasn't planning a release while hykem, on the other hand, revealed that was working on a CFW-like setup for a public release.
During the following weeks, hykem massively documented his findings on the wiiubrew wiki, but ended up disappearing before releasing his announced project.
Then came an awkward period of drama and mystery that marked the Wii U hacking scene for good.
Eventually, smealum announced he had no plans on developing further for the Wii U and released "iosuhax", a collection of patches for the IOSU that would, in theory, create a CFW-like environment (privileged memory access and direct communication with the IOSU in a RPC setup). However, no one released an actual exploit to take over the system and allow for firmware patching.
Earlier this year, someone documented two vulnerabilities found by hykem, naehrwert and plutoo in the wiiubrew wiki and yet again, a community-driven effort began to build an exploit based on these bugs.

This led us to today. The current state of the Wii U hacking scene is a fragmented playground with two dominant groups using this now public information to build an exploit chain and achieve the goal of building a CFW for the Wii U:
- Team SALT: a collective of hackers/developers mainly active on the 3DS scene that began working on a CFW solution quite early this year. They are developing everything in private and share a few details occasionally on their progress.
- Team wiiubru: formed under the public eye by early contributors and developers of the PowerPC homebrew scene that sparked after libwiiu. They develop everything in public and have achieved popularity among the community.

After learning about all this, I still felt motivated to research and work by myself for quite a few reasons:
 - The Wii U's security scheme is nowhere near the Vita's and is much easier to take down;
 - Public vulnerabilities already exist and there's plenty of documentation out there as well;
 - My vision conflicts with team SALT's and team wiiubru's.

That last point is crucial. Team SALT chose to work privately, which is understandable when dealing with something that can potentially brick your console, but it's not possible to evaluate what they have accomplished thus far. Also, one of their members was allegedly responsible for leaking team libwiiu's OSDriver exploit which doesn't inspire much trust in the group.
On the other hand, team wiiubru is doing everything in public, but using low standards in terms of code quality and organization, which is to be expected when working in large groups.

Not satisfied with either groups' approaches, I decided to come up with my own: hexFW.
The name may not be very original, but to be honest, I didn't put much effort into making it up.

So, in order to build a CFW of my own, I made two important decisions: use smealum's iosuhax as the base for IOSU patching and implement my own exploit chain.
In my opinion, iosuhax does the job fairly well. It's a set of patches and a script that replaces sections of a firmware image and re-encrypts it back to be run on the Wii U. The idea is to start with smealum's code base and evolve from there.

But that only covers the process of creating a CFW image, the hard part is to exploit the system and make it load this image.
I looked into wiiubru's public implementation of the two publicly disclosed bugs, but found many issues with it.
By far, what seemed to make less sense in their implementation was the fact that the exploit is launched using the "homebrew_launcher" (or HBL). This felt like an overkill: why exploit the PPC kernel and run userspace code to exploit the IOSU when the disclosed bugs can be used without kernel privileges?
After playing around with the bugs for a while I finally understood their point: it's really hard to pull this off solely from the browser.

Hacker yellows8 released a stagefright exploit compatible with the Wii U a long time ago which became the standard for userspace code execution under the most recent firmware versions (the bugs libwiiu had used in the past have all been patched). It's code is quite clean and does the job perfectly, so I decided to use it as the main entry point to achieve userspace code execution.
For compiling the code I resorted to libwiiu for it's simplicity and began writing the exploit chain from scratch. Soon enough I came across an issue that might have been why wiiubru decided to ditch direct code execution and resort to HBL and ELF loading: the userspace payload's size is limited due to the nature of the stagefright exploit.

Nonetheless, I came up with some creative ways of squeezing all the necessary steps into a single payload and managed to get it working.
This means hexFW is launched solely from the browser without the need for installing or using HBL!
Of course, this came with a small cost: it's very likely that running the exploit will crash once. This is due to how I'm locating a target thread inside the IOSU to modify it's stack pointer. However, if you reset the console it is guaranteed to launch the second time. After the launcher runs, the actual CFW will be booted into the system from the SD card.

I have released a preview version of hexFW on Github: https://github.com/hexkyz/hexFW
It contains my own exploit chain ("fwboot") and a copy of smealum's original iosuhax minus the filesystem patches (IOS-FS). This is merely to demonstrate it's usage, but the overall structure is very likely to stay the same. The following commits will consist in the actual firmware patches that make hexFW a CFW (SLC/MLC/OTP/SEEPROM dumping with/without compression, redNAND, USB read/write/install, coldboot, etc.).
If you're familiar with iosuhax, you can setup hexFW and it will generate a patched firmware image that contains smealum's wupserver. You can then communicate with it using the wupclient.py script and play around with it.

In the next days I'll be pushing several components of hexFW I've been working on. My next post will be a detailed write-up on the two vulnerabilities exploited and how I constructed the exploit chain from scratch.

Stay tuned!

The aftermath: What's next?

Even though HENkaku's KOTH challenge has ended, this is far from being the end!

First and foremost, I want to personally congratulate Team molecule for their achievements and show my appreciation for what they have done.
In the past few days we've got a full fledged CFW framework (taiHEN), an upgraded HENkaku payload (taiHENkaku), a SDK with plugin and kernel module support and a handful of valuable documentation on the Vita's internals.
I've taken the time to analyze everything and I simply stared at my laptop's screen in awe. The level of dedication that has been put into this project is mind blowing.
Their talent is unquestionable, sure, but the professional and meticulous way they handled everything is unparalleled.
Many successful companies (which actually profit from their work) don't have this level of coordination and dedication. I can easily tell this is a labor of pure love. Love for knowledge, curiosity and overall fun.
I can't thank you enough for what you have done here. I hope this will help cement how much hackers' love their work and promote an healthy coexistence between companies and individuals.
Four talented individuals brought interest back to a forsaken video game console and did it in the most respectful and professional way I've ever seen.
I can't stress this enough, everything was orchestred in such a way that the intentions of Team molecule became fully transparent and resulted in something that opens many doors without hurting the (almost non-existent anyway) market.

History has been made. Yes, we are talking about hacking video game consoles, but the principles behind this go way past that. We are actually seeing talented people doing something good and showing the world we have the right to explore, research and reverse-engineer in the name of knowledge.
Actions like this help building and supporting those who defend our rights and I eager for the day we have the freedom to explore without being afraid.
Thank you Yifan Lu, Davee, Proxima and xyz. Your work will never be forgotten.

So, what's next?
As you've probably noticed, the challenge's prize was the access to the now public Vita Wiki. Me and st4rk (the other winner of the challenge) were granted write access to this collaborative Wiki and I plan to put that privilege to good use.
I've been spending a lot of time researching the Vita's kernel and planning TrustZone attacks. All the information I'm acquiring in the process will be gradually added to the Wiki not only to fill in the gaps but also to continue Team molecule's work for as long as I can.
Even though we are running code inside the Vita's kernel by now, there's still a lot to do. Team molecule themselves have already defeated the secure kernel (a.k.a. TrustZone) and learned about it's true purpose: communicate with the security processor (dubbed F00D due to ELF headers).
I plan on following their steps and, hopefully, defeat the secure kernel next. After that comes the massive challenge of defeating the obscure F00D processor.
Considering how little is known about it and how small it's attack surface is, we may never even be able to take it down. Still, the process is guaranteed to provide critical information just like Team molecule has proven.

Parallel to all this, I've been introduced to a whole different "scene" recently. Earlier this week, a friend of mine came to me complaining about how disappointed he was with his latest purchase: a Wii U.
Apparently he hated the console and watching the new Switch's trailer was enough for him to get rid of it. Believe it or not, he wanted to toss it in the trash (yeah, money is not a problem there) but asked me first if I wanted it.
The last Nintendo console I ever had was a SNES so, why not?
Naturally, I began looking into hacking it and learned a looooong story about it in the process. Popular video game console hackers fail0verflow were the first to crack this device back in 2012, but only recently people have managed to defeat it's most critical security components.
Unfortunately, it appears that hacking efforts for this device are heavily fragmented with a large group of people working on it publicly and a smaller one working in private. Useful vulnerabilities are already public and a few shaky implementations are out in the wild as well.
After researching the console and analyzing what's been documented already I've decided to take a shot at it and over the last few days I've spent plenty of time digging into it and working on cool stuff for it (emuNAND, USB read/write, boot-time launching, to name a few).
I'll be publishing a few articles about this along with a crude CFW and some tools. I'm working on top of other hackers' work, so I'll be sure to cover and credit their work as well.

Stay tuned!

HENkaku - Exploit teardown - Stage 3

Here it is, Stage 3, the last stage of HENkaku.
This was by far the toughest to crack, so, let's dive in!

HENkaku - Stage 3

In Stage 2, we analyzed how HENkaku exploits two distinct kernel bugs to achieve code execution: a memory leak bug (in the sceIoDevctl function) to defeat KASLR and a use-after-free (in the sceNetIoctl function) to break into the kernel and do ROP.
However, since the execution flow switches over to a ROP chain planted into the kernel, we still couldn't figure out what was happening next.

Like I mentioned in the previous write-up's ending note, dumping the kernel (more specifically, the SceSysmem module) was now necessary. Team molecule did not provide any additional vulnerability that we could use for this purpose, so, it was up to the participants to figure it out themselves.

I had already found a potential memory leak vulnerability while playing around with Stage 2 but, unfortunately, due to it's nature (out-of-bounds read) it wasn't enough to reach the SceSysmem module.
Frustrated, I began looking for other plausible entry-points. It took me several attempts and required analyzing several key components of the Vita's system:
- Network:
    The SceNet module was the origin of the use-after-free and I had already an OOB read there, so, what else could be in there?
- Filesystem:
    The SceDriverUser module exposes a decent amount of unique system calls for the filesystem. Some of them crash. Can I leak memory here?
- Audio:
    Developers don't pay much attention to security when it comes to implement media handling. Some specific audio handling features are taken care by the kernel itself. Can I compromise it?
- Graphics:
    Just like with audio, graphics are a common source of flaws. The Vita has plenty of libraries with unique system calls for this (SceGpuEs4User, SceGxm, ScePaf). Will this help?
- Application:
    User applications are managed by modules that heavily communicate with the kernel (SceAppUtil and SceDriverUser via SceAppMgr calls). Perhaps this can be taken down?
   
Eventually, one of those gave me what I wanted and I was able to dump the entire Vita's kernel memory. After locating the SceSysmem module among the dumped binaries I became able to solve the rest of the challenge.
On a side note, I did attempt blind ROP at first by relocating a few gadgets and taking wild guesses, but team molecule made sure it wouldn't be that easy. The gadgets' placement makes it very difficult to predict what each one will do.

Anyway, here is the result:

So, random comments and mistakes aside, this gives us a clear view of what the kernel ROP chain is doing:

If you recall, the kernel loader was an encrypted chunk of 0x100 bytes that was appended to the bottom of the ROP chain we copy into a kernel stack using sceIoDevctl:

• // NULLs for padding at the bottom of the chain
  0x00(x_stack + 0x00008D7C) = 0x00000000;
  0x00(x_stack + 0x00008D80) = 0x00000000;
  0x00(x_stack + 0x00008D84) = 0x00000000;
  
  // Code starts here
  0x00(x_stack + 0x00008D88) = ...;

The kernel ROP decrypts this chunk using AES-256-ECB and the key is a piece of code from SceSysmem itself.
This is what the kernel loader looks like (note that base offset is set to 0x00000000):

In sum, the loader allocates two memory blocks, one for data and another for code. Then it fetches the HENkaku's payload from user memory (using copy_from_user) and decrypts it in place using a static key (stored inside the kernel loader binary data). Finally, it copies the decrypted payload into an executable memory block, set's PC and SP and jumps to it.

Now we have HENkaku running on our system!
As proof, here are the SHA-1 hashes of the two crucial keys for the entire process:
Kernel loader key (AES-256-ECB): f1a8e9415bf3551377a36a1a5b25ba64f2d96494
Kernel payload key (AES-128-ECB): eacac4a780065c8c106349e412696aabd1b1b8d1

And that's it! This concludes the final stage of the HENkaku's KOTH challenge.
I don't plan on dwelving much into how I leaked the kernel's memory and I don't plan on releasing the keys themselves out of respect for other groups attempting to complete the challenge and for the developers themselves.
I believe the goal of this challenge was not to simply crown the first person to crack HENkaku, but to get the whole community engaged and bringing new ideas to the table.
By not releasing the decrypted binaries or the method I used to leak memory, others still have the chance to solve the challenge themselves.
I may publish a few more posts detailing some interesting features of the HENkaku's payload, but I will leave the full source code reveal to the developers themselves.

Until next time!

HENkaku - Exploit teardown - Stage 2

HENkaku - Stage 2

 

Stage 2's payload is composed by another ROP chain and data.

It creates two userland threads (each one with it's own ROP chain), that take care of leaking kernel pointers (by issuing devctl commands to "sdstor0:") and breaking the userland sandbox (by exploiting sceNet functions).

  

Stage 2 leverages a bug in sceIoDevctl in order to leak 2 distinct kernel pointers. These 2 pointers refer, respectively, to SceSysmem module's base address and SceIoFilemgr(?) thread's stack address:

•     // Store leaked kernel pointer 1
      // Comes from devctl_outbuf + 0x3D4
      scesysmem_base = 0x00(x_stack + 0x00007308) + 0xFFFFA8B9
      // Store leaked kernel pointer 2
      // Comes from devctl_outbuf + 0x3C4
      sceiofilemgr_stack_base = 0x00(x_stack + 0x000072F8) + 0xFFFFF544

When preparing to write the kernel ROP chain, we can see a few pointers being set. These translate to:

•     // Kernel ROP inside sceiofilemgr
      // This is where our ROP chain gets copied to inside the SceIoFilemgr module
      kern_rop = sceiofilemgr_stack_base + 0x000006F8
      // Encrypted kernel code
      kern_code = kern_rop + 0x300

Now we write down our kernel ROP chain in the stack, but we can see that some values only get written afterwards.

This is because these values are directly related to the decryption of the next kernel level stage! Team molecule likely only writes them into the ROP chain later so they can easily update the encrypted stage without having to change the kernel ROP chain directly.

So:

•     // Overwrite specific NULLs in the ROP chain
      0x00(x_stack + 0x00008C04) = 0x00(x_stack + 0x00008EAC)     // kern_code
      0x00(x_stack + 0x00008B48) = 0x00000090
      0x00(x_stack + 0x00008CC0) = 0x00000240
      0x00(x_stack + 0x00008D58) = 0x00000200
      0x00(x_stack + 0x00008D14) = 0x00008FC0                     // kern_next_payload

And our final ROP chain should look like this:

Now we copy the chain from the stack into the buffer that's being sent through sceIoDevctl:

•     // Copy kernel ROP chain
      memcpy(x_stack + 0x00007448, x_stack + 0x00008A8C, 0x300);
      // Copy the first 0x400 bytes of "obfuscated" data
      // and append them at the bottom of the ROP chain
      memcpy(x_stack + 0x00007744, x_stack + 0x00008EB8, 0x400);

And so, the final input buffer will look like this:

•     // SceSysmem address
      // Unknown pointer written right on top of the input buffer
      0x00(x_stack + 0x00007444) = scesysmem_base + 0x0001E460
      // Kernel ROP chain
      0x00(x_stack + 0x00007448) = 0xXXXXXXXX;
      ...
      0x00(x_stack + 0x00007734) = 0xXXXXXXXX;
      // A few NULLs for padding
      0x00(x_stack + 0x00007738) = 0x00000000;
      0x00(x_stack + 0x0000773C) = 0x00000000;
      0x00(x_stack + 0x00007740) = 0x00000000;
      // Encrypted kernel code
      // Only the first 0x100 bytes will fit in the buffer
      0x00(x_stack + 0x00007744) = 0xXXXXXXXX;
      ...
      0x00(x_stack + 0x00007B44) = 0xXXXXXXXX;

Finally, we must craft a buffer that will have our ROP chain's SP and PC. This is the buffer we feed the SceNet exploit with:

•     // Set kernel thread SP, PC, UNK
      0x00(x_stack + 0x0000884C) = sceiofilemgr_stack_base + 0x000006F8 + 0x00000004      // SP
      0x00(x_stack + 0x00008850) = scesysmem_base + 0x00000347                            // PC
      0x00(x_stack + 0x00008858) = sceiofilemgr_stack_base + 0x000006DC                   // UNK

When the SceNet exploit finishes, we should have hijacked a kernel thread inside the SceNetPs module and overwritten it's stack contents with our own.

This results in the kernel jumping to scesysmem_base + 0x00000347 (which is very likely a POP {PC} gadget) and executing our ROP chain at sceiofilemgr_stack_base + 0x000006F8 + 0x00000004 (which translates to kern_rop + 0x04).

  

To further reverse the exploit, one must dump the target kernel modules, rebuild the kernel ROP and deobfuscate/decrypt the rest of HENkaku's code.

Next up, stage 3!

HENkaku - Exploit teardown - Stage 1

This post aggregates my previously published write-ups that detail the first stage of HENkaku's exploit chain.
Information has been reorganized to reflect the original developers' naming scheme.

HENkaku - Stage 1

Visiting http://henkaku.xyz and pressing the "Install" button results in a server side useragent check.

If the browser's useragent matches the one of a PS Vita/PSTV on the latest firmware version (3.60), the user is redirected to http://go.henkaku.xyz and an exploit is deployed.

This exploit re-uses elements from the older public exploits (heap spraying method, sort() bug, scrollLeft attribute manipulation) and pairs them with a new heap corruption technique.

Team molecule renamed variables and methods to provide a simple obfuscation layer on the HTML code.

Partially reversed HTML:

Similarly to older exploits, this allows to corrupt an object's vtable and achieve ROP inside the SceWebkit module.

Offsets for libraries and relevant ROP gadgets are fetched from a javascript file (http://go.henkaku.xyz/payload.js) during the last stage of the exploit.

Team molecule implemented a dynamic method to relocate gadgets and functions' offsets for each module after their base addresses' are found (by looking at SceWebkit's import stubs).

The payload.js file contains two arrays, one containing the payload's binary data and another containing the relocation type for each word.

By crossing this information the exploit reads the payload and relocates all code offsets to their target module's address space by adding the module's base address to them:

    Relocation type 0 -> Plain data stored inside the ROP space itself. No relocation needed.

    Relocation type 1 -> Offset inside the ROP payload's stack.

    Relocation type 2 -> Offset inside the SceWebkit module.

    Relocation type 3 -> Offset inside the SceLibKernel module.

    Relocation type 4 -> Offset inside the SceLibc module.

    Relocation type 5 -> Offset inside the SceLibHttp module.

    Relocation type 6 -> Offset inside the SceNet module.

    Relocation type 7 -> Offset inside the SceAppMgr module.

Payload's generated binary data:

This payload is responsible for taking care of a few things like:

After the payload is done, an HTTP request is sent to the server using the following template:

    http://go.henkaku.xyz/x?a1=stack_base&a2=webkit_base&a3=libkernel_base&a4=libc_base&&a5=libhttp_base&a6=net_base&a7=appmgr_base&

Example:

    http://go.henkaku.xyz/x?a1=89f02000&a2=81b009a0&a3=e000dd00&a4=811c0cc0&&a5=e0607c80&a6=e01302b0&a7=e0047bf0&

The "x" script on the server side collects the base addresses for each module and generates a second payload to be run on the Vita.

This second payload is composed by another ROP chain and obfuscated ARM code.

A preliminary analysis of this payload reveals a few interesting things:

Next up, stage 2!

HENkaku - KOTH Challenge Recap

As most of you know, Team Molecule (a collective of console hackers composed by Yifan Lu, Davee, Proxima and xyz) released what I consider to be a true work of art in the realm of videogame console hacking.

They released "HENkaku", a homebrew enabler for the PS Vita, almost 3 months ago.
Along with the release, they posed a challenge to the community: reverse what we've done in HENkaku and become "king of the hill".

As soon as HENkaku was out, I began looking into it and slowly tried to pick it apart. Most of my progress and findings were published in a collection of "pastebins" I scattered around the web.

The road was harsh, but I finally managed to crack down the full HENkaku's code flow a few weeks ago.
Now that I've finally had the time to put it all up together, I decided to publicly claim my title by posting the hash of a very important key. ;)

To make all the effort complete, I'm going to publish in this blog all of my previous progress first (as an attempt to aggregate the data from the "pastebins") and end it with a detailed write-up of the last and final stage of this awesome challenge.

Stay tuned!

Hello World

Welcome.

This blog is meant to be a collection of posts, documentation and overall information on my feats in the world of cyber security.
Nothing too fancy, just a more organized way to publish my ideas and accomplishments.

Let's start!