Prevent CSV Injection via formulas (Symfony Blog)


Description

CSV Injection, also known as Formula Injection, occurs when websites embed untrusted input inside CSV files. When a spreadsheet program opens a CSV, any cell starting with = is interpreted by the software as a formula and could be abused by an attacker.

In Symfony 4.1, we’ve added the opt-in csv_escape_formulas option in CsvEncoder, to prefix all cells starting by =, +, - or @ by a tab t.

Since then, OWASP added 2 chars in that list:
– Tab (0x09)
– Carriage return (0x0D)

This makes our previous prefix char (Tab t) part of the vulnerable characters, and OWASP suggests using the single quote ' for prefixing the value.

Resolution

Symfony now follows the OWASP recommendations and use the single quote ' to prefix formulas and adds the prefix to cells starting by t, r as well as =, +, - and @.

The patch for this issue is available here for branch 4.4.

Credits

We would like to thank Jake Barwell for reporting the issue and Jérémy Derussé for fixing the issue.

If you have found a security issue in Symfony, please send the
details to security [at] symfony.com and don’t
disclose it publicly until we can provide a fix for it.



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