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Attacking Thick Client Applications

Introduction

  • Thick client applications are the applications that are installed locally on our computers

  • Unlike thin client applications that run on a remote server and can be accessed through the web browser, these applications do not require internet access to run, and they perform better in processing power, memory, and storage capacity

  • Thick client applications are usually applications used in enterprise environments created to serve specific purposes

  • Such applications include project management systems, customer relationship management systems, inventory management tools, and other productivity software.

  • These applications are usually developed using Java, C++, .NET, or Microsoft Silverlight.

  • Thick client applications can be categorized into two-tier and three-tier architecture

Information Gathering

  • penetration testers have to identify the application architecture, the programming languages and frameworks that have been used, and understand how the application and the infrastructure work

  • The following tools will help us gather information.

    • CFF Explorer

    • Detect It Easy

    • Process Monitor

    • Strings

Client Side attacks

- we can reverse-engineer and examine .NET and Java applications including EXE, DLL, JAR, CLASS, WAR, and other file formats. - Dynamic analysis should also be performed in this step, as thick client applications store sensitive information in the memory as well. - Ghidra - IDA - OllyDbg - Radare2 - dnSpy - x64dbg - JADX - Frida

Network Side Attacks

  • Wireshark

  • tcpdump

  • TCPView

  • Burp Suite

Server Side Attacks

  • Server-side attacks in thick client applications are similar to web application attacks, and penetration testers should pay attention to the most common ones including most of the OWASP Top Ten.

Retrieving hardcoded Credentials from Thick-Client Applications

  • Scenario is you have a exe file

  • Using ProcMon64 from SysInternals and monitoring the process reveals that the executable indeed creates a temp file in C:\Users\Matt\AppData\Local\Temp.

  • In order to capture the files, it is required to change the permissions of the Temp folder to disallow file deletions.

  • To do this,

    • we right-click the folder

      • C:\Users\Matt\AppData\Local\Temp

      • and under Properties -> Security -> Advanced -> cybervaca -> Disable inheritance -> Convert inherited permissions into explicit permissions on this object -> Edit -> Show advanced permissions,

      • we deselect the Delete subfolders and files, and Delete checkboxes.

      • Finally, we click OK -> Apply -> OK -> OK on the open windows.

      • Once the folder permissions have been applied we simply run again the Restart-OracleService.exe and check the temp folder.

      • The file 6F39.bat is created under the C:\Users\cybervaca\AppData\Local\Temp\2.

      • The names of the generated files are random every time the service is running.

  • dir C:\Users\cybervaca\AppData\Local\Temp\2
    ...SNIP...
04/03/2023  02:09 PM         1,730,212 6F39.bat
04/03/2023  02:09 PM                 0 6F39.tmp

    • Listing the content of the 6F39 batch file reveals the following.

    @shift /0
@echo off

if %username% == matt goto correcto
if %username% == frankytech goto correcto
if %username% == ev4si0n goto correcto
goto error

:correcto
echo TVqQAAMAAAAEAAAA//8AALgAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA > c:\programdata\oracle.txt
echo AAAAAAAAAAgAAAAA4fug4AtAnNIbgBTM0hVGhpcyBwcm9ncmFtIGNhbm5vdCBiZSBydW4g >> c:\programdata\oracle.txt
<SNIP>
echo AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA >> c:\programdata\oracle.txt

echo $salida = $null; $fichero = (Get-Content C:\ProgramData\oracle.txt) ; foreach ($linea in $fichero) {$salida += $linea }; $salida = $salida.Replace(" ",""); [System.IO.File]::WriteAllBytes("c:\programdata\restart-service.exe", [System.Convert]::FromBase64String($salida)) > c:\programdata\monta.ps1
powershell.exe -exec bypass -file c:\programdata\monta.ps1
del c:\programdata\monta.ps1
del c:\programdata\oracle.txt
c:\programdata\restart-service.exe
del c:\programdata\restart-service.exe

    • Inspecting the content of the file reveals that two files are being dropped by the batch file and being deleted before anyone can get access to the leftovers.

    • We can try to retrieve the content of the 2 files, by modifying the batch script and removing the deletion.

    @shift /0
@echo off

echo TVqQAAMAAAAEAAAA//8AALgAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA > c:\programdata\oracle.txt
echo AAAAAAAAAAgAAAAA4fug4AtAnNIbgBTM0hVGhpcyBwcm9ncmFtIGNhbm5vdCBiZSBydW4g >> c:\programdata\oracle.txt
<SNIP>
echo AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA >> c:\programdata\oracle.txt

echo $salida = $null; $fichero = (Get-Content C:\ProgramData\oracle.txt) ; foreach ($linea in $fichero) {$salida += $linea }; $salida = $salida.Replace(" ",""); [System.IO.File]::WriteAllBytes("c:\programdata\restart-service.exe", [System.Convert]::FromBase64String($salida)) > c:\programdata\monta.ps1

    • After executing the batch script by double-clicking on it, we wait a few minutes to spot the oracle.txt file which contains another file full of base64 lines, and the script monta.ps1 which contains the following content, under the directory c:\programdata\.

    • Listing the content of the file monta.ps1 reveals the following code

    cat C:\programdata\monta.ps1
    $salida = $null; $fichero = (Get-Content C:\ProgramData\oracle.txt) ; foreach ($linea in $fichero) {$salida += $linea }; $salida = $salida.Replace(" ",""); [System.IO.File]::WriteAllBytes("c:\programdata\restart-service.exe", [System.Convert]::FromBase64String($salida))

    • This script simply reads the contents of the oracle.txt file and decodes it to the restart-service.exe executable.

    • Running this script gives us a final executable that we can further analyze.

     ls C:\programdata\
    
Mode                LastWriteTime         Length Name
<SNIP>
-a----        3/24/2023   1:01 PM            273 monta.ps1
-a----        3/24/2023   1:01 PM         601066 oracle.txt
-a----        3/24/2023   1:17 PM         432273 restart-service.exe

    • Now when executing restart-service.exe we are presented with the banner Restart Oracle created by HelpDesk back in 2010.

    • Inspecting the execution of the executable through ProcMon64 shows that it is querying multiple things in the registry and does not show anything solid to go by.

    • start x64dbg, navigate to Options -> Preferences, and uncheck everything except Exit Breakpoint:

    • By unchecking the other options, the debugging will start directly from the application's exit point, and we will avoid going through any dll files that are loaded before the app starts.

      • Then, we can select file -> open and select the restart-service.exe to import it and start the debugging.

      • Once imported, we right click inside the CPU view and Follow in Memory Map:

      • Checking the memory maps at this stage of the execution, of particular interest is the map with a size of 0000000000003000 with a type of MAP and protection set to -RW--.

      • Memory-mapped files allow applications to access large files without having to read or write the entire file into memory at once. Instead, the file is mapped to a region of memory that the application can read and write as if it were a regular buffer in memory.

        • This could be a place to potentially look for hardcoded credentials.

        • If we double-click on it, we will see the magic bytes MZ in the ASCII column that indicates that the file is a DOS MZ executable.

        • return to the Memory Map pane, then export the newly discovered mapped item from memory to a dump file by right-clicking on the address and selecting Dump Memory to File.

          • Running strings on the exported file reveals some interesting information.

C:\TOOLS\Strings\strings64.exe .\restart-service_00000000001E0000.bin

<SNIP>
"#M
z\V
).NETFramework,Version=v4.0,Profile=Client
FrameworkDisplayName
.NET Framework 4 Client Profile
<SNIP>

  • Reading the output reveals that the dump contains a .NET executable.

  • We can use De4Dot to reverse .NET executables back to the source code by dragging the restart-service_00000000001E0000.bin onto the de4dot executable.

.

de4dot v3.1.41592.3405

Detected Unknown Obfuscator (C:\Users\cybervaca\Desktop\restart-service_00000000001E0000.bin)
Cleaning C:\Users\cybervaca\Desktop\restart-service_00000000001E0000.bin
Renaming all obfuscated symbols
Saving C:\Users\cybervaca\Desktop\restart-service_00000000001E0000-cleaned.bin


Press any key to exit...

  • we can read the source code of the exported application by dragging and dropping it onto the DnSpy executable.

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