UPX (Ultimate Packer for eXecutables) is a free and open source executable packer supporting a number of file formats from different operating systems.

UPX uses a data compression algorithm called UCL, which is an open-source implementation of portions of the proprietary NRV (Not Really Vanished) algorithm.

UCL has been designed to be simple enough that a decompressor can be implemented in just a few hundred bytes of code. UCL requires no additional memory to be allocated for decompression, a considerable advantage that means that a UPX packed executable usually requires no additional memory.

UPX (since 2.90 beta) can use LZMA on most platforms; however, this is disabled by default for 16-bit due to slow decompression speed on older computers (use --lzma to force it on).

Starting with version 3.91, UPX also supports 64-Bit (x64) PE files on the Windows platform. This feature is currently declared as experimental.

UPX supports two mechanisms for decompression: an in-place technique and extraction to temporary file.

The in-place technique, which decompresses the executable into memory, is not possible on all supported platforms. It has the advantage of being more efficient in terms of memory, and that the environment set up by the OS remains correct.

The rest uses extraction to temporary file. This procedure involves additional overhead and other disadvantages; however, it allows any executable file format to be packed. The extraction to temporary file method has several disadvantages: