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A simple SUID tool written in C++


Why kos?

  • Very fast compilation times
  • Small amount of dependencies
  • GPLv3 License
  • Quite simple
  • Does not use PAM

Third party software support

  • Bash completion (Install completions/kos.bash into /usr/share/bash-completion/completions/kos)

Known issues

  • Only supported on Linux
shadow.h is one of the requirements of kos which is a "linux thing",
so automatically won't work on something like OpenBSD, MacOS, SerenityOS or
any other OSes.

Kos will not work on any non-unix OSes as kos uses a lot of
unix stuff, for example pwd.h, meaning will not work on stuff
like Windows and other non-unix OSes.

Though this is not really a big issue, this app is meant to
be ran on linux and was made with the intention to be used on
and with linux...
  • Overcomplicated input_no_echo() function
I used that function because it's literally the only
way I know how to disable eching of STDIN in linux with
C++ without using some huge lib like GNU readline or something...

People packaging kos for non-corporate use

I, the creator of kos, permit you to use any of these licenses:

- GPLv3
- BSD 3-clause
- ArAr2

If you do not want to bundle ArAr2 license together you are free
to use any of the other ones

Building and installing

If you are root you do not need to use su, just run commands directly



CXXFLAGS='-D_KOS_VERSION="1"' CXX=g++ ./scripts/  # Compiles with GCC instead of Clang (default)

CXXFLAGS='-D_KOS_VERSION="1"' is important as it will define version argument


./scripts/ kos
su -c 'mkdir -p /usr/local/bin'
su -c 'chown root:root ./kos'
su -c 'install -Dm4711 ./kos /usr/local/bin'

Man page installation

su -c 'mkdir -p /usr/share/man/man1'
su -c 'install -Dm0644 kos.1 /usr/share/man/man1/kos.1'
su -c 'mandb -qf /usr/share/man/man1/kos.1'



su -c 'cp completions/kos.bash /usr/share/bash-completion/completions/kos'


Before running the script you can optionally:

  • Set the DO_STRIP environment variable to strip the binary after compilation
  • Set the INSTALL_MAN environment variable to also install man page
  • Set the INSTALL_BCOMP environment variable to also install bash completion
chmod a+rx ./scripts/
su -c './scripts/'

Note for packagers

  • Arch Linux
Permission issues (ERROR: Failed getting groups for user ...)
    The issue can be solved with one install command:
    $ install -Dm4755 -o root "$srcdir/$pkgname-$pkgver/kos" "$pkgdir/usr/bin/kos"



  • If you're building for size make sure to build with -Os or -Oz in CXXFLAGS as it barely touches start times but it decreases the size largely, stripping can help too
  • If you every want to debug kos use -g -O0 CXXFLAGS


There are two scripts in the testing scripts directory, one is and other, * are just libs.

If you want to test it you just run the scripts, though which ones?

  • If you have access to root run:
  • If you have access to a non-privileged user run:
  • If you have access to both run.. Well both

Highly recommended

CXXFLAGS='-Og -g' ./scripts/
valgrind ./kos
valgrind -s ./kos

Or run testing script, will test all compilers, tools and stuff, in general a much more in-depth test

Returns code 127 on failure (detection of a memory leak) and you can see the log file in valgrind.log


  • Clang
  • GCC
  • Coreutils
  • Bash
  • Net-tools (or a hostname command)

Net-tools is not a thing for me!

You can easily make your own hostname command which is the only thing testing depends on:

#!/usr/bin/env sh
cat /etc/hostname

Add this to /usr/bin/hostname and make it executable:

su -c 'chmod 755 /usr/bin/hostname'

Using as a header

Just define KOS_H before including the main.cpp file

Inputting password from external sources

This section only applies if HAVE_PIPE is set

Kos supports piping to STDIN so you can easily just pipe (|) the password to STDIN :)

For example:

echo 'Top-secret-passw0rd' | kos id

Or even

printf '' | dmenu -l 0 -p 'Password: ' | kos id