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For the rail link service with the reporting mark UPX, see Union Pearson Express.
Ultimate Packer for eXecutebles.png
Initial release May 26, 1998; 18 years ago (1998-05-26)
Stable release
3.09.1 (aka. 3.91) / September 30, 2013; 3 years ago (2013-09-30)
Written in C++, Assembly
Operating system Microsoft Windows, Linux, Mac OS X, MS-DOS, Atari
Platform i386, MIPS, AMD64, ARM, PowerPC, m68k
Available in English
Type Executable compression
License GPL with exception for compressed executables[1]
Website upx.github.io

UPX (Ultimate Packer for Executables) is a free and open source executable packer supporting a number of file formats from different operating systems.[2][3]


UPX uses a data compression algorithm called UCL,[4] which is an open source implementation of portions of the proprietary NRV (Not Really Vanished[5]) algorithm.[6]

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) executable files on the Windows platform.[7] 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. The rest use 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:

  • Special permissions are ignored, such as suid.
  • argv[0] will not be meaningful.
  • Multiple running instances of the executable are unable to share common segments.

Unmodified UPX packing is often detected and unpacked by antivirus software scanners. UPX also has a built-in feature for unpacking unmodified executables packed with itself. The default license for the existing stubs explicitly forbids modification that prevent manual unpacking.[8] Most antivirus products will raise an alarm when UPX header is detected.[citation needed]

Supported formats[edit]

UPX does not currently support PE files containing CIL code intended to run on the .NET Framework.


  1. ^ https://upx.github.io/upx-license.html
  2. ^ Marak, Victor (2015). Windows Malware Analysis Essentials. Packt Publishing. p. 188. ISBN 978-1-78528-151-8. Retrieved November 22, 2015. Packers such as Ultimate Packer for Executables (UPX) are more of executable compressors as size reduction is the primary goal, not obfuscation, which can be a byproduct ... 
  3. ^ Blunden, Bill (2013). The Rootkit Arsenal (Second ed.). Jones & Bartlett Learning. pp. 353–355. ISBN 978-1-4496-2636-5. Retrieved November 22, 2015. One of the most prolific executable packers is UPX (the Ultimate Packer for executables). Not only does it handle dozens of different executable formats, but also its source code is available online. 
  4. ^ http://www.oberhumer.com/opensource/ucl/
  5. ^ http://www.oberhumer.com/products/nrv/
  6. ^ https://github.com/upx/upx/blob/master/README.SRC
  7. ^ https://upx.github.io/upx-news.txt
  8. ^ https://upx.github.io/upx-license.html

External links[edit]