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UPX - the Ultimate Packer for eXecutables

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                The Ultimate Packer for eXecutables

Copyright (c) 1996-2021 Markus Oberhumer, Laszlo Molnar & John Reiser


Welcome to UPX !

Please don't forget to read the file LICENSE - UPX is distributed under the GNU General Public License (GPL) with special exceptions allowing the distribution of all compressed executables, including commercial programs.


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 95/98/ME/NT/2000/XP/CE programs and DLLs, DOS programs, and Linux executables and kernels.

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 Agreement for details.

UPX is rated number one in the well known Archive Comparison Test. Visit .

UPX aims to be Commercial Quality Freeware.


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 --brute program.exe'.

Please see the file UPX.DOC for the full documentation. The files NEWS and BUGS also contain various tidbits of information.


UPX comes with ABSOLUTELY NO WARRANTY; for details see the file LICENSE.

Having said that, we think that UPX is quite stable now. Indeed we have compressed lots of files without any problems. Also, the current version has undergone several months of beta testing - actually it's almost 8 years since our first public beta.

This is the first production quality release, and we plan that future releases will be backward compatible with this version.

Please report all problems or suggestions to the authors. Thanks.


  • AArch64 (64-bit ARM) for Linux and iOS.

  • The Linux approach could probably get ported to a lot of other Unix variants, at least for other i386 architectures it shouldn't be too much work. If someone sends me a fresh hard disk and an official FreeBSD/OpenBSD/NetBSD/Solaris/BeOS... CD I might take a look at it ;-)

  • We will NOT add any sort of protection and/or encryption. This only gives people a false feeling of security because by definition all protectors/compressors can be broken. And don't trust any advertisement of authors of other executable compressors about this topic - just do a websearch on "unpackers"...

  • Fix all remaining bugs - keep your reports coming ;-)

  • See the file PROJECTS in the source code distribution if you want to contribute.


Copyright (C) 1996-2021 Markus Franz Xaver Johannes Oberhumer Copyright (C) 1996-2021 Laszlo Molnar Copyright (C) 2000-2021 John F. Reiser

This program may be used freely, and you are welcome to redistribute it under certain conditions.

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. See the UPX License Agreement for more details.

You should have received a copy of the UPX License Agreement along with this program; see the file 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. ]

[ Feel free to contact us if you have commercial compression requirements or interesting job offers. ]

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