When an infected program is first executed, Riihi becomes memory
resident, and infects COM files as they are executed. Riihi does
not decrease available DOS memory, and it does not show up in
When the virus is resident, it will check every executed program.
If the file extension is COM, and the program does not start with
the 'M' letter, the virus infects it. The 'M' check is done in order
not to infect EXE files that have been renamed to COM.
Infected programs grow by 132 bytes. The virus does not do anything
except spread, and it does not contain any texts.
The exact origin of Riihi is not known, but it is suspected to be
written in Finland - it caused several large-scale infections in
Finland during December 1993 and January 1994.
[Analysis: Mikko Hypponen, F-Secure]