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Unpacking upx and common packers

Skill meltedinhex/analyst-ai-pack/skills/unpacking-upx-and-common-packers

Identifies and unpacks packed binaries: detecting packers from section names and entropy, automatically unpacking UPX, and manually unpacking custom packers by finding the OEP and dumping. Activates for requests to unpack a packed sample, detect a packer, or recover the original binary from UPX or a custom packer.From its SKILL.md

Install
npx -y skills add meltedinhex/analyst-ai-pack --skill unpacking-upx-and-common-packers

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SKILL.md

3.1 KB, 615 tokens by cl100k_base, as published. Nobody here has run it

Unpacking UPX and Common Packers

When to Use

  • A sample shows high entropy, few imports, or packer section names, and you need the original code for static analysis or RE.
  • You want to unpack UPX automatically or manually unpack a custom/modified packer.
  • You are preparing a sample for Ghidra/IDA (decompiling packed code is pointless).

Do not use generic unpackers as a guarantee — modified UPX and custom packers defeat upx -d. Fall back to debugger-based manual unpacking.

Prerequisites

  • Packer detection (Detect It Easy, or the bundled section/entropy heuristics).
  • upx for standard UPX; x64dbg + Scylla for manual unpacking (see the x64dbg skill).
  • Isolated lab.

Workflow

Step 1: Detect the packer

python scripts/analyst.py detect sample.bin

Signals: section names (UPX0/UPX1, .aspack, .themida), high section entropy, a tiny import table, and a small .text with a large high-entropy section.

Step 2: Try automated UPX unpacking

If standard UPX:

upx -d -o sample_unpacked.bin sample.bin

If upx -d fails on a UPX-like sample, the header was likely tampered (e.g. UPX! magic or version bytes altered) — repair the marker or unpack manually.

Step 3: Manual unpacking for custom packers

In x64dbg: break on VirtualAlloc/VirtualProtect, run until the unpacked region is written and made executable, find the tail jump to the original entry point (OEP), and dump there.

Step 4: Dump and rebuild imports

Use Scylla to dump the process and reconstruct the Import Address Table (IAT) so the dumped PE loads in a disassembler.

Step 5: Verify the unpacked result

Confirm the dump is a clean PE with a populated import table and readable strings/code.

Validation

  • The unpacked file has restored imports and meaningful strings absent from the packed sample.
  • Entropy of the unpacked .text drops to normal code levels.
  • The disassembler resolves API calls (IAT rebuilt correctly).

Pitfalls

  • Running upx -d on tampered UPX and concluding it is not UPX; check for renamed/edited markers.
  • Dumping before reaching the OEP, capturing still-encrypted code.
  • Forgetting IAT reconstruction, leaving the dump unusable in Ghidra/IDA.

References

What ships with it: 3 files

4.6 KB alongside SKILL.md, 1 of them executable

references/

scripts/

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