GetModuleFileNameEx And Infinite Loops

In this post i am going discuss an infinite loop bug in some functions of the psapi.dll. I will take the "GetModuleFileNameExW" function as an example.

I have chosen the "GetModuleFileNameExW" function of psapi.dll v5.1.2600.5512, the one shipped with Windows XP SP3.

The function's prototype is:

Let's have a look at the function in OllyDbg.

As you can see in the image above, almost everything is done in the non-exported "_FindModule@12" function. The "_FindModule@12" function's source code looks something like this.

As you can see from the source code above, if the function is called with the "hMod" parameter set to Zero, the "ImageBaseAddress" value in the PEB (Process Environment Block) is retrieved and then the "InMemoryOrderModuleList" doubly linked list is traversed and each "LDR_MODULE" structure is queried for its "BaseAddress" field . If the "BaseAddress" field is equal to the "ImageBaseAddress" value, the function successfully returns.

And if the "hMod" parameter is set to nonzero, the doubly linked list is directly traversed and each "LDR_MODULE" structure is queried for its "BaseAddress" field. If the "BaseAddress" field is equal to the "hMod" value, the function successfully returns.

So, if we manipulate any LDR_MODULE structure to point at itself instead of pointing at the next structure, then we can cause any application that uses the function against our process to go into an infinite loop.

Here is a demo that causes OllyDbg v1.10 to go into an infinite loop when you open the "Select process to attach" dialog box. You can also use the second demo for OllyDbg v2.0.

Trying to test these demos on Windows 7, you will notice that everything is okay and no infinite loops occur. So, let's check the "_FindModule@12" function in psapi.dll shipped with windows 7 (WOW64).

If we compare the source code of both versions, we will easily see that Microsoft fixed that bug by defining a value for the maximum number of modules that can be loaded in a process. The value is 0x2710.

Any ideas or comments are very welcome.

You can follow me on Twitter @waleedassar 

OllyDbg Fake ImageName Bug

I have recently found a weird behavior in OllyDbg, which can further be used as an anti-debugging / anti-attaching trick. The problem occurs when enumerating the running processes if the "Select a process to attach" dialog box is opened.

The psapi "EnumProcesses" function is called to get the list of process identifiers (PIDs). For each PID, the psapi "EnumProcessModules" and "GetModuleFileNameExA" functions are called to extract the image base and full name of the main executable.

As i have shown in previous posts, the values in  PEB.LoaderData can easily be manipulated. In this case i will manipulate only the full name of the main executable to be of an existing but malformed file. Surprisingly, OllyDbg trusts the new file name and starts to extract essential information from it. Information extracted includes MZ signature, optional header values, section table data, etc.

The interesting thing about the forged executable is that it is rejected by the OS loader but still used by OllyDbg.

To create a one-file demo for this bug, i had to embed the malformed executable into the original one as a binary resource.

 

As you can see in the image below, the number of sections is set to 0xFFFF (malformed executable).

 

The demo can be found here.  The virustotal report can be found here.

N.B. This has been tested on OllyDbg v1.10 only.

Update:
Another demo, that crashes OllyDbg upon debugging or attaching, has been created. You can find it here.

Update:
The source code for the demos above can be found here.

You can follow me on Twitter @waleedassar 

Debuggers Anti-Attaching Techniques - Part 5

In this post, i will explain another anti-attaching trick. The trick is that if we manipulate the _PEB_LDR_DATA structure pointed to by PEB.LoaderData, we can cause functions like EnumProcessModules and GetModuleFileNameExA to fail.


Consequently, ollydbg would not be able to see the process in the "Select process to attach" dialog box.

You can play with this demo.

N.B. This trick can't be reliably used unless you carefully choose APIs in your application. Try to avoid APIs which read or write to the _PEB_LDR_DATA structure.

Update:
I have made a tiny plugin for OllyDbg v1.10. The plugin enables debugging those applications, which don't show in the "Select process to attach" dialog box. The plugin first checks the integrity of the target process's _PEB_LDR_DATA structure. If a manipulated structure is detected, a new typical one will be created.

The plugin can be downloaded from here and its source code from here.

Update:
Variants of this trick manipulate PEB.LoaderData so that an infinite loop occurs in OllyDbg or any other application which tries to use the "EnumProcessModules" function or the likes. See the image below.

The demo can be found here.

You can follow me on Twitter @waleedassar