asp防止上传图片木马原理解析

if file.filesize10 then
  Response.Write("script>alert('您没有选择上传文件')/script>")
  Response.Write("script>history.go(-1)/script>")
  Response.End()
end if

set MyFile = server.CreateObject("Scripting.FileSystemObject")
set MyText = MyFile.OpenTextFile(FilePath, 1) '读取文本文件
sTextAll = lcase(MyText.ReadAll)
MyText.close
set MyFile = nothing
sStr=".getfolder|.createfolder|.deletefolder|.createdirectory|.deletedirectory|.saveas
|wscript.shell|script.encode|server.|.createobject|execute|activexobject|language="
sNoString = split(sStr,"|") 
for i=0 to ubound(sNoString)
  if instr(sTextAll,sNoString(i)) then
   set filedel = server.CreateObject("Scripting.FileSystemObject")
   filedel.deletefile FilePath
   set filedel = nothing
   Response.Write("script>alert('您上传的文件有问题，上传失败');window.close();/script>")
   Response.End()
  end if
next

%
'***************************************************************
'CheckFileType 函数用来检查文件是否为图片文件
'参数filename是本地文件的路径
'如果是文件jpeg,gif,bmp,png图片中的一种，函数返回true，否则返回false
'***************************************************************
 
const adTypeBinary=1
 
dim jpg(1):jpg(0)=CByte(HFF):jpg(1)=CByte(HD8)
dim bmp(1):bmp(0)=CByte(H42):bmp(1)=CByte(H4D)
dim png(3):png(0)=CByte(H89):png(1)=CByte(H50):png(2)=CByte(H4E):png(3)=CByte(H47)
dim gif(5):gif(0)=CByte(H47):gif(1)=CByte(H49):gif(2)=CByte(H46):gif(3)=CByte(H39):gif(4)=CByte(H38):gif(5)=CByte(H61)
Response.Write CheckFileType(Server.MapPath("2.gif"))
 
function CheckFileType(filename)
on error resume next
CheckFileType=false
dim fstream,fileExt,stamp,i
fileExt=mid(filename,InStrRev(filename,".")+1)
set fstream=Server.createobject("ADODB.Stream")
fstream.Open
fstream.Type=adTypeBinary
fstream.LoadFromFile filename
fstream.position=0
select case fileExt
case "jpg","jpeg"
stamp=fstream.read(2)
for i=0 to 1
if ascB(MidB(stamp,i+1,1))=jpg(i) then CheckFileType=true else CheckFileType=false
next
case "gif"
stamp=fstream.read(6)
for i=0 to 5
if ascB(MidB(stamp,i+1,1))=gif(i) then CheckFileType=true else CheckFileType=false
next
case "png"
stamp=fstream.read(4)
for i=0 to 3
if ascB(MidB(stamp,i+1,1))=png(i) then CheckFileType=true else CheckFileType=false
next
case "bmp"
stamp=fstream.read(2)
for i=0 to 1
if ascB(MidB(stamp,i+1,1))=bmp(i) then CheckFileType=true else CheckFileType=false
next
end select
fstream.Close
set fseteam=nothing
if err.number>0 then CheckFileType=false
end function
%>