Rock Lobster Contact Form 7 before 3.7.2 allows remote attackers to bypass the CAPTCHA protection mechanism and submit arbitrary form data by omitting the _wpcf7_captcha_challenge_captcha-719 parameter.
https://www.cvedetails.com/cve/CVE-2014-2265/
https://www.acunetix.com/vulnerabilities/web/wordpress-plugin-contact-form-7-security-bypass-3-7-1/