24 Apr

Sniffing correct serial in .NET crackmes

Introduction

In this tutorial I’ll show you a generic way how to break most of the crackmes written in VB.NET. It uses the fact that most crackmes made by beginners will calculate correct serial and do a simple comparison “if enteredSerial = correctSerial then”

To break such a crackme, you only need to find this comparison and sniff the correct serial. This is a very common approach in x86 world but in .NET world it’s not that popular yet.

As for my target, I’m using “RDG Simple Crackme .NET v4 2015

GetProcAddress in .NET

In x86 world you can use GetProcAddress function to get address of any API function from any DLL. Can we do something similar in managed environment like .NET? It turns out that we can, but it’s a little bit harder.

So, for example, to get address of Assembly.Load(byte[]) you need to do:

This works well with static classes and static methods. How about non-static methods like RijndaelManaged.CreateDecryptor(byte[], byte[])?

That’s doable as well, like this:

To make this reference almost complete – here’s how to get address of .ctor:

There are a few gotchas, however..

  • In case your target type is located in assembly that’s not NGEN’ed yet, I suggest that you use ngen and install the assembly in cache. That can prevent certain problems later.
  • Addresses of functions are obviously different in .NET 2.0 and 4.0. You must compile for correct framework version and target the correct .NET assembly.
  • Addresses of functions are different for x86 and x64 framework versions, too. Make sure your assembly is compiled correctly.

Sniffing string compare

Suprisingly, string comparison in VisualBasic.NET and other .NET languages is different. It’s caused by Option Compare statement present in Visual Basic language. So, if the crackme is made in VB.NET, you need to examine Operators.CompareString(string,string,bool) function. For crackmes made in other languages, you’ll need to examine string.Equals(string) or some other variation of this method.

So, using the code I mentioned above, I learned that address of Operators.CompareString(string,string,bool) on my PC is 599F1D30. Now I need to sniff data passed to this function.

There are several possible approaches. You can try using VisualStudio & Reflector plugin as SpoonStudio tried, you can try using ILSpy and it’s debugger plugin, or you can inject DLL into crackme process, as suggested by noth!ng – but I prefer to use OllyDbg.

Load crackme in OllyDbg, make sure that all the anti-anti-debug plugins are working, all the exceptions ignored, put a breakpoint on 599F1D30 and hope for the best.

Nope. Operators.CompareString is called literally thousands of times. So, we need to do something smarter.

For example, we can use conditional logging breakpoints in Olly. Those breakpoints are quite slow, but it’s still faster than to write some sort of hooking DLL and inject it into crackme. So, we need to set 2 logging breakpoints – one for each string compared. Here is first one:
crackme_conditional_breakpoint
Place second breakpoint at the next instruction (59CD1D31) and log string at edx+8.

Run the crackme, enter some fake but easily recognizable serial and few minutes later we have the answer:
crackme_logged_results
My entered serial was “1234567890123456789012345678901234567890” and it’s being compared to “C49476D583364356253377056314435396D456F44796C7A55746431564433544″. Hmm, could that be the correct serial for my nickname? ;) Yes, it is!

Final notes

This was quite nice crackme and I only showed the simplest way to beat it. When you start looking into it, you’ll find some nice anti-debug tricks, some nice anti-patching tricks and pretty nicely obfuscated code.

But that’s a matter for another story. Have fun!

16 Apr

About e-governments

Two days ago Google released Chrome 42. It’s the answer to life, the universe and everything. And among other things, it disables all NPAPI plugins by default, finally putting that Java nightmare into it’s grave. Good riddance!

But what about other NPAPI plugins? Like, you know, the ones used for electronic documents, digital signatures and other e-government thingies?

Well, here are 2 ways how government agencies approach the same problem:
e-governments compared

Welcome to the 21st century. If you’re Estonian, that is..

08 Apr

Catch me when you can

Introduction

Exception filters have been part of ECMA-335 specification since the very beginning. I’m guessing, they were added because Visual Basic used them extensively and therefore Visual Basic.NET had to support them as well. They look something like this:

Until now C# supported try/catch but did not have support for filters. That’s going to change in C# 6.0/VS2015.

How does it work

In early versions of VS2015 the syntax was “catch-if”, as you can see in the initial announcement. In the latest VS2015 CTP builds, they changed syntax to “catch-when”, and there’s a good reason for it.

So, how does it work and what does it mean for reversers?

It’s a compiler-level feature

As I mentioned before, .NET Framework has supported exception filters since the very beginning. So, this feature works even in .NET 2.0 – if you decide to target .NET 2.0 Framework in VS2015 project settings. Not that you really want to do that..

It’s very useful for debugging

catch-when is implemented as an IL exception filter. So, when an exception is thrown, exception filters are processed before the stack is unwound. This means that filter method has created an error report that included the current stack trace, it would show the frame in which the exception occurred. Sounds complicated? It isn’t.

Let’s implement exception filtering in the “old” way:

and this is how the stack looks when we get to filter(ex):
stack_trace_old
You can’t see much here. All the context is gone, you must rely on exception stack trace and message. That’s what we’ve always done, right? :)

If we write it in a “new” way, the code looks like this:

and stack trace will give us full context of exception:
stack_trace_new
Much better, isn’t it? You can see which method threw the exception, on which line, you have access to local variables and everything else. Yummy! :)

Decompiler support for exception filters is crappy

They say, a picture is worth thousand words.. In a very simple example, Reflector gets the code structure right, just filter conditions are missing:
catch_when_reflector
ILSpy handles it slightly worse, filters are messed up and unreadable. Filter code is gone, too:
catch_when_ilspy
And the latest JustDecompile just throws an exception:
catch_when_justdecompile

Have fun with it!

Here is a small keygen-me for you to play with: https://www.mediafire.com/?k5b9vy0p9dfgb97

The difficulty is 2/10, you should be able to solve it in 30 minutes or so. The entire protection is designed to show you try-catch-when feature, so avoid patching – you can’t learn anything by nopping-out few instructions. ;)

31 Mar

The malware arms race

Today’s Slashdot features a very nice question:

We’ve been in a malware arms race since the 1990s. Malicious hackers keep building new viruses, worms, and trojan horses, while security vendors keep building better detection and removal algorithms to stop them.

My question: will the balance continue, or is one side likely to take the upper hand over the next decade or two? Which side is going to win?

In the comments you’ll see a lot of libertarian psychobabble about how NSA/CIA/{whatever-3-letter-organization} caused this, how you are going to surrender your fundamental rights to a few corporations who pretend to protect you, how everything sucks and will suck even more in the future.

Well, that IS Slashdot, afterall.

But this comment made my day so much better:
Idiots will lose

Have fun and keep your schlongs safe!

23 Mar

.NET, ScyllaHide and HEAP_CREATE_ENABLE_EXECUTE

While doing some research on ILProtector, I loaded my test executable in Olly. To much of my surprise, it refused to run and all I could see in Olly log, was this:

Something smells fishy! ;)

I disabled all non-standard plugins, and I was still getting the exception. It was only after I removed the remaining 2 plugins (ScyllaHide and ODBGScript) that my test application launched. Few more restarts and I was sure that ScyllaHide is the one causing the trouble.

OK, I’ve found a bug in ScyllaHide. But where is it? Which option is causing it? And how can I fix it?

Unfortunately, there is no easy way. Just go through one option after another, until you trigger the bug. 10 minutes and 100 rude words later I was sure that “HeapFlags” is the culprit.

A side note from Captain Obvious

If you’re seeing access violation in Olly and want to know where it’s happening, make sure you uncheck Ignore Memory access violation in Debugging Options:
olly debugging options
and then run your target:
crash on jit stub
Here we can see that there is a real code at this address – small stub calling mscorwks.dll and that the call comes from ILProtector’s protect32.dll.

It immediately gives you plenty of useful information about what’s happening. Unfortunately I debugged one instance of Olly from another instance of Olly – got the same results but it took me much longer.

Meet HEAP_CREATE_ENABLE_EXECUTE

It turns out that .NET Runtime Execution Engine (mscoreei.dll) loves to put executable code on heap:

but ScyllaHide prefers to mark all heaps as non-executable:

and these 2 options kinda conflict with each other. :)

Workaround & fix

This small bug can be used to detect ScyllaHide, as it’s enabled by default in all configurations, and tooltip explicitly suggests to leave it as-is:

Very important option, a lot of protectors check for this value.

Here is a suggested patch:

If you don’t want to recompile the entire Scylla, here’s the binary patch for the official ScyllaHideOlly1.dll from ScyllaHide_v1.3fix_Olly1.rar package:

As a simple workaround, you could uncheck “HeapFlags” in ScyllaHide when debugging .NET applications. However, I would really suggest to fix ScyllaHide instead.

Have fun and keep it safe!

16 Mar

Fighting comment spam

When I started this blog, I was aware that comment spam exists. What I didn’t know, is how common it really is. :)

Current statistics are:

Comment type Count %
Spam 39 56
Normal 29 41
Trashed 2 3
Total 70 100

What can be done?

WordPress has several anti-spam plugins. Some of the add captchas, some rely on JavaScript and others rely on continually updated blacklists for spammer IPs and/or keywords. I hate captchas, I respect users that use NoScript, and my webhost is running with allow_url_fopen = false which prevents automatic blacklist updates. Crap!

So, I’m left with a very few options, like blocking spammer IP address ranges using .htaccess file and mod_rewrite.

mod_rewrite magic

When you know what you’re doing, mod_rewrite does wonders. When you don’t, you might lock yourself out of web-admin interface. Trust me, it’s not fun! ;)

In the very simplest form, we can block one IP address:

RewriteCond %{REMOTE_ADDR} ^(123\.456\.789\.666)$
RewriteRule (.*) - [F,L]

First line is a condition – if visitor comes from IP address 123.456.789.666, then apply the rule. Keep in mind that mod_rewrite is matching IP address against regexp, so do not forget backslashes! Otherwise you might accidentally block more than you wanted..

Second line is the rule – whatever URL it tries to access, send response “403 Forbidden”. (.*) is a regexp matching anything1. [F] forbids access and [L] stops any other rules from applying, making it the last rule.

It will work, but my webhost does not allow custom 403 pages. So, we can adjust the example a bit:

RewriteCond %{REMOTE_ADDR} ^(123\.456\.789\.666)$
RewriteCond %{REQUEST_URI} !/error.html$
RewriteRule (.*) /error.html [R=302,L]

Now there are 2 conditions, first is matching IP address, 2nd is checking if requested page is not error.html. Note that by default all conditions must match (logical “and”).

Also, [R=302] is used to redirect users with Error 302 Found to error.html instead of sending Error 403 Forbidden.

It’s better, but we need to block several IP blocks. That’s easy too!

RewriteCond %{REMOTE_ADDR} ^123\.456\.789 [OR]
RewriteCond %{REMOTE_ADDR} ^555\.666
RewriteCond %{REQUEST_URI} !/error.html$
RewriteRule (.*) /error.html [R=302,L]

Flags [OR] say we’re checking if IP address begins with 123.456.789 or 555.666. Also, the regexp was changed to check only beginning of IP address, and ignore the rest.

That’s it. Easy, right? :)

Identifying spammer-friendly IP blocks

I just went through my inbox and looked at the “Please moderate” emails:

Author : Adrienne (IP: 104.168.70.107 , 104-168-70-107-host.colocrossing.com)
E-mail : hekhwrjjrab@mail.com
URL : http://Adrienne
Whois : http://whois.domaintools.com/104.168.70.107
Comment:
Hi, my name is Adrienne and I am the sales manager at {Spammer Company}. I was just looking at your When software is good enough | Life In Hex website and see…

So, the offending IP address is 104.168.70.107.

DomainTools tells us it’s owned by ColoCrossing, and how large the IP block is:
IP Location: United States United States Williamsville Proxy R Us.com
ASN: United States AS36352 AS-COLOCROSSING - ColoCrossing (registered Dec 12, 2005)
Resolve Host: 104-168-70-107-host.colocrossing.com
Whois Server: whois.arin.net
IP Address: 104.168.70.107
NetRange: 104.168.0.0 - 104.168.127.255

Going through other notification emails, I identified 2 more spammer-friendly proxy/vps services: AS15003 and Krypt. It covers almost all comment spam, the rest are residental IP addresses in China and Vietnam – most likely part of some botnet and not really worth blacklisting.

Putting it all together

Armed with basic knowledge about mod_rewrite and offending IP addresses, I put it all together:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_URI} !/403.html$
RewriteCond %{REQUEST_URI} !/403.png$
RewriteCond %{REQUEST_URI} !/403.css$
RewriteCond %{REQUEST_URI} !/sad.png$
RewriteCond %{REMOTE_ADDR} ^23\.108\.170 [OR]
RewriteCond %{REMOTE_ADDR} ^23\.94 [OR]
RewriteCond %{REMOTE_ADDR} ^104\.168 [OR]
RewriteCond %{REMOTE_ADDR} ^98\.126
RewriteRule (.*) /403.html [R=302,L]
</IfModule>

So, anyone coming from those IP address blocks will get redirected to http://lifeinhex.com/403.html. Problem (hopefully) solved! :)

Further reading

These sites were invaluable in adding simple spam block to my blog:
How to redirect requests from particular IP addresses or networks with mod_rewrite – basic usage.
System: mod_rewrite: Examples – great examples, explained well.
mod_rewrite Cheat Sheet – all I ever wanted to know, and little bit more.
How To Ban And Block Proxy Servers? – I didn’t have to take this approach yet. And it wouldn’t work against “elite” proxies anyway.

Footnotes

1. Actually, the pattern in the RewriteRule does not need to match the _whole_ URL, so you might encounter "$", "(.*)", "." and many more variations in these kinds of rules.

11 Mar

Miserable state of open source code

Yesterday I wanted to make a small API hook detector in C#. It has to parse PE file, find exported functions, read bytes from the beginning of function and then compare them with the bytes in process memory. Sounds simple, right?

Well, good luck finding a PE parser that actually works!

Looking for PE parser

Most of PE parsers stop at parsing DOS header, NT headers and section headers. But I needed something that would also parse export table for me. After a couple of Google searches I ended up with PEReader by DKorablin. From the first look it’s decent and even has a demo application. What else could you want?

Hmmm, how about working correctly on really simple files? ;)

Sorry, nope.

Results from PEReader and CFF
It sure finds exported functions but it mismatches function names & RVAs. So, if you wanted to examine, say, CreateFileW, you will end up examining DeleteFileA. Or some other random API. Great job!

But it’s opensource. Just fix it and submit a patch!

Umm, no. I was looking for a PE parser that I can take, load it in VS and use it. I don’t want to spend days hunting down bugs and fixing them – this stops me from doing what I really want to do.

So, dear opensourcer, if you are publishing your code, make sure it actually works. If it doesn’t work, please don’t publish it at all – it’s not helping anyone. Don’t waste other people’s time..

P.S. I ended up with using DNLib and writing my own PE export parsing. At least, I know it works properly..

10 Mar

Gigaom suspends operations

This day started with some sad news:

Gigaom recently became unable to pay its creditors in full at this time. As a result, the company is working with its creditors that have rights to all of the company’s assets as their collateral. All operations have ceased.

Gigaom is (was?) a very nice technology site. I liked it. And I’ll certainly be sad to see it go. However, this failure proves something I’ve said for years, yet everyone else seemed to disagree:

If your web-based business depends on ad revenue, you’re going to fail.

Sure, some stay at home moms can get few hundreds of bucks per month by blogging about the color of their baby poop. And some other stay at home moms will read this shit blog, looks at the ads, and possibly even click on Pimpers(tm) picture to buy some. Yes, that can work.

But you can’t build a real business based on ad revenue.

Nobody loves ads

Face it – people hate ads. That’s why people install adblockers. It’s not because they hate you, the poor publisher. It’s not because adblocker speeds up web page loading time. And it’s certainly not because people need to save some traffic.

You can remind them nicely:
flightradar

You can nag and deny access to your precious content, like ArsTechnica did in 2010, or lots of sites try to do today:
playindiafilms

But no matter how hard you try, you can’t make a long-term business based on ads.

R.I.P. GigaOm, I hope others will learn from your failure.

05 Mar

Improved static Enigma Virtual Box unpacker

Last few weeks have been really hectic. I moved to a new apartment, so lots of time was spent on packing, unpacking, cleaning, and other non-computer related chores. Finally it’s done, I got a great new place to live and I’m happy. :)

To relax and get back into shape, I spent an evening with one of my old projects – Enigma Virtual Box unpacker. I fixed few little bugs and added support for x64 executables.

EnigmaVB unpacker

Get it here: https://www.mediafire.com/?mie1wida3aicrrt

26 Feb

When software is good enough

Microsoft, Google and Apple make software for everybody. Millions of users run this software every day. It must be stable and user friendly, so that Aunt Judy and Average Farmer Joe can use it. If it crashes, clueless user can’t do much about it – and that’s bad. That’s why these companies spend thousands of hours in testing and improving usablility.

On the other hand, reversers make tools. A specialized software for solving small and nasty problems, like hiding debugger, defeating specific protection or bypassing some authorization check. Tools are made by a reverser for a reverser, so there are completely different expectations for them. Nobody expects that today’s DNGuard unpacker will work with next year’s DNGuard binaries, or that DRM authors won’t change their encryption mechanisms.

That’s why reversers make tools that are just “good enough”.

Olly, Confuser and de4dot

Funny thing happens when reversing tools suddenly become extremely popular. Newbies start using them, ordinary users start using them – and the expectations change. Suddenly the author is overwhelmed with extremely helpful “bug reports” like “cannot unpack latest reactor” or “obfuscation fails for my application”. It’s annoying, wastes reverser’s time and is not helpful in any way. Therefore I totally understand 0xd4d’s reaction:

There’s no support. Don’t email me if you can’t use it or if it fails to deobfuscate a file obfuscated with an updated obfuscator.

Instead, try to update de4dot yourself. It’s a lot easier than you think. If you can’t, search the Internet and you should find a couple of forums where you can ask your question.

TitanHide is good enough

Earlier this month I made few posts about bugs in TitanHide. Are these real bugs? Yes. Is it important to fix them? Not really. Let’s face it – there are literally dozens of ways to detect TitanHide. But until commercial protectors start doing that, nobody cares.

TitanHide works and does its job well – that’s all that matters. :)

The two bugs I mentioned earlier

First bug was a confusion about CONTEXT_DEBUG_REGISTERS flags. You see, CONTEXT_DEBUG_REGISTERS is defined as

which is quite unexpected. :) So, the code

was accidentally removing CONTEXT_i386 flag from ContextFlags. Such call to should fail, I’m pretty sure it did fail in some cases in my VMWare, but in real world it works just fine.

Second bug is in checking if CONTEXT structure is writeable when calling SetThreadContext. Why should it be – SetThreadContext is only reading from it.. So, this pseudo-code lets you defeat TitanHide hardware breakpoint protection with ease:

Again, it’s a small bug, nobody is abusing it yet, so there is no real reason to fix it.