HNCTF2024逆向部分题解

Baby_OBVBS:

第一次做vbs的逆向题目，根本不知道怎么解决，看了看网上的教程，发现这个文件有很多Chr加密，所以我当时想要使用python将其提取出来，但是提取之后发现数据不全，所以只能放弃，看了大佬的WP之后，发现有更简单的方法，就是将前面的Execute改成 wscript.echo，然后就可以直接输出脚本内容了，真的厉害，tql了

修改前缀
HNCTF2024逆向部分题解插图

[Visual Basic] 纯文本查看 复制代码

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eAqi = "59fc6b263c3d0fcbc331ade699e62d3473bbf85522d588e3423e6c751ca091528a3c0186e460483917192c14"ANtg = "baacc7ffa8232d28f814bb14c428798b"
Function Base64Decode(base64EncodedString)
    Dim xml, elem
    Set xml = CreateObject("MSXML2.DOMDocument")
    Set elem = xml.createElement("tmp")
    elem.dataType = "bin.base64"
    elem.text = base64EncodedString
    Dim stream
    Set stream = CreateObject("ADODB.Stream")
    stream.Type = 1 'Binary
    stream.Open
    stream.Write elem.nodeTypedValue
    stream.Position = 0
    stream.Type = 2 'Text
    stream.Charset = "utf-8"
    Base64Decode = stream.ReadText
    stream.Close
End Function
Function Initialize(strPwd)
    Dim box(256)
    Dim tempSwap
    Dim a
    Dim b
    For i = 0 To 255
        box(i) = i
    Next
    a = 0
    b = 0
    For i = 0 To 255
        a = (a + box(i) + Asc(Mid(strPwd, (i Mod Len(strPwd)) + 1, 1))) Mod 256
        tempSwap = box(i)
        box(i) = box(a)
        box(a) = tempSwap
    Next
    Initialize = box
End Function
Function Myfunc(strToHash)
    Dim tmpFile, strCommand, objFSO, objWshShell, out
    Set objFSO = CreateObject("Scripting.FileSystemObject")
    Set objWshShell = CreateObject("WScript.Shell")
    tmpFile = objFSO.GetSpecialFolder(2).Path & "\" & objFSO.GetTempName
    objFSO.CreateTextFile(tmpFile).Write(strToHash)
    strCommand = "certutil -hashfile " & tmpFile & " MD5"
    out = objWshShell.Exec(strCommand).StdOut.ReadAll
    objFSO.DeleteFile tmpFile
    Myfunc = Replace(Split(Trim(out), vbCrLf)(1), " ", "")
End Function
Function EnCrypt(box, strData)
    Dim tempSwap
    Dim a
    Dim b
    Dim x
    Dim y
    Dim encryptedData
    encryptedData = ""
    For x = 1 To Len(strData)
        a = (a + 1) Mod 256
        b = (b + box(a)) Mod 256
        tempSwap = box(a)
        box(a) = box(b)
        box(b) = tempSwap
        y = Asc(Mid(strData, x, 1)) Xor box((box(a) + box(b)) Mod 256)
        encryptedData = encryptedData & LCase(Right("0" & Hex(y), 2))
    Next
    EnCrypt = encryptedData
End Function
msgbox "Do you know VBScript?"
msgbox "VBScript (""Microsoft Visual Basic Scripting Edition"") is a deprecated Active Scripting language developed by Microsoft that is modeled on Visual Basic."
msgbox "It allows Microsoft Windows system administrators to generate powerful tools for managing computers without error handling and with subroutines and other advanced programming constructs. It can give the user complete control over many aspects of their computing environment."
msgbox "Interestingly, although VBScript has long since been deprecated, you can still run VBScript scripts on the latest versions of Windows 11 systems."
msgbox "A VBScript script must be executed within a host environment, of which there are several provided with Microsoft Windows, including: Windows Script Host (WSH), Internet Explorer (IE), and Internet Information Services (IIS)."
msgbox "For .vbs files, the host is Windows Script Host (WSH), aka wscript.exe/cscript.exe program in your system."
msgbox "If you can not stop a VBScript from running (e.g. a dead loop), go to the task manager and kill wscript.exe/cscript.exe."
msgbox "cscript and wscript are executables for the scripting host that are used to run the scripts. cscript and wscript are both interpreters to run VBScript (and other scripting languages like JScript) on the Windows platform."
msgbox "cscript is for console applications and wscript is for Windows applications. It has something to do with STDIN, STDOUT and STDERR."
msgbox "OK! Now, let us begin our journey."
key = InputBox("Enter the key:", "CTF Challenge")
if (key = False) then wscript.quit
if (len(key)<>6) then
    wscript.echo "wrong key length!"
    wscript.quit
end if
If (Myfunc(key) = ANtg) Then
    wscript.echo "You get the key!Move to next challenge."
Else
    wscript.echo "Wrong key!Try again!"
    wscript.quit
End If
userInput = InputBox("Enter the flag:", "CTF Challenge")
if (userInput = False) then wscript.quit
if (len(userInput)<>44) then
    wscript.echo "wrong!"
    wscript.quit
end if
box = Initialize(key)
encryptedInput = EnCrypt(box, userInput)
If (encryptedInput = eAqi) Then
    MsgBox "Congratulations! You have learned VBS!"
Else
    MsgBox "Wrong flag. Try again."
End If
wscript.echo "bye!"

根据代码尝试解密一下：

RC4解密：

根据初始化代码，我们可以发现这是一个RC4加密，前面已经给出了密文和key，其中key是MD5值，使用CMD5解密一下，得到key，H&NKEY，然后进行RC4解密

得到flag是H&NCTF{VBS_1s@s0_7unny_an4_pow3rfu1_t00l!}

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