UPX - the Ultimate Packer for eXecutables
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The Ultimate Packer for eXecutables
Copyright (c) 1996-2026 Markus Oberhumer, Laszlo Molnar & John Reiser https://upx.github.io
Welcome to UPX !
UPX is a free, secure, portable, extendable, high-performance executable packer for several executable formats.
UPX is an advanced executable file compressor. UPX will typically reduce the file size of programs and DLLs by around 50%-70%, thus reducing disk space, network load times, download times and other distribution and storage costs.
Programs and libraries compressed by UPX are completely self-contained and run exactly as before, with no runtime or memory penalty for most of the supported formats.
UPX supports a number of different executable formats, including Windows programs and DLLs, and Linux executables.
UPX is free software distributed under the term of the GNU General Public License. Full source code is available.
UPX may be distributed and used freely, even with commercial applications. See the UPX License Agreements for details.
IMPORTANT NOTE: UPX inherits the security context of any files it handles.
This means that packing, unpacking, or even testing or listing a file requires the same security considerations as actually executing the file.
Use UPX on trusted files only!
'upx program.exe' will compress a program or DLL. For best compression results try 'upx --best program.exe' or 'upx --brute program.exe'.
Please see the file doc/upx-doc.txt for the full documentation. The files NEWS and doc/BUGS.txt also contain various tidbits of information, as does the output of 'upx --help'.
Stay up-to-date with ongoing OS and executable format changes
ARM64 for Windows (help wanted)
We will NOT add any sort of protection and/or encryption. This only gives people a false feeling of security because all "protectors" can be broken by definition.
Fix all remaining bugs - please report any issues https://github.com/upx/upx/issues
Copyright (C) 1996-2026 Markus Franz Xaver Johannes Oberhumer Copyright (C) 1996-2026 Laszlo Molnar Copyright (C) 2000-2026 John F. Reiser
UPX is distributed with full source code under the terms of the GNU General Public License v2+; either under the pure GPLv2+ (see the file COPYING), or (at your option) under the GPLv2+ with special exceptions and restrictions granting the free usage for all binaries including commercial programs (see the file LICENSE).
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
You should have received a copy of the UPX License Agreements along with this program; see the files COPYING and LICENSE. If not, visit the UPX home page.
Share and enjoy, Markus & Laszlo & John
Markus F.X.J. Oberhumer Laszlo Molnar [email protected] [email protected]
John F. Reiser [email protected]
[ The term UPX is a shorthand for the Ultimate Packer for eXecutables and holds no connection with potential owners of registered trademarks or other rights. ]