Add target _blank to external links in Neos – Included in https://speakerdeck.com/aertmann/tasty-recipes-for-every-day-neos

gistfile1.txt

<?php
namespace Acme\Acme\TypoScript;

use TYPO3\Flow\Annotations as Flow;
use TYPO3\Neos\Domain\Exception;
use TYPO3\Neos\Service\LinkingService;
use TYPO3\TYPO3CR\Domain\Model\NodeInterface;
use TYPO3\TypoScript\TypoScriptObjects\AbstractTypoScriptObject;

/**
 * A TypoScript Object that converts link references in the format "<type>://<UUID>" to proper URIs
 *
 * Right now node://<UUID> and asset://<UUID> are supported URI schemes.
 *
 * Usage::
 *
 *   someTextProperty.@process.1 = TYPO3.Neos:ConvertUris
 *
 * The optional property ``forceConversion`` can be used to have the links converted even when not
 * rendering the live workspace. This is used for links that are not inline editable (for
 * example links on images)::
 *
 *   someTextProperty.@process.1 = TYPO3.Neos:ConvertUris {
 *     forceConversion = true
 *   }
 */
class ConvertUrisImplementation extends \TYPO3\Neos\TypoScript\ConvertUrisImplementation {

	/**
	 * Convert URIs matching a supported scheme with generated URIs
	 *
	 * If the workspace of the current node context is not live, no replacement will be done unless forceConversion is
	 * set. This is needed to show the editable links with metadata in the content module.
	 *
	 * @return string
	 * @throws Exception
	 */
	public function evaluate() {
		$text = parent::evaluate();

		if ($text === '' || $text === NULL) {
			return '';
		}

		if (!is_string($text)) {
			throw new Exception(sprintf('Only strings can be processed by this TypoScript object, given: "%s".', gettype($text)), 1382624080);
		}

		try {
			$processedContent = preg_replace_callback(
				'~<a.*?href="(.*?)".*?>~i',
				function ($matches) {
					$uriHost = parse_url($matches[1], PHP_URL_HOST);
					if (
						(is_string($uriHost) && $uriHost != $_SERVER["HTTP_HOST"]) ||
						strpos($matches[1], '_Resources') !== false
					) {
						if (!preg_match_all('~target="(.*?)~i', $matches[0], $targetMatches)) {
							return str_replace('<a', '<a target="_blank"', $matches[0]);
						}
					}
					return $matches[0];
				},
				$text
			);
		} catch (\Exception $e) {
			die($e->getMessage());
		}

		return $processedContent;
	}

}

When doing this, adjust it to add rel="noopener" as done in https://github.com/neos/neos-development-collection/pull/1729/files as of Neos 4.0