31 Oct

Unity3D, Mono and invalid PE files, part 2

In the first part of the series I explained how some cheat authors try to protect their work against other cheaters. It was a quick introduction to Unity3D and bugs in Mono that cheat authors exploit.

Last week someone emailed me another example of a game cheat. My tool from the previous article failed to fix invalid metadata, so I decided to look at it again.

Cheats by BlackMod.net

The cheat I received was made by Mod4U from BlackMod.net team. It appears that Mod4U is one of the most active members of the team, judging by number of the releases. His/her cheats use invalid PE file tricks and are encrypted, as you'll see later in the article.

After looking at different mods from other team members, I've confirmed that Rito, Aurora and Legend also are using invalid PE files for hiding their work. But none of their cheats encrypt Assembly-CSharp.dll.

So, let's look at the cheats and see what new tricks BlackMod team members have found!

Just like in my previous article, I'll be using Mono sources from commit 74be9b6, so that they would more or less match the code used by Unity3D.

Unchecked CLI header values

Mono ignores pretty much every field from CLI header in mono_image_load_cli_header:

.NET runtime validates ch_size, ch_runtime_major, ch_runtime_minor fields. Other reversing tools validate some of the fields.

Unchecked MetaDataRoot values

Mono completely ignores md_version_major and other values of the MetaDataRoot in load_metadata_ptrs:

.NET runtime and tools based on older dnLib validate md_version_major and md_version_minor fields.

How long is the version string?

ECMA-335 specification paragraph II.24.2.1 specifically says

Call the length of the string (including the terminator) m (we require m <= 255); the length x is m rounded up to a multiple of four.

Mono throws specification out the window and does it in another, completely broken way:

Mono accepts version_string_len that is not rounded to multiple of four and rounds it up automatically. It also accepts string length larger than necessary.

This is a very interesting bug, as it affects most of the commonly used .NET reversing tools.

  • .NET 2.0 runtime doesn't actually follow ECMA-335 specification. It doesn't require string length to be aligned and doesn't align it. But it does require that metadata streams start at dword boundary.
  • dnLib/dnSpy will accept unaligned strings but will not align them. It will accept extra long strings. dnLib doesn't require streams to start at dword boundary.
  • CFF Explorer will happily read file with unaligned string length and will align it automatically. It will also accept file with extra long strings.

Invalid HeapSizes/MDStreamFlags value

Mono processes only the lowest 3 bits of HeapSizes in load_tables:

and completely ignores flag value 0x40 which is quite crucial.

See the dnLib sources:

When ExtraData flag is set, there is one more field present in .NET metadata::

.NET runtime and tools based on dnLib respect the flag but Mono completely ignores it. D'oh!

Invalid size of #~ stream

Mono doesn't check size of #~ stream at all. Really. It accesses all the #~ fields with no validation whatsoever:

So, you can set size of #~ stream to a random number and Mono will be happy. CFF will be happy as well. But any other tool will complain.

Invalid size of #Blob stream

Mono correctly reads offset and size for #Blob stream:

but doesn't check values for correctness. So, it will also accept insanely large values for heap_blob.size.

The only checks are done when reading from #Blob stream and only ensure that index is smaller than the declared #Blob size:

Assembly-CSharp.dll encryption

As I already mentioned, Mod4U is the only Blackmod team member who uses this type of protection. To learn how exactly the protection works, please read my previous post.

Today I'd like analyze different encryption methods used by Mod4U. Data show that Mod4U keeps stealing the protection mechanisms from others!

Not operation

On 24-Jun-2018, Mod4U released Dragon Spear v1.25 MOD. APK contains Assembly-CSharp.dll encrypted using simple NOT operation. It also contains libmono.so for armeabi-v7a architecture which is protected by some protector that I can't recognize.

Thanks to VirusTotal, we can track this libmono.so back to a Chinese game from year 2016, called Lance Town. Stolen! smile

Xor 0x71

On 12-Jul-2018, Mod4U uses different encryption in his/her Noblesse M Global v1.2.0 release. APK contains libmono.so for armeabi-v7a architecture and it's not protected at all. This is the exact same encryption that was used by AndroidThaiMod and described in my previous post.

Additional VirusTotal search shows that this protection has been used by different teams since August 2017. Stolen! smile

Sub 0x0D

Starting from 24-Jul-2018 release of Boxing Star v1.1.2, Mod4U uses yet another version of libmono.so. Now there are 2 libmono.so versions - one for armeabi-v7a and one for x86 architecture.

This version of libmono.so seems to be stolen from a different Chinese game! smile How else can you explain presence of the exact same code described in Pediy post about Chinese game and reference to ThreeKindom.dll? And the function name rvlt_modify_data_with_name - a reference to RevolutionTeam who released the game.

VirusTotal search confirms the theory. Stolen again! bigsmile

Cheats by platinmods.com

Platinmods is another Android modding team using pretty much the same tricks.

I took just a quick look at their releases (eg. BLEACH Brave Souls v7.3.1) and found that they employ another encryption method (Xor 0xFB).

According to VirusTotal, that particular version of libmono.so has been used by Vietnamese modding team W4VN.NET since April-2017.

Update for my tool

My original intention was to make a SIMPLE tool that helps you to fix the broken PE file and metadata. My tool does exactly that.

Most of the tricks that I described are trivial to fix. Check the value, if it's incorrect, overwrite it with correct value. Repeat until done. Something like this:

However, "Invalid #~/#Blob size" tricks are particularly difficult to fix. My tool cannot magically support all possible cases, not without writing full .NET metadata reader/writer. And I don't want to do that, sorry. wink

So, if you really, really need to fix invalid stream sizes, try running de4dot on the decrypted Assembly-CSharp-fixed.dll. de4dot versions from years 2016-2017 work best, versions from 2018 most likely will complain about "invalid parameter" or something like that. Same advice applies for dnSpy - versions from 2017 will open most (or all) decrypted files, versions from year 2018 will not.

Download here:

Conclusion

As predicted in my previous article, cheat authors are willing to exploit any and all bugs in Mono to hide their work from others. Some of the bugs they've discovered are really surprising and affect more than just one tool. I expect cheat authors to continue finding new and innovative ways to hide their code in the future as well.

Next time I'll look into... well, I have no idea! bigsmile

P.S. If you still can't make some DLL work, please send me an email (or leave a comment below). I'll look at it and try to help you.

15 May

Unity3D, Mono and invalid PE files

Some time ago, Reoto asked a very nice question on Black Storm forum:

Can someone fix the .dll (.net) pe header to MS DOS?
How can I do that?
If you know about protecting .net files for Android, please help me.
I have another question.
Can I fix dnspy to resolve .dll pe header isn't .net?

Obviously, English is not author's first language but it seemed like an interesting problem, so I decided to look into it.

Here is one of the files in question: https://mega.nz/#!0g4VHaIR!KmpQirte4_3lv8MSxyjETiufjFGb-CITpFGrXwxSgGY

TL;DR: Mono loader used by Unity3D accepts invalid PE files. It can be used to break most .NET decompilers. dnlib and tools based on dnlib (dnSpy, de4dot) were updated on 20-Apr-2018 but the rest of the tools still can't handle such files.

Quick background on Unity3D and Mono

I quickly checked file in CFF and it looked like the file doesn't have proper PE header.

I fixed that. But even then it was not recognized as a .NET file.

So, the file is clearly invalid, yet it works just fine in Android! How is that possible?

Well, Android has no clue about PE files or .NET Framework. When you build your program in Unity3D and deploy it to Android, it uses Mono to run your code. Mono is supposed to be open-source alternative of .NET Framework. And it is incredibly buggy.

I'm not Unity3D or Android wizard but the whole monstrosity works something like this:

To make matters worse, even current versions of Unity3D are using a very old Mono version. Like 3+ years old. You can easily tell it by looking at the PE loader error messages. This is how it looks in IDA:

The commit df51163 is clearly missing.

Therefore, for the rest of the article I'll be using Mono sources from commit 74be9b6, so that they would more or less match the code used by Unity3D.

Cause of the problem

A good place to start looking for bugs would be Mono implementation of PE loader:

do_mono_image_load calls function mono_verifier_verify_pe_data which should filter out any invalid PE file and stop loading it. For some reason (which I really don't care about), this function does nothing in Unity3D and returns success.

After that, buggy mono_image_load_pe_data takes over and uses PE parser to load PE structures and .NET metadata. It is followed by equally buggy processing of .NET metadata in both mono_verifier_verify_cli_data and mono_image_load_cli_data.

But first things first..

Invalid PE signature

First of the errors is inside do_load_header. It gets called indirectly from mono_image_load_pe_data:

It checks only first 2 bytes of PE signature, so you can change it to "PE\0\1" or "PEPE" and it will still work under Mono.

Invalid NumberOfRvasAndSizes value

Next bug is located get_data_dir used indirectly by mono_verifier_verify_cli_data.

and get_data_dir looks like this:

Both verify_cli_header and get_data_dir ignore NumberOfRvasAndSizes field in PE header and access "CLR Runtime Header Entry".

Identical bug is in load_cli_header called from mono_image_load_cli_data:

They do not use broken get_data_dir function but the check of NumberOfRvasAndSizes field is still missing. That's why our PE file can have NumberOfRvasAndSizes == 0xA and it still works under Mono.

Invalid metadata size

Mono does a very limited checking of .NET metadata size in load_metadata_ptrs.

However, this check is insufficient. For example, you can set .NET metadata size = 0, most of .NET reversing tools will break but Mono will happily accept the file.

Invalid number of .NET streams

.NET metadata streams are loaded by load_metadata_ptrs.

You can use an arbitrary large number of streams in .NET Metadata header, as Mono will ignore all the invalid data. It probably will spam Android log with "Unknown heap type" messages, but the file will still run.

Invalid number of rows in .NET tables

Final nail in the coffin is the incorrect processing on .NET metadata tables. Tables are loaded in load_tables method which seems to be correct on the first look:

However, the definition of .NET metadata table information (_MonoTableInfo) is wrong:

As a result of this definition, they ignore high-order byte in row count. You can set number of rows to, say, 0xCC000001, .NET metadata will be invalid but Mono happily accepts the file. WTF?

Extra protection by AndroidThaiMod

Game hacks created by AndroidThaiMod.com take the Unity3D/Mono abuse to the next level. They exploit all the bugs I already mentioned. In addition to that, they replace the original libmono.so with their own version. In their version there are few extra lines of code in the function mono_image_open_from_data_with_name:

This change allows AndroidThaiMod to distribute their DLLs encrypted with "extra-strong" XOR encryption. smile

Another fun fact - their cheats only hack ARM version of Unity3D runtime. x86 version, even if present in original APK, gets removed from the hacked APK. So, if you have an Android device with x86 CPU, you're out of luck - AndroidThaiMod cheats won't work there.

Few AndroidThaiMod files I was able to find on Google:
https://drive.google.com/file/d/1g2_43rP9LLrXSmGL-Lw36GTehSUmg3CJ/view
https://drive.google.com/file/d/0B7jRiqM-QmgUeUF2RldrRUFmcUk/view

Other possibilities of abuse

As I mentioned, Mono PE loader is extra buggy. Pretty much all fields in PE header are not validated properly. Plenty of fields in .NET structures are ignored. PE32+ header processing is broken beyond belief. File/section alignment is not enforced. You can have several streams named "#~" and Mono will happily use one of them. Or you could just search for a phrase "FIXME" in Mono sources - you'll find lots of other dirty hacks that can be exploited.

Can it be fixed?

Probably. But considering how broken and messy the entire Mono codebase is, I wouldn't bet on it.

For example, on 13-Apr-2018 Mono developers made this awful commit called "Verify all 4 bytes of PE signature.":

There was no explanation why it was done, what are the side effects and whether they plan to fix all the broken code or if this was just a one-off fix. What a surprise!

And, as I already mentioned, Unity3D is using 3+ years old version of Mono. So, you'll probably have to wait until year 2021 until Unity3D gets they necessary fixes. bigsmile

Workarounds

Even if Mono and Unity3D was fixed, it doesn't help us to analyze existing broken files.

One workaround was to update dnlib (commit c40e148) and reversing tools to support such broken files. It allows to analyze existing files using dnSpy and de4dot. But Reflector and other tools still won't work. So, I decided to make my own tool which will fix broken files and will allow you to use any .NET reversing tool you like.

I needed a PE/.NET reading library which is very simple and low-level. dnLib/Mono.Cecil is way too abstract. After a quick search, I decided to use parts of McCli - but any other library would do just fine.

The code is very straightforward. Read PE structure->validate->fix->repeat.. wink

Conclusion

Games and cheats made using Unity3D are great. It's possible to make very unique protections that will stop most Windows/.NET reversing tools but the program will still work on Android. Enjoy the fun times! smile

Unity3D fixer with source code: