Update ActivityPubFetchService, enforce stricter Content-Type validation

app/Services/ActivityPubFetchService.php

@@ -28,7 +28,8 @@ class ActivityPubFetchService
         $headers['User-Agent'] = 'PixelFedBot/1.0.0 (Pixelfed/'.config('pixelfed.version').'; +'.config('app.url').')';
 
         try {
-			$res = Http::withOptions(['allow_redirects' => false])->withHeaders($headers)
+            $res = Http::withOptions(['allow_redirects' => false])
+                ->withHeaders($headers)
                 ->timeout(30)
                 ->connectTimeout(5)
                 ->retry(3, 500)
@@ -40,9 +41,31 @@ class ActivityPubFetchService
         } catch (Exception $e) {
             return;
         }
+
         if(!$res->ok()) {
             return;
         }
+
+        if(!$res->hasHeader('Content-Type')) {
+            return;
+        }
+
+        $acceptedTypes = [
+            'application/activity+json; charset=utf-8',
+            'application/activity+json',
+            'application/ld+json; profile="https://www.w3.org/ns/activitystreams"'
+        ];
+
+        $contentType = $res->getHeader('Content-Type')[0];
+
+        if(!$contentType) {
+            return;
+        }
+
+        if(!in_array($contentType, $acceptedTypes)) {
+            return;
+        }
+
         return $res->body();
     }
 }