According to F-Secure, Invalid is an Internet worm written in pure Assembly. The worm's file is a 12288 bytes long PE EXE file. The worm's file is not compressed.

When the worm's file is run it first checks for available Internet connection. If a connection is not found the worm starts to recursively look for '*.exe' files. If an EXE file is found, the worm gets external cetrificate from Windows crypto library, generates a new key. If key generation fails, the worm exits. Otherwise the worm encrypts a found file with a generated key. When the worm reaches root directory, the encryption process stops and the worm exits. There seems to be a bug in recursive scan routine, so it should encrypt files only in current directory and directories listed in PATH variable.

If Internet connection is found, the worm gets information about its own file, allocates 2 memory buffers, reads itself into the first memory buffer and then encodes itself with BASE64 encoding (encoding subroutine is inside the worm's file) into the second memory buffer. After that the worm gets the special folder location and looks for '*.ht*' (*.HTM, *.HTML, etc.) files there. When an appropriate file is found, the worm loads it into memory and starts looking for 'mailto:' strings inside the file. If this string is found the worm gets an e-mail address after it and sends itself to this address. Then the worm continues to search for 'mailto:' string in the same file and will send itself out if other e-mail addresses are found. If no more addresses are found, the worm looks for more HTML files.