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manually-unpacking-a-packed-binary

Manually unpacks a runtime-packed Windows binary by finding the original entry point (OEP) through tail-jump and entropy analysis, then guiding a memory dump at OEP. Activates for requests to manually unpack a sample, find the OEP, or unpack a custom/unknown packer that generic tools cannot handle.

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Files included (4)

  • SKILL.md2.8 KB
  • LICENSE340 B
  • references/api-reference.md1.2 KB
  • scripts/analyst.py2.9 KB

SKILL.md (original)

Review the instructions the agent will receive before you install.

Manually Unpacking a Packed Binary

When to Use

  • A sample is runtime-packed by a custom/unknown packer that UPX and generic unpackers cannot handle.
  • You need to locate the original entry point (OEP) and dump the unpacked image for static analysis.

Do not use generic auto-unpackers here — this is the manual path for cases where they fail. Run the sample only inside an isolated, instrumented debugger VM.

Prerequisites

  • A debugger (x64dbg) on an isolated VM and the packed sample.

Safety & Handling

  • The sample executes during unpacking; do this only in a disposable, isolated VM with snapshots.

Workflow

Step 1: Characterize the packer statically

python scripts/analyst.py inspect packed.exe

Reports section entropy, suspicious section names (UPX, .vmp, random), small import tables, and a write-then-execute section (classic unpacker stub).

Step 2: Find candidate OEP heuristics

Set a memory breakpoint on the unpacked code section's execution; the packer's tail jump (a far jmp/ret into a different section) typically transfers control to the OEP.

Step 3: Dump at OEP

When stopped at OEP, dump the process image and proceed to import reconstruction (see the dump-and-rebuild skill).

Step 4: Verify the dump

Confirm the dumped image has a sane entry point, restored imports, and readable code.

Validation

  • The unpacking stub is identified by a write→execute section transition.
  • The tail jump lands in a different section than the stub (OEP candidate).
  • The dumped image disassembles cleanly with recognizable runtime startup code.

Pitfalls

  • Stopping at a stage-2 stub rather than the true OEP in multi-layer packers.
  • Hardware-breakpoint detection by the packer — vary the breakpoint strategy.
  • Dumping before the import table is fully resolved.

References

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