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.
