1e. Advanced UPX Methods for Modifying Binary Structure

UPX offers several packing, obfuscation, and evasion techniques beyond basic compression.

$ upx-ucl --version
upx 4.2.2
UCL data compression library 1.03
zlib data compression library 1.3.0.1-motley
LZMA SDK version 4.43

UPX Method	Command	Purpose
Basic UPX Compression	`upx-ucl -9 payload.exe`	Maximum compression
Fast UPX Packing	`upx-ucl --best payload.exe`	Faster execution
Brute-force Packing	`upx-ucl --ultra-brute payload.exe`	Best compression
Encrypt UPX Stub	`upx-ucl --lzma payload.exe`	Harder to detect
Change UPX Signature	`hexedit payload.exe`	Modify `UPX!` signature
Strip UPX Headers	`upx-ucl --strip-relocs=0 payload.exe`	Removes relocation info
Exclude Sections	`upx-ucl --overlay=copy payload.exe`	Leaves overlays intact
Manual UPX Stub Editing	`upx-ucl --extract-stub=stub.bin`	Modify decompression stub
Unpacking UPX	`upx-ucl -d payload.exe`	Restore original binary

1️. Basic UPX Compression

This is the most common way to pack an executable:

upx-ucl -9 payload.exe

-9 → Maximum compression
Effect: Reduces file size and modifies PE structure.

Use Case: Reduces detection by changing the binary’s hash.

2️. Using `--best` Mode

Optimizes the compression ratio while maintaining execution speed:

upx-ucl --best payload.exe

Effect: Balances speed and compression.
Use Case: Suitable when execution performance matters.

3️. Using `--ultra-brute` for Maximum Compression

upx-ucl --ultra-brute payload.exe

Effect: Tries all available compression methods for the smallest possible file.
Detection Evasion: Changes the binary’s entropy, which may confuse heuristic detection.

4. Encrypting the UPX Header

By default, UPX leaves headers partially readable, which makes it easy for security tools to detect a UPX-packed file. You can encrypt the UPX stub to evade detection:

upx-ucl --lzma payload.exe

Effect: Uses LZMA compression, making unpacking harder.
Detection Evasion: Some EDRs struggle to analyze LZMA-packed binaries.

5️. Changing UPX Magic Signature

Some AV/EDR tools detect UPX-packed binaries by looking for the UPX! signature in the PE header.
To modify this:

hexedit payload.exe

Locate UPX!, UPX1, UPX2 in the hex editor.
Change it to XYZ! or another unique signature.
Save the changes.

Effect: Makes the file undetectable by signature-based scanners.

6️. Stripping the UPX Header

To further evade detection, you can strip out the UPX headers so that the executable appears normal in static analysis:

upx-ucl --strip-relocs=0 payload.exe

Effect: Removes unnecessary relocation data.
Detection Evasion: Some AV engines use relocation info for analysis.

7️. Packing Only Certain Sections

UPX lets you choose which sections of the PE file to compress:

upx-ucl --compress-icons=1 payload.exe

Effect: Only compresses icon resources, leaving the main code uncompressed.

You can also exclude some sections:

upx-ucl --overlay=copy payload.exe

Effect: Keeps overlays (extra data) intact, making it look more like a normal binary.

8️. Manual UPX Stub Modification

Pack the binary using UPX:
```
upx-ucl -9 payload.exe
```

Extract the stub from a packed file:

upx-ucl --extract-stub=stub.bin payload.exe

Modify the stub (e.g., using a hex editor or assembler).
Reinsert the stub into a new binary.

dd if=modified_stub.bin of=payload.exe bs=1 seek=0 count=25674 conv=notrunc

Effect: Breaks static detections looking for known UPX stubs.

9️. Manually Obfuscating UPX-Packed Files

After using UPX, further obfuscate the binary:

XOR Encode the File:

xxd -p payload.exe | tr -d '\n' | xxd -r -p > payload_obfuscated.exe

Rename Sections in the PE Header:
- Use PE-bear or CFF Explorer to rename sections from UPX0 and UPX1 to CODE and DATA.

10 . Unpacking and Repacking a UPX Binary

Unpacking a UPX-Packed Binary

If you receive a packed binary and need to analyze it:

upx-ucl -d payload.exe

Effect: Fully restores the original file.

Repacking After Modifications

Once changes are made, repack it to retain obfuscation:

upx-ucl -9 payload_modified.exe