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because it lacks inline citations please help improve this article by introducing more precise citations july 2025 learn how and when to remove this message shell code redirects here for code written in a shell s command language see shell script alphanumeric executable redirects here for executable code presented in hexadecimal format see hex file disambiguation shellcode is executable code intended to be used as a payload for exploiting a software vulnerability the term includes shell because the attack originally described an attack that opens a command shell that the attacker can use to control the target machine but any code that is injected to gain access that is otherwise not allowed can be called shellcode for this reason some consider the name shellcode to be inaccurate 1 an attack commonly injects data that consists of executable code into a process before or as it exploits a vulnerability to gain control the program counter is set to the shellcode entry point so that the shellcode runs deploying shellcode is often accomplished by including the code in a file that a vulnerable process downloads and then loads into its memory common wisdom dictates that to maximize effectiveness a shellcode payload should be small 2 machine code provides the flexibility needed to accomplish the goal shellcode authors leverage small opcodes to create compact shellcode 3 4 types edit local a local shellcode attack allows an attacker to gain elevated access privilege on their computer in some cases exploiting a vulnerability can be achieved by causing an error such as buffer overflow if successful the shellcode enables access to the machine via the elevated privileges granted to the targeted process remote a remote shellcode attack targets a process running on a remote machine on the same local area network intranet or on the internet if successful the shellcode provides access to the target machine across the network the shellcode normally opens a tcp ip socket connection to allow access to a shell on the target machine a remote shellcode attack can be categorized by its behavior if the shellcode establishes the connection it is called a reverse shell or a connect back shellcode on the other hand if the attacker establishes the connection the shellcode is called a bindshell because the shellcode binds to a certain port on the victim s machine a bindshell random port skips the binding part and listens on a random port a a socket reuse shellcode is an exploit that establishes a connection to the vulnerable process that is not closed before the shellcode runs so that the shellcode can re use the connection to allow remote access socket re using shellcode is more elaborate since the shellcode needs to find out which connection to re use and the machine may have many open connections 5 a firewall can detect outgoing connections made by connect back shellcode as well as incoming connections made by bindshells and therefore offers some protection against an attack even if the system is vulnerable a firewall can prevent the attacker from connecting to the shell created by the shellcode one reason why socket re using shellcode is used is that it does not create new connections and therefore is harder to detect and block download and execute a download and execute shellcode attack downloads and executes malware on the target system this type of shellcode does not spawn a shell but rather instructs the machine to download a certain executable file from the network and execute it nowadays it is commonly used in drive by download attacks where a victim visits a malicious webpage that in turn attempts to run such a download and execute shellcode in order to install software on the victim s machine a variation of this attack downloads and loads a library 6 7 advantages of this technique are that the code can be smaller that it does not require the shellcode to spawn a new process on the target system and that the shellcode does not need code to clean up the targeted process as this can be done by the library loaded into the process staged when the amount of data that an attacker can inject into the target process is too limited to achieve the desired effect it may be possible to deploy shellcode in stages that progressively provide more access the first stage might do nothing more than download the second stage that then provides the desired access egg hunt an egg hunt shellcode attack is a staged attack in which the attacker can inject shellcode into a process but does not know where in the process it is a second stage shellcode generally smaller than the first is injected into the process to search the process s address space for the first shellcode the egg and executes it 8 omelet an omelet shellcode attack similar to egg hunt looks for multiple small blocks of data eggs and combines them into a larger block omelet that is then executed this is used when an attacker is limited on the size of injected code but can inject multiple 9 encoding edit shellcode is often written in order to work around the restrictions on the data that a process will allow general techniques include optimize for size optimize the code to decrease its size self modifying code modify its own code before executing it to use byte values that are otherwise restricted encryption to avoid intrusion detection encode as self decrypting or polymorphic character encoding an attack that targets a browser might obfuscate shellcode in a javascript string using an expanded character encoding 10 for example on the ia 32 architecture here s two unencoded no operation instructions used in a nop slide 90 nop 90 nop as encoded percent encoded unescape u9090 unicode literal u9090 html xml character reference 邐 or 邐 null free shellcode must be written without zero value bytes when it is intended to be injected into a null terminated string that is copied in the target process via the usual algorithm i e strcpy of ending the copy at the first zero byte called the null character in common character sets if the shellcode contained a null the copy would be truncated and not function properly to produce null free code from code that contains nulls one can replace machine instructions that contain zeroes with instructions that don t for example on the ia 32 architecture the instruction to set register eax to 1 contains zeroes as part of the literal 1 expands to 0x00000001 b8 01000000 mov eax 1 the following instructions accomplish the same goal eax containing 1 without embedded zero bytes by first setting eax to 0 then incrementing eax to 1 33c0 xor eax eax 40 inc eax text an alphanumeric shellcode consists of only alphanumeric characters 0 9 a z and a z 11 12 this type of encoding was created by hackers to obfuscate machine code inside what appears to be plain text this can be useful to avoid detection of the code to allow the code to pass through filters that scrub non alphanumeric characters from strings b a similar type of encoding is called printable code and uses all printable characters alphanumeric plus symbols like a similarly restricted variant is echoable code not containing any characters which are not accepted by the echo command it has been shown that it is possible to create shellcode that looks like normal text in english 13 writing such shellcode requires in depth understanding of the instruction set architecture of the target machines it has been demonstrated that it is possible to write alphanumeric code that is executable on more than one machine 14 thereby constituting multi architecture executable code a work around was published by rix in phrack 57 11 in which he shows that it is possible to turn any code into alphanumeric code often self modifying code is leveraged because it allows the code to have byte values that otherwise are not allowed by replacing coded values at runtime a self modifying decoder can be created that initially uses only allowed bytes the main code of the shellcode is encoded also only using bytes in the allowed range when the output shellcode is run the decoder modifies its code to use instructions it requires and then decodes the original shellcode after decoding the shellcode the decoder transfers control to it it has been shown that it is possible to create arbitrarily complex shellcode that looks like normal english text 13 modern software uses unicode to support internationalization and localization often input ascii text is converted to unicode before processing when an ascii latin 1 in general character is transformed to utf 16 16 bit unicode a zero byte is inserted after each byte character of the original text obscou proved in phrack 61 12 that it is possible to write shellcode that can run successfully after this transformation programs that can automatically encode any shellcode into alphanumeric utf 16 proof shellcode exist based on the same principle of a small self modifying decoder that decodes the original shellcode compatibility edit generally shellcode is deployed as machine code since it affords relatively unprotected access to the target process since machine code is compatible within a relatively narrow computing context processor operating system and so on a shellcode fragment has limited compatibility also since a shellcode attack tends to work best when the code is small and targeting multiple exploits increases the size typically the code targets only one exploit none the less a single shellcode fragment can work for multiple contexts and exploits 15 16 17 versatility can be achieved by creating a single fragment that contains an implementation for multiple contexts common code branches to the implementation for the runtime context analysis edit as shellcode is generally not executable on its own in order to study what it does it is typically loaded into a special process a common technique is to write a small c program that contains the shellcode as data i e in a byte buffer and transfers control to the instructions encoded in the data function pointer or inline assembly code another technique is to use an online tool such as shellcode_2_exe to embed the shellcode into a pre made executable husk which can then be analyzed in a standard debugger specialized shellcode analysis tools also exist such as the idefense sclog project originally released in 2005 in the malcode analyst pack sclog is designed to load external shellcode files and execute them within an api logging framework emulation based shellcode analysis tools also exist such as the sctest application which is part of the cross platform libemu package another emulation based shellcode analysis tool built around the libemu library is scdbg which includes a basic debug shell and integrated reporting features see also edit computer security protection of computer systems from information disclosure theft or damage heap overflow software anomaly metasploit project computer security testing tool pages displaying short descriptions of redirect targets shell shoveling computer network security concept stack buffer overflow software anomaly notes edit the bindshell random port is the smallest stable bindshell shellcode for x86_64 available to date in part such filters were a response to non alphanumeric shellcode exploits references edit foster james c price mike 2005 04 12 sockets shellcode porting coding reverse engineering exploits and tool coding for security professionals elsevier science technology books isbn 1 59749 005 9 anley chris koziol jack 2007 the shellcoder s handbook discovering and exploiting security holes 2 ed indianapolis indiana usa wiley isbn 978 0 470 19882 7 oclc 173682537 foster james c 2005 buffer overflow attacks detect exploit prevent rockland ma usa syngress isbn 1 59749 022 9 oclc 57566682 tiny execve sh assembly language linux x86 github retrieved 2021 02 01 bha 2013 06 06 shellcode socket reuse retrieved 2013 06 07 skylined 2010 01 11 download and loadlibrary shellcode released archived from the original on 2010 01 23 retrieved 2010 01 19 download and loadlibrary shellcode for x86 windows 2010 01 11 retrieved 2010 01 19 skape 2004 03 09 safely searching process virtual address space pdf nologin retrieved 2009 03 19 skylined 2009 03 16 w32 seh omelet shellcode skypher com archived from the original on 2009 03 23 retrieved 2009 03 19 javascript large number of unescape patterns detected archived from the original on 2015 04 03 1 2 rix 2001 08 11 writing ia32 alphanumeric shellcodes phrack 0x0b 57 phrack inc 0x0f of 0x12 archived from the original on 2022 03 08 retrieved 2022 05 26 1 2 obscou 2003 08 13 building ia32 unicode proof shellcodes phrack 11 61 phrack inc 0x0b of 0x0f archived from the original on 2022 05 26 retrieved 2008 02 29 1 2 mason joshua small sam monrose fabian macmanus greg november 2009 english shellcode pdf proceedings of the 16th acm conference on computer and communications security new york ny usa pp 524 533 archived pdf from the original on 2022 05 26 retrieved 2010 01 10 10 pages multi architecture x86 and 64 bit alphanumeric shellcode explained blackhat academy archived from the original on 2012 06 21 eugene 2001 08 11 architecture spanning shellcode phrack phrack inc 0x0e of 0x12 archived from the original on 2021 11 09 retrieved 2008 02 29 nemo 2005 11 13 osx multi arch shellcode full disclosure archived from the original on 2022 05 26 retrieved 2022 05 26 cha sang kil pak brian brumley david lipton richard jay 2010 10 08 2010 10 04 platform independent programs pdf proceedings of the 17th acm conference on computer and communications security ccs 10 chicago illinois usa carnegie mellon university pittsburgh pennsylvania usa georgia institute of technology atlanta georgia usa pp 547 558 doi 10 1145 1866307 1866369 isbn 978 1 4503 0244 9 archived pdf from the original on 2022 05 26 retrieved 2022 05 26 12 pages see also external links edit shell storm database of shellcodes multi platform an introduction to buffer overflows and shellcode the basics of shellcoding pdf an overview of x86 shellcoding by angelo rosiello an introduction to shellcode development contains x86 and non x86 shellcode samples and an online interface for automatic shellcode generation and encoding from the metasploit project a shellcode archive sorted by operating system microsoft windows and linux shellcode design tutorial going from basic to advanced windows and linux shellcode tutorial containing step by step examples designing shellcode demystified alpha3 a shellcode encoder that can turn any shellcode into both unicode and ascii uppercase and mixedcase alphanumeric shellcode writing small shellcode by dafydd stuttard a whitepaper explaining how to make shellcode as small as possible by optimizing both the design and implementation writing ia32 restricted instruction set shellcode decoder loops by skylined archived 2015 04 03 at the wayback machine a whitepaper explaining how to create shellcode when the bytes allowed in the shel...