Took me a week to complete 2 tracks in Palo Alto CTF, Windows and Docs. Will be sharing my humble little solutions here. Hope you guys will learn something useful here. Some of the challenges uses malware techniques! Kudos to the organizer.
Description: This challenge uses UPX to pack but modded it to prevent upx -d from working. It also uses some anti debugging techniques to deter the use of debugging tools. Flag is not in clear, one would need to analyze the math behind and decode the flag.
It seems like the binary is packed with UPX. Let’s try to use the official tool to unpack. However it seems like we are unable to unpack =(
Loading the binary in IDA Pro, we can observe that the binary is indeed packed. From the screenshot below, we can only see 1 function; an indication of a packed binary. Next, we can clearly see that there is a tail jump. To unpack this binary manually, we just need to break at the tail jump and dump the process.
But before that, I would want to disable ASLR to make my job easier while debugging it. Using CFF Explorer tool, simply uncheck “DLL can move” checkbox and apply the changes.
Run the patched AntiD.exe in ollydbg. Set a breakpoint @ 0x4091AC. Run the program till it breaks and step into the next instruction.
Just dump the process as shown below. However running the dumped process would popup an error.
The dumped process’s IAT need to be fixed. For this, I use Scylla tool to delete the invalid imports and clicking on the fix dump button. Now we should have a working copy of the binary.
Loading the unpacked binary into IDA Pro, we can see that it looks unpack now. We can see more functions, imports and plain string.
X-referencing from the string, we would derive to the following function. Seems like 0x004011B0 function is the key to solving this puzzle.
In 0x004011B0 function, we can see some encoded variables; 40 of them and the following mathematical transformation consisting of XOR, addition and subtraction.
I simply converted the codes into PHP and attempt to bruteforce the flag.
As presented in RECON 2016 by James Forshaw. He demonstrated that he is able to overwrite an application’s physical file while it is still running as a process. According to his presentation, NtCreateUserProcess returns a file handle when it is called. The file handle can be used to overwrite the running process His POC is in the form of C#/Powershell. I simply converted it to C/Executable.
To demonstrate that it really work, I used an executable that simply popup a message box; named demo.pdf. You can see the hexadecimal of the binary file in the HxD GUI below. On executing Overwrite.exe, demo.pdf will be created as a process and while it is still running, Overwrite.exe will then overwrite demo.pdf with an actual pdf file (Process Failure modes – Final.pdf).
Usage of the program is as follows: Overwrite.exe [Full path to executable] [input file]
As we can see from the above screenshot, demo.pdf is executed as a process but HxD has already reflected the new bytes in Demo.pdf. We can also observe that demo.pdf file size has changed to 3MB from 165KB proving that this trick did indeed work.
From a RE point of view, we should keep a look out on the CREATE_INFO’s AdditionalFileAccess mask and the usage of undocumented API as shown below.
As presented in RECON 2016 by James Forshaw. He demonstrated that he is able to execute a DLL as a standalone process. According to his presentation, the reason why a DLL can’t be executed as a process is due to a check on the ProhibitedImageCharacteristics field. His POC is in the form of C#/Powershell. I simply converted it to C/Executable.
So by simply resetting this field to 0, we could then run a DLL as a process!
The following is a DLL with a .txt extension; just for fun… using regsvr32 is one of the way to execute a DLL and as you can see, a messagebox shows up stating that it is a DLL.
Now using James approach, we can now execute pikachu.txt as an executable!
Looking it from IDA Pro
Note that one do not necessary need to clear the field with 0. Other values work as well.
The first project will be on Process Mode Failures by James Forshaw as presented in Recon 2016. The POC given by James is in Powershell/C#. I reproduced some of his work into C. Over the next few days you will find the following executables in github.
Executing DLL as a process [Uploaded]
Overwriting the physical file of a binary while it is still running as a process [Uploaded]