Virus Database

Hmyr.1800

Description Hmyr.1800

It is a not a dangerous memory resident encrypted parasitic virus. It hooks INT 21h and on selecting a new directory searches for .COM and .EXE files and writes itself to the end of the file. On January 1st, February 23rd, March 8th, April 1st, May 1st, June 12th, and Fridays that falls on 13th the virus creates in directories files with name in Cyrillic - this name is pronounced like "Hmyr".

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Plutto.602

Description Plutto.602

This is a very dangerous nonmemory resident parasitic virus. it searches for .COM files of the current directory and writes itself to the end of the file. If a .COM file has EXE format, the virus corrupts it. The virus writes the string "pLuTto_B" to the the header of such file (EXE header), and that file is not restorable.

Ply.4224

Description Ply.4224

This is a dangerous, non memory-resident parasitic virus. It searches for all EXE files in the current directory, then writes itself to the end of the file.
The virus is not encrypted, but it appears as polymorphic virus. Its codes in different infected files have very few constant bytes, and as a result, there is no constant scan-string to detect this virus. To do this, the virus uses a rather complex engine that "mixes" the code in the virus body.
The virus contains three blocks: the main code, data, and redirected calls.
+----------+
ƒMain Code ƒ
ƒ ƒ
ƒ----------ƒ
ƒData ƒ
ƒ----------ƒ
ƒRedirectedƒ
ƒCalls ƒ
ƒ ƒ
+----------+

All assembler instructions in the main code are not more than 3 bytes in length, and all instructions occupy three bytes in the virus code. If the length of instruction is less than 3 bytes, free bytes contain NOP instructions. As a result, all instructions in the viruses occupy 3-bytes blocks.
While infecting a file, the virus "move" the instructions to the 3-bytes block, if there is a NOP command, then:
8C C8 MOV AX,CS <--> 90 NOP
90 NOP 8C C8 MOV AX,CS

There are also data that contain 6-byte blocks to copy the instructions to redirected calls and replace them with CALL or JMP commands:
Replaced with CALL Replaced with JMP Original code
------------------ ----------------- -------------
E8 xx xx CALL -+ <--> E9 xx xx JMP -+ <--> 90 NOP
all <-ƒ---+ ... <-ƒ---+ 8C C8 MOV AX,CS
... ƒ ƒ ... ƒ ƒ
... V ƒ ... V ƒ
8C C8 MOV AX,CS ƒ 8C C8 MOV AX,CS ƒ <marked as free
90 NOP ƒ 90 NOP ƒ block<
C3 RET ---+ E9 xx xx JMP back -+

Therefore, any instruction can be shifted in the 3-byte blocks, it can be copied to a randomly selected address in the virus, and then replaced with a CALL or JMP command, and existing CALLs and JMPs redirectors can be replaced with the original code. No byte is encrypted, and there are very few constant bytes to detect the virus.
Such a complex engine is not bugs-free, and the virus often corrupts files while infecting them.
The virus checks the names of the files before infecting them, and do not infect the following files:
AVP AVPLITE AVPVE EMM386 F-PROT FV386 FV86 MSAV MVTOOL10 SCAN TBSCAN TBAV
TBCHECK TBCLEAN TBDISK TBDRIVER TBFILE TBGENSIG TBKEY TBLOG TBMEM TBSETUP
TBSCANX TBUTIL VALIDATE VIRSTOP VPIC VSAFE.

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