F-Secure Virus Descriptions : 5lo

The first discovery of this virus was made in late October, 1992.

The virus increases file size with approx. 1000-1100 bytes. A typical value for this is 1032 bytes. Directly at the end of a infected file is found the string "92.05.24.5lo.2.23MZ". This is the reason for the odd name of this virus. Also, the virus carries the name of the infected file within itself. Strings "????????.EXE" and "*.EXE" are visible inside the virus code as well.

The virus stays resident and infects only EXE files. Whenever a EXE file is ran the virus infects it and also one other EXE file. The virus leaves its infection date and time into the directory entry, which makes it easier to trace the virus path of infections.

The virus increases a counter within itself after every infection. The counter is never checked, so the virus never activates. It is well written and contains code which makes it impossible to dissassemble automatically with programs such as Sourcer and DIS-DOC. This slows down the examination of the code a bit.

The virus appends its code at the end of all infected files. It recognises infected files by changing the field 0Ch in the EXE header (maximum paragraphs to allocate in addition to executable's size) to value of FFAAh. Also, the virus identifies itself from memory by using an interrupt call of INT 21, AX=3521h which it has hooked. All the checks work correctly and the virus won't infect files multiple times and it installs itself to memory only once.

When the virus is active in the memory it is not discoverable by using the command MEM /C. This is because the virus installs itself seemingly to be part of the operating system. Free memory decreases by approx. two kilobytes, though.

5lo does not do anything else than spreads.