If your application exports CSV, be advised that Excel and other spreadsheet applications treat certain cells (those starting with =, +, - or @) as formulas.

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This is an issue if you output user input. Not only is it probably not what you want, it also poses a security risk. See the link for attack vectors.

Note that current Excel versions will warn the user when opening the file. At least for the code execution vulnerability, these three warnings seems adequate to me.

Code execution warnings:

External links:

Solution

If possible, always try to avoid CSV and output XLS(X) files directly as this does not suffer from this issue and has other advantages as well.