asp防止上传木马原理：

首先判断文件大小：

if file.filesize<10 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

将文件上传到服务器后，判断用户文件中的危险操作字符:
 

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

如何防止木马性图片上传

这个代码我检验过没有问题，可以阻挡木马性图片的上传
 

<%
'***************************************************************
'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
%>

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