Didier Stevens

Thursday 28 January 2010

Quickpost: Shellcode to Load a DLL From Memory

Filed under: Hacking,My Software,Quickpost — Didier Stevens @ 3:08

I finally took time to develop shellcode to load a DLL, not with LoadLibrary, but directly from memory. Not storing the DLL on disk prevents it from being detected by AV software; not using LoadLibrary bypasses HIPS software that monitors this system call.

My shellcode is based on Joachim’s code.

In previous posts, I showed how to load a DLL or shellcode with VBA in Excel. This is a combination of both techniques: a VBA macro loads and executes shellcode in Excel’s process space, and the shellcode loads a DLL from memory into Excel’s process memory.

With the code of the previous post, the DLL appears in the list of loaded DLLs:

With this shellcode, it doesn’t:


Quickpost info


Monday 28 September 2009

Quickpost: SAFER and Malicious Documents

Filed under: My Software,Quickpost — Didier Stevens @ 17:50

I wasn’t going to mention SAFER to restrict the rights of an application, because Software Restriction Policies can be bypassed. But a Tweet by Edi Strosar made me review my viewpoint. In this particular case, bypassing SRP is a non-issue, because the user is already local admin!

Software Restriction Policies allow you to force specific applications to run with a restricted token. As Michael explained it with AD GPOs, I’ll show it with local policies.

Enable SAFER policies for SRPs by adding DWORD registry key Levels (value 0x31000) to HKLM\SOFTWARE\Policies\Microsoft\Windows\Safer\CodeIdentifiers:

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Start the Local Security Policy administration tool and go to the Software Restriction Policies. You’ll have to create new policies if this is the first time you configure SRPs.

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Create a new rule in Additional Rules. We’ll identify the application to restrict by its path and name, so create a Path Rule:

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For the security level, select Basic User:

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If you have no Basic User option, you forgot to update the registry before launching the administration tool:

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Select the application to restrict:

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This rule will force Adobe Reader to run with a restricted token:

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Writing to SYSTEM32 is denied:

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Quickpost info


Thursday 23 October 2008

Excel Exercises in Style

Filed under: Hacking — Didier Stevens @ 10:34

I developed another variant of my “Excel macro injects embedded DLL” script.

In stead of creating and loading a temporary DLL from VBScript, I inject and execute shellcode directly from the VBA application.

Some HIPS would prevent my previous script from running, because it loaded an unapproved DLL. But my new version doesn’t load a DLL.

Of course, writing shellcode is more difficult than developing a PE executable.

Monday 9 June 2008

Quickpost: Embedding an Executable in a VBscript

Filed under: My Software,Quickpost — Didier Stevens @ 11:53

My latest bpmtk post got some people to ask me for the VBscript. I’ll do better, I’m posting the Python program I wrote to generate the script. You can download it here.

You have to provide it 2 arguments: the name of the executable to embed and the name of the VBscript to generate, like this:

file2vbscript cmd.exe cmd.vbs

This will generate a VBscript that will write cmd.exe to the current directory and execute it (create a new process). If you want to load a DLL in stead of executing an EXE, use the -l option:

file2vbscript -l mydll.dll mydll.vbs

And to use it in an Office application (Office VBA doesn’t take long subs), use the option -o:

file2vbscript -ol mydll.dll mydll.vbs

This will split the embedded file over several subs, to accommodate for the size limitation of Office VBscripts.


Quickpost info


Thursday 6 March 2008

bpmtk: Replacing Gpdisable

Filed under: Hacking,My Software,Reverse Engineering — Didier Stevens @ 8:52

Gpdisable is a tool to bypass group policy as a limited user, posted by Marc Russinovich on his blog when he was still the owner of Sysinternals. But now that Sysinternals is owned by Microsoft, the tool is not available anymore.

My Basic Process Manipulation Tool Kit can replace Gpdisable, I’ll show how and give you one more trick.

LikeMarc did, you can inject a DLL that will patch the IAT to subvert NtQueryValueKey, but I’ll leave this technique for an upcoming post.

My example doesn’t require you to program a DLL to inject: since we want to hide the TransparentEnabled registry key, we will just rename the key in the process memory of the programs that impose Software Restriction Policies on us (like explorer.exe). Here is the bpmtk config file to achieve this goal:

dll-name advapi32.dll
#rename TransparentEnabled to AransparentEnabled
search-and-write module:. unicode:TransparentEnabled ascii:A

This will patch each process you’ve rights to and who has loaded advapi32.dll (this DLL enforces SRP).

But as Mark writes in his blog, this will not work for running processes because they have already cached the value of TransparentEnabled and are thus not querying the registry anymore. This is why many people reported that Gpdisable didn’t work for them. Gpupdate /force will force a refresh of the policies, and invalidate the cache.

But if you’re in a restricted environment, there’s a chance you’re prevented from doing a gpupdate. Here’s another way: set the variable _g_bInitializedFirstTime to 0, this will also invalidate the cache. For advapi32.dll version 5.1.2600.2180, this variable is at address 77E463C8. Our script becomes:

dll-name advapi32.dll
#rename TransparentEnabled to AransparentEnabled
search-and-write module:. unicode:TransparentEnabled ascii:A
write version:5.1.2600.2180 hex:77E463C8 hex:00
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