PE-bear – version 0.3.8 available

It has been a long time since I abandoned PE-bear project (version 0.3.7 was released in 2014!). But due to the fact that it still has new downloads, and I keep getting messages from its users, I understood it would be a shame to leave it without any support. A tool is alive as long as someone wants to use it, so, here is an update for PE-bear.

As I wrote in the release notes, the latest release fixes several bugs . In this post I will elaborate on the most important changes and illustrate them with examples.

  1. Fixed bugs in parsing Delay Load Imports (64bit)

So, this is the old, incorrect version (example: winnet.dll, 64bit)


And in the new, corrected one:

delayed_imp_new2. Fixed bugs in parsing Load Config Directory (64bit)

This is the old, incorrect version:

load_config_old The fields ProcessHeapFlags and ProcessAffinityMask should be flipped, otherwise their sizes are incorrectly identified. It is fixed in the new release:


3. While adding a new section, the selected access rights were applied only if the section was loaded from the file. Also, in some alignments, there was a cave appearing between the previous section and the added one, that needed to be fixed manually in headers, or otherwise the application won’t run. This all is fixed in the current version.


Section test added by new version:


I fixed also some other, smaller bugs here and there. So if you like PE-bear, it’s time to update. And if you don’t know it yet, feel free to give it a try, because from now onward I am not gonna leave this app without support, and if you find any bug it will be fixed as soon as possible. However, I will do only minimalistic mantainance, so don’t ask me for some super cool new extra features. (Or maybe I get tempted for more… No, I won’t 😉)


Posted in PE-bear | 4 Comments

White Rabbit crackme!

UPDATE: We already got the three winners. Good job guys! However, we are waiting for the writeups to select the reward for the best one – so if you are still in between of doing the crackme, don’t give up!

UPDATE2: We got first writeups! All the upcoming ones will be linked under the section “Writeups”. The submission for the contest closes 20th February.

UPDATE3: Contest closed! The winner is @Eleemosynator with his writeup available here. Both writeups were very good and detailed, so we decided that the second one, by @pieceofsummer, also deserved a distinction and a bonus reward. Big thanks to both authors!

This time I would like to introduce a small contest organized by me and Grant Willcox. I wrote a small crackme and he volunteered to sponsor the rewards. The first 3 solutions will be rewarded by books chosen by the winners.

The crackme is a 32bit PE file. It shouldn’t be too difficult, but I didn’t want to make it boring either, so it has few tricks.


Disclaimer: I am not an author of the graphics used in the application, such as ASCII arts, icons and others. I don’t claim any rights to them.


You need to find the flag in format flag{...} and submit it ASAP to any of us as a DM on twitter (@hasherezade or @tekwizz123). After we announced that the contest is closed, we would like you to make a writeup explaining how did you solved it.

There will be an additional reward for the best writeup – so even if you was not the fastest, you still have a chance to get a book for free.

If you have any questions, you can write them as comments to this post and I will be answering them. I am not giving hints via private messages – I want the contest to be fair for everyone.

At the end I will publish my own writeup with a detailed explanation.

Download: (password: crackme)

Mind the fact, that the crackme contains some small obfuscation and malware-like tricks, so it may be flagged by some of the AV systems as malicious. False positives are very common when it comes to crackmes – it can’t be helped, sorry! I recommend you to run it on a Virtual Machine.


check_mark  Finished? You can rate it!


Posted in CrackMe | Tagged , | 9 Comments

Unpacking a malware with libPeConv (Pykspa case study)

In one of the recent episodes of “Open Analysis Live!” Sergei demonstrated how to statically unpack the Pykspa Malware using a Python script. If you haven’t seen this video yet, I recommend you to watch, it is available here – and the full series is really cool.

The video inspired me to use the same sample and demonstrate an alternative solution, applying my library, libPeConv . The advantage of using libPeConv is that you don’t have to spend time on understanding and rewriting the unpacking algorithm. Instead, you can import the original unpacking function from the original malware. It can speed up the work and be helpful also in the cases when the function of our interest is obfuscated.

Analyzed sample


Static analysis

The static analysis of this malware is already well demonstrated in the mentioned video. I will just recall the important points to which we are going to refer.

Function definition

The function that is responsible for unpacking is available at RVA 0x4520. It has the following prototype:


By analyzing how it is applied, we can find out what are the arguments that should be passed:


The first one is a blob of data (BYTE*), second – size of the blob (DWORD), next comes the name of the file where the output will be written, and last one is some magic char. This is how the function declaration should look:

int __cdecl *unpack_func(BYTE* blob, DWORD blob_size, LPCSTR lpFileName, char magic_val);

Function arguments

The function is applied twice, to decrypt two blobs of data (I call them blob1 and blob2). Important things to note are: the offsets of the blobs, their sizes and the passed magic values.

Decrypting blob1:


  • Blob1 RVA: 0xC030
  • Blob1 size: 0x11000

By following the code before the function call, we can find that the last argument (the magic char) must have the value ‘r’.


Decrypting blob2:


  • Blob2 RVA: 0x1D038
  • Blob2 size: 0x50000

Again, the magic value is ‘r’:


Now we have all the data to implement a static unpacker.

Writing a unpacker

Setting up the project

For this part you need to have Visual Studio, CMake and Git installed.

I already prepared a template that you can use to make a libPeConv-based project, so it is enough to fetch it from my Github:

git clone --recursive

Now use the CMake to generate a VisualStudio project:


The malware is 32bit, so it is important to generate a project for 32bit build, otherwise we will not be able to import the sample. Example:


Click “Finish” then  “Generate” and finally you can open the project in Visual Studio.

Unpacker’s code

Code of the full unpacker is very short:

Firstly, we load the original malware (by a function from peconv). We need it to be loaded with all the dependencies and ready to be executed. A function that allows to achieve it is load_pe_executable:

BYTE* peconv::load_pe_executable(LPCSTR path_to_pe, size_t &out_size);

This malware sample has no relocation table, so we not only need it loaded, but it must be loaded at it’s original base. This operation may fail on some runs, so we have to keep it in mind.

size_t v_size = 0;
BYTE *malware = peconv::load_pe_executable(mal_path, v_size);
if (!malware) return -1;

Then, using the known offset and the reconstructed declaration of the unpacking function, we are importing it from the loaded malware.

ULONGLONG func_offset = (ULONGLONG)malware + 0x4520;
unpack_func = (int (__cdecl *) (BYTE*, DWORD, LPCSTR, char)) func_offset;

We also use the known offsets of the blobs, and make pointers to the data. After we called the unpacking function with appropriate arguments, our payloads will be dumped to files with the supplied names.

DWORD res1 = unpack_func((BYTE*)((ULONGLONG) malware + blob1_offset), blob1_size, "blob1_unpack.bin", 'r');
std::cout << "Unpacked blob1, res:" << res1 << std::endl;

DWORD res2 = unpack_func((BYTE*)((ULONGLONG) malware + blob2_offset), blob2_size, "blob2_unpack.bin", 'r');
std::cout << "Unpacked blob2, res:" << res2 << std::endl;

At the end we can free the loaded malware:

peconv::free_pe_buffer(malware, v_size);

That’s all, the unpacker is ready. One last thing we can do is preparing a .bat file that will run the unpacker until the malware get loaded (remember the loading base issue caused by the missing relocation table).

Example of the batch script:

@echo off
peconv_project.exe malware.bin

The full package (except the malware) is available here:

Finally, let’s see it in action:

Posted in Malware, Programming, Tutorial | Tagged | Leave a comment

Solving a PyInstaller-compiled crackme

I got this crackme from one of my readers, who asked me for the help in understanding how to solve it. As he wrote in the e-mail, it comes “from last year competition by the CheckPoint company”. I promised to make a writeup, so here it is :). I hope it will benefit others also.

The crackme is for the beginners, so don’t expect any fireworks ;). But it was relaxing and fun to solve.

The crackme can be found here (password: crackme), also available at HA: 8ee7382cfdf632c29df5f2d9d3286614


This is how the application looks:


When we run in, it asks for a username:


And when we give an invalid one, it responds with a text:

“Go away, you are not me”

The first important step in solving the crackme, is noticing how exactly was it made and what tools are to be applied. As the icon hints, it seems to be an application written in Python and converted into EXE. But let’s confirm it by looking inside. The main process runs another instance of itself:

Let’s attach the debugger to the child process and see the loaded modules:

We can find that indeed Python2.7 is loaded to interpret the code (the module is marked red on the picture).

At this moment we can confirm that this EXE is in reality a wrapper for a Python script. There are several applications that allows to achieve it. Depending on which application produced the wrapping, the output has a bit different format and requires a different decompiler.

The popular converters of Python scripts into EXE format, are, i.e. Py2Exe and PyInstaller. This time, PyInstaller was applied.

Tools required

Step 1 – Unwrapping the exe

Unpacking the EXE is easy with the appropriate tool. In this case I used PyInstallerExtractor, written in Python.

python pycrackme.exe

This is the output:

[*] Processing pycrackme.exe
[*] Pyinstaller version: 2.1+
[*] Python version: 27
[*] Length of package: 2604972 bytes
[*] Found 20 files in CArchive
[*] Beginning extraction...please standby
[+] Possible entry point: pyiboot01_bootstrap
[+] Possible entry point: black_box
[*] Found 196 files in PYZ archive
[*] Successfully extracted pyinstaller archive: pycrackme.exe
You can now use a python decompiler on the pyc files within the extracted directory

The script directly hints, that the next step will be to use a Pyhon decompiler and turn the obtained pyc files into Python scripts.
It also hints about the possible entry point of the application. This information helps us to find where the code of our interest is located.

Step 2 – Decompiling the pyc

The produced output is stored in the directory corresponding to the name of the input executable. We can see there multiple modules extracted, but the interesting one seems to be the file called “black_box”:


The black_box is a pyc file with a magic number removed, so we can just copy this part  from some other pyc file that we found in the extracted set, i.e.


Let’s paste it at the beginning of the black_box:


After this step we are ready to save it as black_box.pyc and decompile:


And here is the result:

The file got decompiled properly, and at this point we can rename it to py and run like any other Python script. We can see the same prompt as it was in the EXE:


Looking at the code we can see that is is mildly obfuscated. The important part is hidden in the variable “c” that is obfuscated by ROT13 and compressed by ZLIB:

c = '789p8594po76n34n0p457s8947p85n0p0q31506093s02nn06654912r30p563o50q367p7q0o3o9qo6q37q6s061r182471246q1qq2o9138q8p3rpssp9rqnn246o638315o9s4oq3580n7o3oso71rr915o8p54n4o2o87s4s6q2pq8661868664pnp0o3853s443n1606907rsqro3oq670np348643s8o65778o5r248r0r7or5350r34p40714601snqps985255so43n278q69s582sq27n92554q919682n941rs05034r241pnqs1nrn40q44p147s73popp45268qqrns2o3os206q6s22ps6p441024r136n9373n32q965ss128p507sqpp78qo63orp2s9017p0379235ps8so96ns2n12169prn9nq7745snq47o71rs2173p39163pp45nn49p844o559p37sq390998267qn5n2712416qp7nq4n591s6867p8n5p9ro1062498678q88n1934s3788q8o7o8rrnr656ps162q72s0423277pp618roopr5q4np71q98p93o645qp6pp093n2sqn546s9171rqs59543rr9ns7n0p575r86p54n5626928320855139r672rrp449o601sn0ro69qpq5p8soppoorq6184590rn071303s6n32s82r857qn1n612q4or4pp8p40orpppn18nqr20279p90r30o12191nq4ns50oq11p8p271883srn3n655sn4177816pr659n3pqs6673so31qo5n985n4rnp0q7s1015q850s71psn99s3npsp01snps6516s4n8p16369r319oq2427628onrp882ooopr019r63q3pq4spnq559po661r2r250969r91rq57q4s6656n5op0n4q681qr3r8sn495555r40nqp27p98p199o5622ppq4q9q022s81p04r067op9151q38o6r65o7r399436r76ro6rsq87poo8557r89opq08s213rrqqsn3sn26q652412ss5r9s6308q612r8471r68po780roo2o22o9661q964824o754750s305sr0rr52oqss6367q7oo32o9rp05770p5ps9813r6324so7575068q4on9r833o1s4nr4ps1p2r6662p6pspo4pr9464or0487p0qs6289475o2o8r5rsnoosn5s751o15537q15343568nors5084sr8npq3o57r4r9s15rn3n0p0p803n35s668q55s00pr8sq6ro715r2sqs70ppqro6qs953nr9p44696n0o55sp9s04o8s9s496p6o23349op863838qrp5p5o1r4rs32297492p6s277r6r6sp303o673sp2or47qnr203qp411s61p308o76104pp5soqp08o9n87s9rs3n5sq016orn986o3o7s5qsp04q865q1o0nn8q1908877ro9pqqo979r460q60rr0210oopssopo32s3r023p9q79r994q32r2555nr8613nor196o3o0272o1sn95s95o698686q0pp0r04p324o2rr0o689s9p483p880592627780s376r1pn8120n98610571po9n7ooo593195r03s2495no223q037361p3qn950op4qq0s4so9q77orq05ss66r7ossqrp1n77rs63qs316nso13o1s19sso07060349qs86s1ss31r8poo9r2nss11530q17ss6688o656525078s784s75q95557nsoo4p54395qrs8no43ns8483sn0os39npn1s7nn38qs6op347n63po2s6p55o33q6rn23993sor0q77009679n44n39q09n0sos00070p6111'
exec base64.b64decode(zlib.decompress(binascii.unhexlify(c.decode('rot13'))))

To understand it better what happens here, we should dump the content after the deobfuscation, rather than executing it. In order to do so, I slightly modified the script. I removed the code responsible for executing the second stage, and substituted it with the function that writes the decompressed result into a file. This is my modified version:

Now, once we execute this script, we get the next stage dumped. And this is how it looks:

from hashlib import sha256
from time import sleep
import socket, sys

PASSWORD = "36949"
HASH = sha256(PASSWORD).hexdigest()
USER = 'Nigel'
CODE = "807290"

PORT = 587

def login():
    print ""
    username = raw_input("Enter First Name: ")
    if username.rstrip(' \n\t') != USER:
        print "Go away! You are not me..."

    print "Hello %s, Good to see you!" % USER
    while True:
        password_guess = raw_input("Enter 5-digit password: ")
        print "[DEBUG]: calculating sha-256 hash"
        print "[DEBUG]: comparing with %s's hash: %s"  % (USER, HASH)
        print "[DEBUG]: performing anti-brute-force delay..."
        if sha256(password_guess).hexdigest() == HASH:
            print "Password OK!"
            print "Wrong password!"

    while True:
        s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, 0)
        s.connect((IPADDR, PORT))

        print "%s, two-factor authentication is required. A one-time code was sent to your email address" % USER
        code_guess = raw_input("Enter code: ")
        if code_guess == CODE:
            print "Success! The code is what you're looking for :)"
            print "Wrong code!"


The script is not further obfuscated.
Once we read it, it’s pretty straight-forward what to do next. So, the username was Nigel. Then, we have to give his password that is 36949 and finaly his code: 807290. This was my final conversation with the crackme confirming that the code is valid.



Enter First Name: Nigel
Hello Nigel, Good to see you!
Enter 5-digit password: 36949
[DEBUG]: calculating sha-256 hash
[DEBUG]: comparing with Nigel's hash: 6912863904dab1ddc332a928bf6df7f365bf1131906f3424aa931c6c85595c34
[DEBUG]: performing anti-brute-force delay...
Password OK!
Nigel, two-factor authentication is required. A one-time code was sent to your email address
Enter code: 807290
Success! The code is what you're looking for :

Exactly the same results we get when we talk with the original EXE:
So, the final answer is 807290.


This crackme can be solved very easily if we know the few tricks. The most important was to find what are the proper tools to be applied. Once we got them, we could easily decompile the code and read the answer.



Posted in CrackMe, Tutorial | Tagged , | Leave a comment

Process Doppelgänging – a new way to impersonate a process

Recently at Black Hat Europe conference, Tal Liberman and Eugene Kogan form enSilo lab presented a new technique called Process Doppelgänging. The video from the talk is available here. (Also, it is worth mentioning that Tal Liberman is an author of the AtomBombing injection).

This technique is a possible substituent of the well-known Process Hollowing (RunPE), that is commonly used in malware. Both, Process Doppelgänging and Process Hollowing, gives an ability to run a malicious executable under the cover of a legitimate one. Although they both serve the same goal of process impersonation, they differ in implementation and make use of different API functions. This is why, most of the current antivirus solutions struggled in detecting Process Doppelgänging. In this post we will take a closer look on how the Process Doppelgänging works and compare it with the popular RunPE.

WARNING: Running this PoC on Windows 10 may cause a BSOD – the reason is a bug in Windows 10. Details here.

Process Doppelgänging vs Process Hollowing (aka RunPE)

The popular RunPE technique substitutes the PE content after the process is created (suspended), but before it is fully initialized. In order to implement this technique, we need to do by our own the step that WindowsLoader took so far: converting the PE file from it’s raw form into a virtual form, relocating it to the base where it is going to be loaded, and pasting into the process’ memory. Then, we can awake the process from the suspended state, and the WindowsLoader will continue loading our (potentially malicious) payload. You can find a commented implementation here.

The Process Doppleganging, in contrary, substitutes the PE content before even the process is created. We overwrite the file image before the loading starts – so, WindowsLoader automatically takes care of the fore-mentioned steps. My sample implementation of this technique can be found here.

NTFS transactions

On the way to it’s goal, Process Doppelgänging uses a very little known API for NTFS transactions.

Transactions is a mechanism commonly used while operating on databases – however, in a similar way it exists in the NTFS file system. It allows to encapsulate a series of operations into a single unit. Thanks to this, multiple operations can be treated as a whole: they can either succeed as a whole – and be committed, or fail as a whole – and be rolled back. Outside of our transaction, the result of the operations is not visible. It starts to be noticeable after the transaction is closed.

Windows API makes several functions available for the purpose of transactions:

Briefly speaking, we can create a file inside a transaction, and for no other process this file is visible, as long as our transaction is not committed. It can be used to drop and run malicious payloads in an unnoticed way. If we roll back the transaction in an appropriate moment, the operating system behaves like our file was never created.

The steps taken

Usage of NTFS transactions

Firstly, we need to create a new transaction, using the API CreateTransaction.

Then, inside of this transaction we will create a dummy file to store our payload (using CreateFileTransacted).

This dummy file will be then used to create a section (a buffer in a special format), which makes a base for our new process.

After we created the section, we no longer need the dummy file – we can close it and roll back the transaction (using RollbackTransaction).

Usage of undocumented process creation API

So far we created a section containing our payload, loaded from the dummy file. You may ask – how are we going to create a process out of this? The well known API functions for creating processes on Windows require file path to be given. However, if we look deeper inside those functions, we will find that they are just wrappers for other, undocumented functions. There is a function Zw/NtCreateProcessEx which, rather than the path to the raw PE file, requires a section with a PE content to be given. If we use this function, we can create a new process in a “fileless” way.

Definition of the NtCreateProcessEx:

Creation of process by this way requires more steps to be taken – there are some structures that we have to fill and setup manually – such as process parameters (RTL_USER_PROCESS_PARAMETERS). After filling them and wring into the space of the remote process, we need to link them to the PEB. Mistake in doing it will cause the process to not run.

After setting everything up, we can run the process by creating a new thread starting from it’s Entry Point.

Despite some inconveniences, creating the process by a low-level API gives also interesting advantages. For example, we can set manually the file path – making an illusion, that this is the file that has been loaded, even if it was not. By this way, we can impersonate any windows executable, but also we can make an illusion, that the PE file runs from a non-existing file, or a file of a non-executable format.

Below you can see an example where the illusion was created, that the PE file runs from a TXT file:

How to detect?

Although this technique may look dangerous, it can be easily detected with the help of any tool that compares if the image loaded in the memory matches the corresponding file on the disk. Example: detection with PE-sieve (former hook_finder):

The process of injection is also not fully stealthy. It still requires writing into the memory (including PEB) of the newly created process, as well as creating a remote thread. Such operations may trigger alerts.

In addition, the mechanism of NTFS  transactions is very rarely used – so, if any executable call the related APIs, it should become an object of a closer examination.

So far this technique is new, that’s why it is not broadly recognized by AV products – but once we are aware of it’s existence, implementing detection should not be difficult.

Posted in Malware, Programming, Techniques | Tagged , | 3 Comments

Hook the planet! Solving FlareOn4 Challenge6 with libPeConv

Recently I started making a small library for loading and converting PE files (libpeconv – EARLY BETA available on my GitHub). In my previous post, I demonstrated how the Challenge 3 from FlareOn4 could be solved with it’s help: I used libPeConv to import the function from the original crackme, so that it can be used as local – without the need of re-implementing it or emulating.

This time, we will have a closer look at challenge 6 from FlareOn4. This challenge is a bit more difficult, so it is a good opportunity to show some other capabilities of libPeConv – not only importing functions, but also hooking the imported code in various ways.

When I solved this crackme for the first time, during the FlareOn competition, my approach was very dirty – it required me to go through 26 MessageBoxes, write down each value, convert them from hex to ASCII and put them together to make the full flag – oh, my! Looking at the write-ups afterwards, I noticed that most of the people did it this way (check appendix for more details). But I was sure that there must be a better solution – and with the help of libPeConv, I finally did it in a way in which I wanted: no pop-ups to click, the flag is automatically composed by the loader.


The end result looks like this:

github  The repository with all the presented loaders (code + compiled binaries) is available here.

The full code of the final loader:

In this post I will to explain in details how I made it, show the experiments and the reasoning behind them.

Tool used

For analyzing the crackme:

For building the solution:


The challenge named payload.dll is a 64bit PE file. When we look at it’s export table, we can find that it exports one function, named EntryPoint:

But if we try to run it in a typical way, by rundll32.exe payload.dll,EntryPoint, it turns out that the function cannot be found:

Pretty weird… So, let’s try to run it by ordinal: rundll32.exe payload.dll,#1:

This way works – however still we are far from getting the flag.

The curious thing is why the exported function was not found by the name? It seems that the export name has been overwritten while the DLL was loading. To check it very fast, we can use hook_finder, a tool that detects modifications of the running PE vs the PE on the disk.

I called the function again by the ordinal, and when the MessageBox popped up, I scanned the running rundll32.exe process by hook_finder. Indeed we can see that the DLL was overwritten:

The modified image has been automatically dumped by the hook_finder, so we can open it by typical tools. First, I use PE-bear to take a look at the exports table:

And yes, now it looks very different… Let’s see this function in IDA.

Looking inside we can confirm that this was the function responsible for displaying the message that saw before:

This message is displayed when the function is called without any parameters. If, in contrary, it is called with proper parameters, some further chunk of code is decrypted and executed:

The name of the function is used as the key for the decryption. So, what are the conditions that the supplied arguments must fulfill?

The exported function expects 4 arguments:

As we can see, the checked argument is the third one of the arguments supplied:

It is compared against the function name. If it is exactly the same as the function name, the decryption proceeds – otherwise, the fail MessageBox (“Insert clever message…”) is shown.

Let’s run the function with proper parameters and see what happens.
This is a small wrapper that will help us call this function from our code:

We can do the same from the command line:

rundll32.exe payload.dll [func_name] [checked_str]

Cool, a new message popped up. It seems to be a chunk of the key: 0x75 -> ASCII ‘u’. But this is just one of the pieces, and we have to get the full key.

For this purpose, we will look inside the DLL to find the the code that was responsible for overwriting the exports table. That function starts at RVA 0x5D30:

This is the pseudocode:

It decrypts the code chunk pointed by the given index and redirects to the new exported function. We want to manipulate the indexes in order to get all the remaining key parts. The index of the chunk is calculated basing on the current time, inside the function at RVA 0x4710:

We can see operation modulo 26, so it means it is the maximal value. There are 26 possible indexed -> 26 pieces of the key. The calculated index is then supplied to the decrypting function. The decrypting function is pretty simple – based on XOR:

First, the random generator is initialized basing on the supplied chunk_index + a constant. Then, pseudo-random values, retrieved by rand() are used as the XOR key. Thanks to the feature of this (weak) random generator, values are not really random – the same seed gives always the same sequence, so it works pretty well as the key.

The simplest to implement (and terribly annoying)  approach to solve this task is to keep changing  the system time, running the DLL and writing down the popping up chunks of the key. Sounds too ugly? Let’s see what libPeConv can do about it…

Importing and hooking function with LibPeConv

Preparations and tests

In the previous post I gave some overview of PeConv library, so if you didn’t read that part, please take a look. This time, I will use the features that I introduced before plus some others. We will not only import and use the code of the original crackme, but also mix it with our own code, to alter some behaviors.

First, I want to import from the crackme the function that overwrites the exports. This function has at RVA 0x5D30 and it’s prototype is:

__int64 __fastcall to_overwrite_mem(__int64 a1);

Let’s make a loader that will load the crackme to the current process. This was my first version:


Everything looks good and should work, but when I run it, I met an unpleasant surprise:


When we try to debug the code, we will find that the exception is thrown from inside the statically linked functions srand() and rand(). They were used by the function dexor_chunk_index, within the function to_overwrite_memory that we imported from payload.dll.

This may happen due to the fact, that  the payload.dll has been loaded manually, not by the windows loader, and some of those low level structures are not filled. Statically linked srand() and rand() tries to query invalid addresses. But this is easy to fix – we can just redirect the calls to the copies of the same functions that resides within the loader.

First, let’s get the addresses where those functions are in the payload.dll:

The function srand has RVA 0x7900:


And rand: 0x78D4:


I will redirect them to local  copies. I added few line of code that does this:


And now everything works smoothly without any crashes. If we want to log the rand values, instead of making redirection to the original function, we can redirect to our own wrapper, i.e.:


Now, instead of running silently, it will print a value each time when the rand was called:


Now, let’s test if the exported function has been overwritten properly. If so, we should be able to use it analogically like in the case of the previous basic loader.
Instead of GetProcAddress, that I would use on the module loaded in a typical way, I used a function from PeConv with analogical API:

peconv::get_exported_func(loaded_pe, MAKEINTRESOURCE(1));

And this is the code of the full loader, this time using libPeConv:

And yes, it works exactly the same:

As we know, the argument that we supply to the function changes depending on the current month and year. For December 2017 it is:


But it would be nice if our loader can fill it automatically.

This argument must me exactly the same as the exported function name. We can take advantage of this and use a libPeConv’ feature of listing exported function names. Code:


This is the improved version of loader:

Everything works fine:

Ok, tests and preparations are over, now is time for the solution.

Manipulating the chunk index

We have everything ready to start manipulating the index. There are various approaches – one of them is to hook the imported function GetSystemTime. But with  libPeConv we can hook also local functions, so let’s make it even simpler.

The function that calculates the index can be found in the payload.dll at RVA 0x4710:


We will use exactly the same function as we used before, to redirect the statically linked rand and srand:


All we need to prepare is our own function returning the index. For example, we can enforce it to return some hardcoded index:


And it works!


But recompiling the loader each time when we want to change the index is not a good idea. So, I made an improved version of the loader that allows you view the chunk at the index supplied as the argument. Code of the loader:

See it in action:

By this way, we can retrieve all the key pieces one by one. But still, we need to encounter those annoying MessageBoxes. Let’s replace them and redirect the message that was going to be displayed to our own function. This time we will be hooking a function linked dynamically – so, installation of the hook will be a bit different.

Hooking IAT with libPeConv

When libPeConv loads executable, it resolves imported function with the help of a specially dedicated class: peconv::t_function_resolver. The library allows to use non-standard resolvers instead of the default one – the only condition is that they have to inherit from this base class.

One of the resolvers that comes in the full package is a hooking_func_resolver. It allows to hook IAT. Basically, when it loads imports, it may substitute some of the imported functions by our own functions (the only condition is that they must have the same API). For now, this resolver supports replacing functions defined by names. So, for example if we want to replace MessageBoxA by our own my_MessageBoxA:


For example, we can replace it by the following function:


Now, instead of displaying the MessageBox, with the character written in hex…


…it will display the same character as ASCII:


This is the code of the full loader:

We can use it along with a small batch script:

@echo off
set loopcount=0
peconv_hooked_msgbox_sol.exe payload.dll %loopcount%
set /a loopcount=loopcount+1
if %loopcount%==26 goto exitloop
goto loop

As the result we get the full flag printed and 0 annoying pop-ups:


In the final version, the key is composed by the application itself:


LibPeConv is my new project, still on very early stage of development, so many things may change – but it already proven that it can be useful in solving some challenges. It gives you possibility not only to import code of other executables to your projects, but also to hook it and modify. I hope you guys will try it and have so much fun with it as I have developing it. I am looking forward to hear some feedback from you!

All the binaries that were used in this demo are here – the password to the zip is “crackme”.


See also other approaches:


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Import all the things! Solving FlareOn4 Challenge 3 with libPeConv

Recently I started making a small library for loading and converting PE files (libpeconv, available on my GitHub). The library is still on early stages of development, so please don’t judge and don’t use it in any serious projects. The API may change anytime! However, I have so much fun developing and testing it, that I wanted to share some of my experiments and ideas.

Some time ago I solved some of the FlareOn4 challenges, i.e. the challenge 3. That time I didn’t have the libpeconv yet, so I solved it by some other method. Now it came to my mind, that with the help of my new library solving it could be way much faster and easier.  In this post I will describe my alternative solution and some of the related experiments.

Tool used

For the static analysis:

  • IDA (demo version is enough)

For building the projects:

  • Visual Studio + CMake
  • Python27 (optional – for the helper script that I used in “Bonus”)


The challenge named greek_to_me.exe is a 32bit PE file. It has stripped relocations.


When we deploy it, it shows the empty console and waits. It is not reading any data from the standard input, so we can conclude that it is using some another way to read the password from the user.

We will start from some static analysis in IDA. The crackme has a very simple and clean structure, it is not obfuscated. We can see that at the beginning of the execution it creates a socket and waits for the input.

The socket listens at localhost on port 2222:


After getting the connection, it reads 4 bytes from the input into the buffer:


After it read 4 bytes, it starts processing the input and uses it for decoding an encrypted buffer:


If the checksum is valid, it means the encrypted code was decrypted properly, and it is further executed.

As we can see, only 1 byte of the input is used for decoding the buffer, so we can easily brutforce it. The code responsible for decoding the buffer is also pretty simple:

const size_t encrypted_len = 0x79;
for (int i = 0; i < encrypted_len; i++) {
    BYTE val = encrypted_code[i];
    encrypted_code[i] = (unknown_byte ^ val) + 0x22;

The only part of the crackme that may be somehow challenging is the checksum – this function is not that simple to reimplement. However, if we want to make a brutforcer, we need to be able to calculate the checksum after every attempt.

In my previous solution, I just reimplemented the checksum – it worked but it was not so much fun 😉 . I saw also some other approaches such as emulating the checksum function by the Unicorn engine, using angr framework, or making a brutforcer that talks to the original program via socket. Can it be done even faster? Let’s see…

LibPeConv comes into play

With PeConv we can convert any PE file from raw format to virtual and back. It also provides a custom PE loader – it’s goal is to provide a possibility of loading any PE file into the current process (even if it is not a DLL and even if it does not have no relocations table – it will be explained in the further part). This loaded PE can be later used as a fully functional PE file that can run from inside the curent process. We can also use any selected function from its code – all we need to know is the function’s RVA and the API.

In this case, I will use libpeconv to load the crackme and import from it the function calculating checksum. Also, rather than copying the encrypted buffer to my code, I will read it directly from the loaded PE.

Preparing the required information

Let’s take a look at the crackme again in IDA. We need to find the appropriate offsets and understand the API of the function that we are going to import.

The function calculating the checksum starts at RVA 0x11E6:


It takes 2 arguments: pointer to the buffer and it’s size.

It returns a WORD type:


Summing up, we can define the function prototype as:

WORD calc_checksum(BYTE *decoded_buffer, size_t buf_size)

It is also worth to note, that this function is self-contained and does not call any imported libraries – that makes importing it even easier (we are not forced to load any imports for the module or to apply relocations).

Another thing that we need is the encrypted buffer. It starts at RVA 0x107C and is 0x79 (121) bytes long:


That’s all! Let’s start coding.

Solving the crackme with libPeConv

The current version of libpeconv allows to load PE file in two ways. By the function load_pe_module and by the function load_pe_executable. The second one: load_pe_executable is a complete loader, that loads given PE to the current process in the RWX memory, automatically applies relocations and load dependencies. The first one (load_pe_module) does not load the dependencies and also gives more control: we may load the PE file in non-executable memory and applying the relocations is optional. More information (and *very* possible updates on the API) you can find here:

As we saw, the function that we want to import is self-contained, so it will not harm if we load the crackme PE without imports and without relocations (to see it loaded as a fully functional PE see the next part of the article). I will use a function load_pe_module

BYTE* loaded_pe = (BYTE*)load_pe_module(
    v_size, // OUT: size of the loaded module
    true,   // executable
    false   // without relocations

Now, let’s import the function. First let’s make a pointer to it:

WORD (*calc_checksum) (BYTE *buffer, size_t buf_size) = NULL;

Calculate the absolute offset to the function within the loaded module:

ULONGLONG offset = DWORD(0x11e6) + (ULONGLONG) loaded_pe;

And filling the pointer:

calc_checksum = ( WORD (*) (BYTE *, size_t ) ) offset;

That’s it, now we can use the function in our application like any other function.

But before we can start brutforcing, we also need to fill the pointer to the buffer:

g_Buffer = (uint8_t*) (0x107C + (ULONGLONG) loaded_pe);

This is the full brutforcer that I perpared:
And it works  🙂  The value that we got is exactly what it was supposed to be:


But still, the found value is just a part of the solution, not the flag that we are searching for. As we know from the static analysis, if this value is given correct, the chunk of code will be decrypted and executed. Would be cool to see how exactly that chunk of code looks when it is written into it’s place, don’t you think?

And also it is very easy to achieve. The PE file is loaded in the RWX memory inside the current process – so we can easily substitute the encrypted chunk of code with the decoded. Simple memcpy will do the job:

memcpy(g_Buffer, g_Buffer2, g_BufferLen);

Then, libPeConv will help us to convert the PE file back to the raw format, so that we can open it in IDA. We can do it with the help of pe_virtual_to_raw from libpeconv:

size_t out_size = 0;
BYTE* unmapped_module = pe_virtual_to_raw(
    loaded_pe, //pointer to the module
    v_size, //virtual size
    module_base, //in this case we need here
                 //the original module base, because
                 //the loaded PE was not relocated
    out_size //OUT: raw size of the unmapped PE

And this is the complete solution:

Comparing the dumped executable with the original one we can see that the buffer was overwritten:
So let’s see the modified exe in IDA:
And yes! At the known offset there is the flag revealed:

Bonus – loading  and running a PE with stripped relocations

Ok, you may say – it was easy – the loaded function was self contained, so we could as well rip it off from the original file, not using any loaders. But what if the function calls several other functions within the given module and also imported functions? Will the same trick work? And could it work even for PE file without relocations?

To answer those questions I prepared another test case. Now, instead of loading one function, I will load and execute the full crackme from inside the brutforcer.

First we will modify few things. This time, instead of using load_pe_module I will use load_pe_executable – to load full executable with dependencies.

BYTE* loaded_pe = (BYTE*)load_pe_executable(path, v_size);

The function will automatically detect that the PE file has no relocations, and enforce loading it at it’s original module base. Mind the fact, that allocating memory at the specific base may not always work – so, sometimes it takes several runs to execute it properly. You must also make sure that the module base of the loader does not collide with the module base required by the payload (if the loader’s base is random it is good enough).

Once the PE file is loaded, we just need to get it’s Entry Point – and then we can call it like any other function*:

// Deploy the payload:
// read the Entry Point from the headers:
ULONGLONG ep_va = get_entry_point_rva(loaded_pe)
    + (ULONGLONG) loaded_pe;

//make pointer to the entry function:
int (*loaded_pe_entry)(void) = (int (*)(void)) ep_va;

//call the loaded PE's ep:
int ret = loaded_pe_entry();

* – but mind the fact that depending on the payload’s implementation details, once you redirected your execution to it’s entry point, it may just exit after finishing it’s job and never return back to your code.

I am going to modify the brutforcer code in such a way, that this time after finding the value the original crackme will be run. This is the code of the full application:

To make sure that everything works fine (the deployed payload really creates socket and gives response in exactly the same way like the one deployed independently), I wrote a small Python script that will communicate with it and display the response:

And now, let’s see it all in action:

This is all what I prepared for today, I hope you enjoyed it! The lib is now under rapid development so many things will get refactored and improved, stay tuned!
The binaries of all the presented loaders, along with the crackme, are available here: – the password to the zip is: crackme


See other approaches to solve the same crackme:

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