Detect YouTube URLs and emit privacy-enhanced iframe

Recognizes youtu.be, watch?v=, shorts, embed, and mobile variants.
Emits an iframe pointing to youtube-nocookie.com with lazy loading,
strict-origin referrer policy, and allowfullscreen.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/Markdown/MediaUrlResolver.php

@@ -14,7 +14,8 @@ public function resolve(string $url): ?string
             return null;
         }
         return $this->detectVideo($url)
-            ?? $this->detectAudio($url);
+            ?? $this->detectAudio($url)
+            ?? $this->detectYouTube($url);
     }
 
     private function detectVideo(string $url): ?string
@@ -35,6 +36,40 @@ private function detectAudio(string $url): ?string
         return "<audio src=\"{$safe}\" controls class=\"kb-audio\"></audio>";
     }
 
+    private function detectYouTube(string $url): ?string
+    {
+        $patterns = [
+            '~^https?://youtu\.be/([A-Za-z0-9_-]{11})(?:[/?#]|$)~',
+            '~^https?://(?:www\.|m\.)?youtube\.com/watch\?(?:[^#]*&)?v=([A-Za-z0-9_-]{11})(?:[&#]|$)~',
+            '~^https?://(?:www\.|m\.)?youtube\.com/shorts/([A-Za-z0-9_-]{11})(?:[/?#]|$)~',
+            '~^https?://(?:www\.|m\.)?youtube\.com/embed/([A-Za-z0-9_-]{11})(?:[/?#]|$)~',
+        ];
+        $videoId = null;
+        foreach ($patterns as $p) {
+            if (preg_match($p, $url, $m)) {
+                $videoId = $m[1];
+                break;
+            }
+        }
+        if ($videoId === null) {
+            return null;
+        }
+        $src = "https://www.youtube-nocookie.com/embed/{$videoId}";
+        return $this->iframeHtml($src, 'youtube');
+    }
+
+    private function iframeHtml(string $src, string $provider): string
+    {
+        $safe = htmlspecialchars($src, ENT_QUOTES, 'UTF-8');
+        return '<iframe src="' . $safe . '" '
+            . 'width="560" height="315" '
+            . 'loading="lazy" '
+            . 'referrerpolicy="strict-origin-when-cross-origin" '
+            . 'allow="autoplay; encrypted-media; picture-in-picture" '
+            . 'allowfullscreen frameborder="0" '
+            . 'class="kb-embed kb-embed-' . $provider . '"></iframe>';
+    }
+
     private function getPathExtension(string $url): string
     {
         $path = parse_url($url, PHP_URL_PATH);

src/tests/Unit/Markdown/MediaUrlResolverTest.php

@@ -87,4 +87,43 @@ public static function audioUrls(): array
             'm4a' => ['/clip.m4a'],
         ];
     }
+
+    #[DataProvider('youtubeUrls')]
+    public function test_youtube_urls_produce_iframe(string $url): void
+    {
+        $html = $this->resolver->resolve($url);
+        $this->assertNotNull($html);
+        $this->assertStringStartsWith('<iframe', $html);
+        $this->assertStringContainsString('youtube-nocookie.com/embed/dQw4w9WgXcQ', $html);
+        $this->assertStringContainsString('class="kb-embed kb-embed-youtube"', $html);
+        $this->assertStringContainsString('loading="lazy"', $html);
+        $this->assertStringContainsString('allowfullscreen', $html);
+    }
+
+    public static function youtubeUrls(): array
+    {
+        return [
+            'short youtu.be' => ['https://youtu.be/dQw4w9WgXcQ'],
+            'watch v=' => ['https://www.youtube.com/watch?v=dQw4w9WgXcQ'],
+            'shorts' => ['https://www.youtube.com/shorts/dQw4w9WgXcQ'],
+            'embed' => ['https://www.youtube.com/embed/dQw4w9WgXcQ'],
+            'mobile' => ['https://m.youtube.com/watch?v=dQw4w9WgXcQ'],
+            'no www watch' => ['https://youtube.com/watch?v=dQw4w9WgXcQ'],
+        ];
+    }
+
+    #[DataProvider('invalidYoutubeUrls')]
+    public function test_invalid_youtube_urls_return_null(string $url): void
+    {
+        $this->assertNull($this->resolver->resolve($url));
+    }
+
+    public static function invalidYoutubeUrls(): array
+    {
+        return [
+            'too short id' => ['https://youtu.be/short'],
+            'host mismatch' => ['https://example.com/watch?v=dQw4w9WgXcQ'],
+            'XSS attempt in id' => ['https://youtu.be/abc"><script>'],
+        ];
+    }
 }