Die_Hard is a resident fast infector which targets COM and EXE files.
It is known to be in the wild especially in India, where it was found
in September 1994. Die_Hard has also been sighted in Singapore,
Indonesia, USA and in many parts of Europe.
When the virus is executed, it goes resident in memory, decreasing
the available DOS memory by 9232 bytes. Die_Hard infects all executed
or opened COM and EXE files. The infected files grow by exactly 4000
Die_Hard hides beneath several layers of encryption. When the virus
is decrypted, the following texts can be seen:
SW DIE HARD 2
Since the virus does not utilize polymorphic encryption techniques,
it is quite easy to find.
Die_Hard activates on the 3rd, 11th, 15th, or 28th of any month,
provided the day is Tuesday and the virus has already infected at
least 13 files. The virus will then wait until some program changes
the screen to graphics mode. At this time the virus will display an
animation of large 'S' and 'W' characters on the screen. It will also
deny write access to files, displaying text 'SW Error'.
Besides infecting COM and EXE files, Die_Hard trojanizes ASM and PAS
source files when they are accessed; in other words, the virus
inserts source code Trojan horses in these files.
[Analysis: Mikko Hypponen, F-Secure]