Editing Shellcode (section)

{{Short description|Small piece of code used as a payload to exploit a software vulnerability}}
{{Redirect|Shell code|code written in a shell's command language|Shell script}}
{{Redirect|Alphanumeric executable|executable code presented in hexadecimal format|Hex file (disambiguation)}}

In [[hacker (computer security)|hacking]], a '''shellcode''' is a small piece of code used as the [[Payload (computing)|payload]] in the [[exploit (computer security)|exploitation]] of a software [[Vulnerability (computing)|vulnerability]].  It is called "shellcode" because it typically starts a [[Shell (computing)|command shell]] from which the attacker can control the compromised machine, but any piece of code that performs a similar task can be called shellcode.  Because the function of a payload is not limited to merely spawning a shell, some have suggested that the name shellcode is insufficient.<ref>{{cite book 
|title=Sockets, Shellcode, Porting, & Coding: Reverse Engineering Exploits and Tool Coding for Security Professionals 
|author-first1=James C. |author-last1=Foster |author-first2=Mike |author-last2=Price |publisher=Elsevier Science & Technology Books |date=2005-04-12 |isbn=1-59749-005-9 |url=https://books.google.com/books?id=ZNI5dvBSfZoC}}</ref> However, attempts at replacing the term have not gained wide acceptance. Shellcode is commonly written in [[machine code]].

When creating shellcode, it is generally desirable to make it both small and executable, which allows it to be used in as wide a variety of situations as possible.<ref name="anley_koziol_2007">{{Cite book |title=The shellcoder's handbook: discovering and exploiting security holes |date=2007 |publisher=Wiley |author-first1=Chris |author-last1=Anley |author-first2=Jack |author-last2=Koziol |isbn=978-0-470-19882-7 |edition=2 |location=Indianapolis, Indiana, UA |oclc=173682537}}</ref> In assembly code, the same function can be performed in a multitude of ways and there is some variety in the lengths of opcodes that can be used for this purpose; good shellcode writers can put these small opcodes to use to create more compact shellcode.<ref>{{Cite book |title=Buffer overflow attacks: detect, exploit, prevent |date=2005 |publisher=Syngress |author-last=Foster |author-first=James C. |isbn=1-59749-022-9 |location=Rockland, MA, USA |oclc=57566682}}</ref> Some have reached the smallest possible size while maintaining stability.<ref>{{Cite web |title=Tiny Execve sh - Assembly Language - Linux/x86 |url=https://github.com/geyslan/SLAE/blob/master/4th.assignment/tiny_execve_sh.asm |access-date=2021-02-01 |website=GitHub}}</ref>