Improve MSO comments processing in incoming emails - closes #5068

Author: freescout-help-desk

app/Thread.php

@@ -316,6 +316,15 @@ public function getCleanBody($body = '')
         // Cut out "collapse" class as it hides elements.
         $body = preg_replace("/(<[^<>\r\n]+class=([\"'][^\"']* |[\"']))(collapse|hidden)([\"' ])/", '$1$4', $body) ?: $body;
 
+        // Take care of MSO-comments.
+        // https://github.com/freescout-help-desk/freescout/issues/5068
+        // Step 1: Completely remove the MSO-specific blocks.
+        // The 's' flag allows '.' to match newlines, and 'i' makes it case-insensitive.
+        $body = preg_replace('/<!--\[if mso\]>.*?<!\[endif\]-->/is', '', $body);
+        // Step 2: Unwrap the standard HTML from the "not mso" comments.
+        // This reveals the standard <a> tag for the purifier.
+        $body = preg_replace('/<!--\[if !mso\]><!--\s*-->(.*?)<!--\s*<!\[endif\]-->/is', '$1', $body);
+
         // Remove only the <!--[if !mso]><!--> and <!--<![endif]--> around the elements.
         // https://github.com/freescout-helpdesk/freescout/pull/3865#issuecomment-1990758149
         $body = preg_replace('/<!\-\-\[if [^>]+\]><!\-\->(.*?)<![ ]+\-\-<!\[endif\]\-\->/s', '$1', $body);