Everything about the CSV Excel Macro Injection

CSV Excel Macro Injection, also known as Formula Injection or  CSV Injection, is an attack technique which we use in the day to day penetration testing of the application.

CSV injection is a vulnerability which affects applications having the export spreadsheets functionality. These spreadsheets generate dynamically from invalidated or unfiltered user inputs. Modern web applications offer spreadsheet export function these days. This allows the user to download data in a .csv file format or .xls file format. This is suitable for handling spreadsheet applications like MS-Excel and OpenOffice Calc as a result of which the cells in the spreadsheets can contain inputs from untrusted source. As a result, the end user who is accessing the exported spreadsheet can be affected.
This vulnerability can be used by an attacker to execute attacks such as client-side command injection or code injection. Basically, the attack scenario for this is purely targeting the user(s) who download the Excel file naturally. We usually disregard this attack as a non-issue. However, websites should still be aware that the information they are exporting can potentially affect the users.

Demonstration of CSV excel macro injection:

CSV excel macro injection can be exploited when the application supports export to excel functionality.
This happens on spreadsheet files which dynamically generate from invalidated input data.

Payloads used to test and exploit:

We can use formulae which we use in excel for carrying out operations to test for formula injection on websites.
Eg: =sum(10+10)

Screenshot_1

 

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As you can see, once we click on the export excel option, the records automatically export to an excel file with the .xls format. Thus, this allows us to download the .xls file

Technical Analysis of the Vulnerability:

This vulnerability occurs due to the concept of dynamic Data exchange(DDE). DDE is a protocol for interprocess communication under windows supported by MS-Excel, LibreOffice and Apache OpenOffice.

The DDE function is in the following format:

=DDE(server; file; item; mode)

So by using some malicious arguments, it is possible to remotely execute applications or commands on the victim’s computer of whoever opens the document.

Screenshot_4

 

So, the most common payload used is:
=cmd|’ /C calc’!A0

Technical Details of the above payload:

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

So our final DDE call becomes:

DDE (“cmd”;”/C calc”;”!A0″)

 

 

Screenshot_6

Screenshot_8

As you can see in the above screenshots, our payload adds to the input fields. Now, once we export this record to an excel file, our payload informs the program to run an application called  cmd.exe with the command /C calc which executes calc.exe from the command line.

Once the excel file is open calc.exe will be executed as shown in the screenshot below.

Screenshot_11

Mostly these payloads are all fine and well. But, sometimes the = character filters out. However we can use some different combinations such as @, + or – . So, current payload of choice for exploiting this as a proof of concept is.

@SUM(1+1)*cmd|' /C calc'!A0

We can use any formula starting with:

  • =
  • +
  • @

Recommendations

Updated on Sunday, 6:30 AM

In conclusion, I recommend that it is always agood practice not to trust user inputs and always encode the output. Also, for the successful execution of the formula, attacker will have to use the ‘-‘ , ‘=’ , and the pipe (|) is used to execute the binary in the excel software. Hence, it is strongly recommended to filter the ‘- ‘, ‘|’ , ‘+’ and  ‘=’  to mitigate this vulnerabilty.

 

References

[1] Download Vulnerable Code here

[2] CSV Injection

[3] Comma Separated Vulnerabilities