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

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. Using LZMA Compression

By default, UPX uses its own compression format. The --lzma flag switches to the LZMA compression algorithm, which changes the compressed data layout and may hinder some static analysis tools. This is not encryption — UPX has no built-in encryption capability; --lzma selects a compression codec only.

upx-ucl --lzma payload.exe

• Effect: Uses LZMA compression, changing the packed data layout compared to the default UCL algorithm.
• Detection Note: Some EDRs struggle to analyze LZMA-packed binaries due to the different decompression stub structure, but the binary is not encrypted and can be unpacked with standard tools.

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

1. Locate UPX!, UPX1, UPX2 in the hex editor.
2. Change it to XYZ! or another unique signature.
3. 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

1. Pack the binary using UPX:

   upx-ucl -9 payload.exe
   

2. Extract the stub from a packed file:

   Note: UPX has no built-in --extract-stub option; use a hex editor or objcopy to extract the stub bytes from the UPX-packed binary. For example:

   objcopy --dump-section UPX0=stub0.bin payload.exe
   # or use a hex editor to manually locate and extract the UPX stub region
   

3. Modify the stub (e.g., using a hex editor or assembler).
4. 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