JezK

Edit File: messages.php

<?php

trait WPCF7_ConfigValidator_Messages {

/**
	 * Runs error detection for the messages section.
	 */
	public function validate_messages() {
		$messages = (array) $this->contact_form->prop( 'messages' );

if ( ! $messages ) {
			return;
		}

if (
			isset( $messages['captcha_not_match'] ) and
			! wpcf7_use_really_simple_captcha()
		) {
			unset( $messages['captcha_not_match'] );
		}

foreach ( $messages as $key => $message ) {
			$section = sprintf( 'messages.%s', $key );

if ( $this->supports( 'html_in_message' ) ) {
				if ( $this->detect_html_in_message( $section, $message ) ) {
					$this->add_error( $section, 'html_in_message',
						array(
							'message' => __( 'HTML tags are used in a message.', 'contact-form-7' ),
						)
					);
				} else {
					$this->remove_error( $section, 'html_in_message' );
				}
			}
		}
	}

/**
	 * Detects errors of HTML uses in a message.
	 *
	 * @link https://contactform7.com/configuration-errors/html-in-message/
	 */
	public function detect_html_in_message( $section, $content ) {
		$stripped = wp_strip_all_tags( $content );

if ( $stripped !== $content ) {
			return true;
		}

return false;
	}

}