In general, in computing, an alphanumeric code is a series of letters and numbers (hence the name) which are written in a form understandable and processable by a computer. One such alphanumeric code is ASCII.

More specifically, in computer underground terminology alphanumeric code is machine code that is written so that it assembles into entirely readable ASCII-letters such as "a"-"z", "A"-"Z", "1"-"9", "#", "!", "@", and so on.

This is possible to do with a (very) good understanding of the assembly language for the specific computer that the code is intended for.

This code is used by crackers with the intent of fooling applications, such as Web forms, into accepting code used for exploit. Typically, such exploits involve buffer overflows.

Sometimes, alphanumeric code could also be used when the programmer needs to write a program but does not have access to any compiler or assembler. All that is needed to write a alphanumeric program is a text editor.

Example DOS-program

%!!%BB-tz-!!-%cP\%!!%BB-=<Pu( ------Written by Kalle

The text above will, when interpretended by an x86 CPU using the DOS operating system overwrite parts of itself with a code used to terminate the execution of the program, and return back to DOS. Just copy the text into an pure text editor (for example edit.exe in DOS or notepad in Windows), save the file and then rename it to "something.com". To execute the program, just type "something" in a DOS command shell.