Virus Name: PUREST Aliases: V Status: New, Research Viron Discovery: July, 1994 Symptoms: None - Pure Stealth - Keyboard Key Lock is Disabled Origin: USA Eff Length: 439 Bytes Type Code: OReE - Extended HMA Memory Resident Overwriting .EXE Infector Detection Method: None Removal Instructions: See Below General Comments: The PUREST virus is a HMA memory resident overwriting direct action infector. The virus is a pure 100% stealth virus with no detectable symptoms. No file length increase; overwritten .EXE files execute properly; no interrupts are directly hooked; no change in file date or time; no change in available memory; INT 12 is not moved; no cross linked files from CHKDSK; when resident the virus cleans programs on the fly; works with all 80?86 processors; VSAFE.COM does not detect any changes; Thunder Byte's (v6.21) Heuristic virus detection does not detect the virus; Windows 3.1's built in warning about a possible virus does not detect PUREST. The PUREST is a variation of the PUREPLUS virus. PUREST was completely rewritten to avoid being able to use the same scan string as PUREPLUS. PUREPLUS was detected with Thunder Byte's (v6.21) Heuristic scanning. PUREST is not. PUREST will disable the keyboard key switch. PUREST now works with Novell DOS 7. The PUREST virus will only load if DOS=HIGH in the CONFIG.SYS file. The first time an infected .EXE file is executed, the virus goes memory resident in the HMA (High Memory Area). The hooking of INT 13 is accomplished using a tunnelling technique, so memory mapping utilities will not map it to the virus in memory. It then reloads the infected .EXE file, cleans it on the fly, then executes it. After the program has been executed, PUREST will attempt to infect 15 .EXE files in the current directory. If the PUREST virus is unable to install in the HMA or clean the infected .EXE on the fly, the virus will reopen the infected .EXE file for read-only; modify the system file table for write; remove itself, and then write the cleaned code back to the .EXE file. It then reloads the clean .EXE file and executes it. The virus can not clean itself on the fly if the disk is compressed with DBLSPACE or STACKER, so it will clean the infected .EXE file and write it back. It will also clean itself on an 8086 or 8088 processor. It will infect an .EXE if it is executed, opened for any reason or even copied. When an uninfected .EXE is copied, both the source and destination .EXE file are infected. The PUREST virus overwrites the .EXE header if it meets certain criteria. The .EXE file must be less than 62K. The file does not have an extended .EXE header. The file is not SETVER.EXE. The .EXE header must be all zeros from offset 73 to offset 512; this is where the PUREST virus writes it code. The PUREST virus then changes the .EXE header to a .COM file. Files that are READONLY can also be infected. To remove the virus from your system, change DOS=HIGH to DOS=LOW in your CONFIG.SYS file. Reboot the system. Then run each .EXE file less than 62k. The virus will remove itself from each .EXE program when it is executed. Or, leave DOS=HIGH in you CONFIG.SYS; execute an infected .EXE file, then use a tape backup unit to copy all your files. The files on the tape have had the virus removed from them. Change DOS=HIGH to DOS=LOW in your CONFIG.SYS file. Reboot the system. Restore from tape all the files back to your system.