UPX

UPX, short for Ultimate Packer for eXecutables, is a free, open-source executable packer designed to reduce the size of binaries for distribution and storage. It supports a broad range of executable formats, including Windows Portable Executables (PE), Linux/Unix ELF binaries, and macOS Mach-O files, as well as many object files and libraries. Packed files contain a small loader that decompresses the original code and data at runtime, allowing the program to run normally after startup.

UPX uses multiple compression backends, notably LZMA-based methods and PPMd, enabling users to choose between higher compression ratios and faster decompression. The typical workflow is to run the tool on an executable to produce a smaller packed file; the tool can also decompress, test, or list contents. Common options include -9 for maximum compression, --best, -d to decompress, -l to list, and -t to test integrity.

While UPX can substantially reduce file size, the decompression step can introduce a small startup delay, and some environments or security tools react negatively to packed binaries. Some software with digital signatures may require re-signing after packing. Certain platforms or security policies may block or hamper unpacked code, and some antivirus products flag UPX-packed files as suspicious.

UPX is distributed under the UPX License, a permissive free software license. It is widely used by developers to shrink distribution size and by researchers for malware analysis, debugging, and software packaging.