Render plain text properly in the default HTML email template

templates/email/html/default.php

@@ -15,8 +15,11 @@
  * @var string $content
  */
 
-$lines = explode("\n", $content);
-
-foreach ($lines as $line) :
-    echo '<p> ' . $line . "</p>\n";
-endforeach;
+// $content is plain text: blank lines separate paragraphs and single line breaks become <br>.
+// (*BSR_ANYCRLF) limits \R to CR and LF; without /u it would also match the byte 0x85 inside UTF-8 characters such as "Å".
+?>
+<?php foreach (preg_split('/(*BSR_ANYCRLF)\R\s*\R/', trim($content)) ?: [] as $paragraph) : ?>
+    <?php if ($paragraph !== '') : ?>
+        <p><?= implode('<br>', array_map(h(...), preg_split('/(*BSR_ANYCRLF)\R/', $paragraph) ?: [])) ?></p>
+    <?php endif; ?>
+<?php endforeach; ?>

tests/TestCase/Mailer/DefaultTemplateTest.php

@@ -0,0 +1,92 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * CakePHP(tm) : Rapid Development Framework (https://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ * @link          https://cakephp.org CakePHP(tm) Project
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace App\Test\TestCase\Mailer;
+
+use Cake\Mailer\Message;
+use Cake\Mailer\Renderer;
+use Cake\TestSuite\TestCase;
+
+/**
+ * Tests the default email templates in templates/email.
+ */
+class DefaultTemplateTest extends TestCase
+{
+    /**
+     * Render content with the default HTML template, without a layout.
+     *
+     * @param string $content The content.
+     * @return string
+     */
+    protected function renderHtml(string $content): string
+    {
+        $renderer = new Renderer();
+        $renderer->viewBuilder()
+            ->setTemplate('default')
+            ->disableAutoLayout();
+
+        return $renderer->render($content, [Message::MESSAGE_HTML])[Message::MESSAGE_HTML];
+    }
+
+    /**
+     * Test blank lines separate paragraphs and single line breaks become <br>.
+     *
+     * @return void
+     */
+    public function testHtmlParagraphs(): void
+    {
+        $html = $this->renderHtml("First line\r\nsecond line\r\n\r\n\r\nNext paragraph\n");
+
+        $this->assertSame(
+            '<p>First line<br>second line</p><p>Next paragraph</p>',
+            preg_replace('/>\s+</', '><', trim($html)),
+        );
+    }
+
+    /**
+     * Test the content is escaped.
+     *
+     * @return void
+     */
+    public function testHtmlEscapesContent(): void
+    {
+        $html = $this->renderHtml('<script>alert("x")</script> & more');
+
+        $this->assertStringContainsString('&lt;script&gt;alert(&quot;x&quot;)&lt;/script&gt; &amp; more', $html);
+        $this->assertStringNotContainsString('<script>', $html);
+    }
+
+    /**
+     * Test multibyte characters containing the byte 0x85 are not split as line breaks.
+     *
+     * @return void
+     */
+    public function testHtmlKeepsMultibyteCharacters(): void
+    {
+        $html = $this->renderHtml("Ångström\nĽahký kôň žerie ďatelinu");
+
+        $this->assertSame('<p>Ångström<br>Ľahký kôň žerie ďatelinu</p>', trim($html));
+    }
+
+    /**
+     * Test empty content renders no paragraphs.
+     *
+     * @return void
+     */
+    public function testHtmlEmptyContent(): void
+    {
+        $this->assertStringNotContainsString('<p>', $this->renderHtml(" \r\n\r\n "));
+    }
+}