fix: only linkify http and https URIs on the error page

escapeButPreserveUris() matched the scheme with "[A-z]+?://", which accepts
any scheme. A frame comment containing javascript://x.co/?%0Aalert(1) survives
htmlspecialchars() untouched and was then wrapped in an anchor, so clicking it
executed script in the origin of the error page. "[A-z]" is also a code point
range rather than a letter class, so it additionally matched the six characters
between "Z" and "a", making "_://x.co/a" a link too.

Match the scheme with "https?" instead, case-insensitively so uppercase
"HTTP://" keeps working. Other schemes stay as escaped plain text. The editor
integration links are not affected, they are emitted directly by the template
and never pass through this method.

Fixes #791

src/Whoops/Util/TemplateHelper.php

@@ -80,6 +80,10 @@ class TemplateHelper
      * Escapes a string for output in an HTML document, but preserves
      * URIs within it, and converts them to clickable anchor elements.
      *
+     * Only http and https URIs are turned into links. Any other scheme is left as
+     * plain text, because schemes such as javascript: and data: would otherwise
+     * become clickable script execution on the error page.
+     *
      * @param  string $raw
      * @return string
      */
@@ -87,7 +91,7 @@ class TemplateHelper
     {
         $escaped = $this->escape($raw);
         return preg_replace(
-            "@([A-z]+?://([-\w\.]+[-\w])+(:\d+)?(/([\w/_\.#-]*(\?\S+)?[^\.\s])?)?)@",
+            "@(https?://([-\w\.]+[-\w])+(:\d+)?(/([\w/_\.#-]*(\?\S+)?[^\.\s])?)?)@i",
             "<a href=\"$1\" target=\"_blank\" rel=\"noreferrer noopener\">$1</a>",
             $escaped
         );