Fork bomb

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.

"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

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 a copy-on-write resource management technique 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.

A classic example of a fork bomb is the Unix shell one :(){ :|:& };:, which can be more easily understood as:

fork() {
    fork | fork &
}
fork

In it, a function is defined (fork()) as calling itself (fork), then piping (|) its result to a background job of itself (&).

The Windows equivalent, given the limitations in system calls, could be written as such in batch:

:loop
start %~nx0
goto loop

An even shorter version of this can be achieved by using anonymous functions:

%0|%0

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.[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] Modern Linux systems also allow finer-grained fork bomb prevention through cgroups and process number (PID) controller.[9]

See also

References
  1. 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. Jielin, Dong (2007). Network Dictionary. p. 200. ISBN 1602670005.
  4. Dhamdhere, Dhananjay M. (2006). Operating Systems: A Concept-based Approach. p. 285. ISBN 0-07-061194-7.
  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.
  9. "Process Number Controller in Documentation/ as appeared in Linux kernel 5.3". October 8, 2019.
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