Debugging Complex Malware that Executes Code on the Heap

Introduction

In this blog, I will share a simple debugging tactic for creating
“save points” during iterative remote debugging of complex multi-stage
samples that execute code in heap memory at non-deterministic
addresses. I’ll share two examples: one contrived, and the other a
complex, modular malware sample (MD5 hash:
830a09ff05eac9a5f42897ba5176a36a) from a family that we call
POISONPLUG. I will focus on IDA Pro and WinDbg, but I’ll explain how
to achieve the same effect with other tools as well. With this tactic,
you can also hand off program execution between multiple debuggers
using the strengths of different tools (e.g. unpacking a binary,
dumping memory maps, combatting anti-RE, or normal debugging).

The essence is merely suspending the malware. To set the stage, I
must first explain how malware influences our debugging tactics to
necessitate this. This explanation will serve as a review of common
techniques that make malware debugging easier and culminate in the
case study of POISONPLUG. If you’re already a seasoned analyst using
IDA Pro to remotely debug malware, and you’re only interested in the
bottom line of how to suspend and snapshot live malware, then skip to
the Summary section at the end.

VMs and Snapshots as Save Points

To prevent malware from doing damage, most malware reverse engineers
debug in an isolated VM. This gives rise to the powerful tactic of
capturing VM snapshots throughout the debugging process to be able to
return to a “save point” after making a mistake. The analyst is then
free to be aggressively experimental about exploring malware behavior.
The only consequence of an error is that the analyst must revert the
VM and avoid making the same mistake again.

Remote Debugging

Debugging malware on the same system where static analysis artifacts
are stored is dangerous; malware (e.g. ransomware) can destroy notes
and disassembly databases, or malware anti-RE measures can inflict
data loss (e.g. by rebooting). Consequently, it makes sense to use
separate systems for debugging versus disassembly and note-taking.
Depending on the tools used, this can force the analyst to flip back
and forth between viewing disassembler output and the debugger, like a
spectator at a tennis match. These transitions are distracting.

Unifying Static and Dynamic Analysis with IDA Pro as a Front-End

Fortunately, IDA Pro (and probably most modern disassemblers) can
act as a debugging front-end, superimposing disassembly annotations
over live memory and register state in a running program. This lets
the analyst see and directly alter disassembly annotations in response
to their observations, without switching back and forth.

Malware that Modifies its Memory Map at Runtime

There is one frequent scenario that further shapes the requirements
for a dynamic analysis methodology: malware that allocates heap
memory, writes code to that memory, and executes that code. Consider
Figure 1, which shows a simple program written in C.



Figure 1: Simple shellcode example program

The program allocates memory using malloc, copies six bytes to that
location using memcpy, logically inverts each byte, calls the buffer
as a function, and finally returns the shellcode’s return value (error
checking omitted both for brevity and realism). Figure 2 shows the
decoded shellcode in memory.



Figure 2: Simple shellcode function
returns 42

Without this code, the disassembly database is missing useful
information about the malware’s code, leaving its behavior as a bit of
a black box. This simple example demonstrates a common pattern, but
its trivial nature isn’t compelling enough to consider this a serious
problem. A more realistic example will provide more substantial
motivation for the debugging tactic at hand.

Case Study: POISONPLUG

For a realistic example, consider the sample with MD5 hash
830a09ff05eac9a5f42897ba5176a36a (which is available from VirusTotal).
This malware creates a thread that decodes and calls shellcode, which
unpacks and calls into the entry point of a modified DLL module. The
module in turn unpacks six additional modules before finally calling a
function within one of those modules. The DllEntryPoint functions of
several modules each create several anti-RE threads that attempt to
detect common analyst tools and terminate the malware in response.
After completely unpacking the malware, tools such as Tyler Dean’s injectfind
for flare-dbg
or my own flare-qdb
(Query-Oriented Debugger)
can expose all the read/write/execute
(R/W/X) mappings in memory that, in this case, point directly to the
malware modules. Figure 3 shows the output from flare-qdb debugging a
subset of the malware to this point and dumping its R/W/X allocations.



Figure 3: POISONPLUG R/W/X memory
locations after unpacking

Figure 4 shows the unpacked shellcode-based loader from this sample,
which is intricate, obfuscated, and time-consuming to annotate.



Figure 4: POISONPLUG’s shellcode-based loader

This shellcode implements several anti-RE features specific to this
malware family, and a copy of this is used to unpack seven modules
altogether with modified/custom PE-COFF headers. A common response to
finding an entire executable file in memory is to dump the file and
create its own disassembly database. However, the modules use a list
of function pointers stashed in a mapping of the paging file to locate
and call into one-another’s function “exports” in spaghetti code
fashion to deliberately obfuscate control flow and functional
semantics. Figure 5 shows an example, where each lane represents one
executable code module, and the boxes inside each lane represent
distinct function entry point RVAs within that module.



Figure 5: Partial interaction diagram for
retrieving and decoding configuration

The code at offset 0x11f2 in module 0 is simply calling into other
modules to eventually arrive at code within its own module (at offset
0x1d42). Dumping to separate disassembly databases creates
distractions for the analyst as they must Alt+Tab between entirely
different disassembly databases to follow the path of execution.

These types of complex samples create a dual problem for the
debugging tactics described so far…

Challenge 1: Syncing Code from the Heap

The first problem is that the code written to memory is generally
not readily available in the original disassembly output, and dumping
to separate disassembly databases is not always appropriate. It can
also be a lot of work to neutralize anti-reversing measures and
shepherd a sample to the point where it has unpacked all its encoded
modules into heap memory. A debugging mistake can entail a lot of
additional work to fix and resume analysis. Live memory is a resource
that could hasten reverse engineering if it can be preserved beyond
the life of the debug session. Luckily, this first problem of making
unpacked modules conveniently available in a single disassembly
database can be solved trivially, at least in IDA Pro:

  1. Visit each dynamically
    allocated code region to change its segment attributes (Alt+S) and
    mark each as a Loader segment
  2. Pull the dynamically
    allocated memory into the disassembly database (Debugger >
    Take memory snapshot > Loader Segments)

If you are following along without having started a debugging
session, IDA’s Change segment attributes dialog will omit the
Loader segment checkbox. Figure 6 shows this dialog during a
debugging session, with the Loader segment checkbox highlighted.



Figure 6: Change segment attributes
dialog during debugging session

After pulling in live memory, it is possible to read and annotate
unpacked modules and code in heap allocations even after terminating
the debugging session, as shown in Figure 7.



Figure 7: Function code from heap saved
from a debug memory snapshot

Challenge 2: Non-Deterministic Memory Maps

A second problem arises from samples that execute code in
dynamically allocated memory. Recovering from a debugging mistake
still requires debugging the program again, but modules frequently
occupy varying addresses across different executions. Consequently,
the helpful annotations created in IDA Pro at the original addresses
are absent from the new code locations. Figure 8 shows an example
containing the same code as in Figure 7, but loaded at a different
address during a subsequent execution of the program. The analyst must
then recognize and/or relabel everything to continue the analysis.
This can be scripted, but it is a time-consuming distraction.



Figure 8: Same code at a different
address lacks annotations

The reason the code appears at varying addresses across debugging
sessions is that Windows’ memory allocation functions such as
VirtualAlloc do not always return consistent addresses from one
execution of a program to the next. For example, the first time a
program runs, it may obtain memory at address 0xe000, the second time
at 0x11a000, et cetera. For complex malware with several modules, this
presents a problem.

We’d like the memory map to be uniform from one debug session to
another so we can continue to build on our existing static analysis
annotations, each of which IDA Pro has associated with a single
virtual address. Alas, even though VirtualAlloc accepts an optional
lpAddress parameter to indicate the starting address of the region to
allocate, this is merely a suggestion unless memory was already
reserved and uncommitted at that address. Forcing the lpAddress
parameter to a desired value rarely (in my experience, never) yields success.

Alternately, it would be nice to go back to using virtual machine
snapshots to create “save points” like before. Unfortunately, when
debugging remotely over a network, the process of reverting a virtual
machine snapshot breaks the TCP connection between the debug server
and IDA Pro and prevents the malware from continuing under the control
of the debugger.

…Where we Lay our Scene

The stage is now set to introduce the new technique. First, a short
recap of how we got here:

  • Need to debug in a VM to
    avoid damage to the host system
  • Prefer to use IDA Pro as
    the debugging front-end to unify static and dynamic analysis
  • Need to use remote debugging to avoid damage to static analysis
    artifacts and documentation
  • Need to debug iteratively
    across multiple debug sessions
  • Disassembly annotations must
    align with the memory map in the debug session to be useful

Malware behavior and analyst preferences seem to have painted us
into a corner. Running the malware repeatedly results in a
non-deterministic memory map that does not align with the annotations
in the disassembly database, and using IDA Pro to unify the static
view with live remote debugging appears impede the use of VM snapshots
to act as save points. What should an analyst do?

Park Your Malware

To capture a VM snapshot that allows us to repeatedly reattach to
and resume debugging, we’ll increase the suspend count of all the
threads in the program and detach the debugger. The debug server will
gracefully close its TCP connection, and the program will stay
suspended until we reattach. We then capture a VM snapshot. Finally,
we can repeatedly revert the VM, reattach, and resume execution to
continue our analysis. This way, you can park your malware once, and
then crash it over and over again until you understand its behavior.

As it turns out, IDA Pro’s facility for suspending threads
(right-click -> Suspend) doesn’t maintain its effect after
detaching the debugger. Instead, we’ll specifically use WinDbg as
IDA’s debugger back-end (see the directions
at Hex-Rays’ site
).

The WinDbg command for viewing thread status is ~ (tilde). The ~
command accepts an optional numeric argument to specify which thread
to display (e.g. ~3), or you can specify ~* to display full status for
all threads. WinDbg also supports commands ~n and ~m for
suspending and resuming threads. These also permit
numeric or asterisk arguments, so we can use ~*n to suspend all
threads before detaching, and ~*m to resume them upon
reattaching. Figure 9 shows IDA/WinDbg output after viewing thread
status, suspending all threads, and finally viewing their status once more.



Figure 9: Viewing thread status,
suspending, and viewing again

The suspend count increases from 1 to 2 after issuing the ~*n
command. Now, when the debugger detaches from the process and
decrements the suspend count of all threads (as usual), the
artificially elevated suspend count of each thread will remain greater
than zero. Consequently, the NT dispatcher will not schedule any
threads in the process to run, and the process will continue to exist
in a suspended state.

Now, we can capture a VM snapshot that can be repeatedly reverted to
resume debugging from where we left off. Figure 10 shows the process
attachment dialog in IDA Pro after reverting the VM snapshot and
clicking Debugger -> Attach to process…



Figure 10: Attaching to the suspended process

You can create these “save points” at various junctures – as many as
you have disk space to store.

The one caveat to this procedure is that it is easy to forget to
resume threads between reattaching and attempting to continue
debugging. If you forget this step, then the “Please wait…” modal
dialog in Figure 11 will appear.



Figure 11: Debugging a suspended process

A reverse engineer might be accustomed to seeing this dialog only
after making a mistake and allowing malware to run free, but in this
case, the program is not actually executing any instructions. To fix
it, simply click the Suspend button in IDA Pro’s “Please wait…” dialog
and then resume all threads (WinDbg: ~*m) to decrease their suspend
count. Then, execution can continue as normal.

Summary

To suspend a program that you are running within an IDA Pro + WinDbg
remote debug session to capture a reusable VM snapshot:

  1. Suspend all threads
    (WinDbg: ~*n)
  2. Detach from the process (IDA Pro:
    Debugger -> Detach from process)
  3. Capture
    your VM snapshot

To resume the suspended program:

  1. Attach to the remote
    process (IDA Pro: Debugger -> Attach to
    process…)
  2. Resume all threads (WinDbg: ~*m)
  3. Resume
    debugging as normal

If you aren’t interested in using WinDbg commands, you can instead
use SysInternals’ Process Explorer to suspend the process in your
debugging VM and simply detach using IDA Pro. You could also write a
Python ctypes script or native program to directly use the relevant
Windows APIs if you prefer (specifically via CreateToolhelp32Snapshot
with the TH32_SNAPTHREAD flag, OpenThread, SuspendThread, and ResumeThread).

This tactic allows us to cope with complex multi-stage shellcode or
modular malware that has several (sometimes cascading) unpacked code
regions. It lets us create save points in our debug session while
maintaining the same memory map so our disassembly annotations always
remain aligned with the memory map in the debug session. It also
allows us to suspend malware execution in one debugger and pick it up
in another, provided each debugger allows thread suspend count to
remain at a non-zero value before detaching.

Before closing, I’d like to give credit to Tarik Soulami for his
explanation of WinDbg thread management in his book, “Inside Windows
Debugging” (Microsoft Press, 2012). If you’re starting to confront
more difficult debugging scenarios in your journey as a reverse
engineer, I strongly encourage you to pick up “Inside Windows
Debugging” to augment your repertoire and further understand the
powerful debugging capabilities of WinDbg and Windows itself.



This is a Security Bloggers Network syndicated blog post authored by Nick Harbour. Read the original post at: Threat Research Blog