Catch-up on Flash XSS exploitation – bypassing the guardians! – Part 1
I had tweeted a few techniques in exploiting XSS in vulnerable flash files a few months ago. I thought it is a good idea to summarise them here and share it with you. I will try to add more parts to this in future…
Bypassing IE protection/feature against SWF reflected XSS:
Instead, it will show you the following error message in the console (press F12 to see it):
The same script is run easily in Firefox (without NoScript) and Google Chrome. Moreover, the other methods that I had invented previously in the following blog post do not work in IE10: “http://soroush.secproject.com/blog/2012/11/xss-by-uploadingincluding-a-swf-file/”.
Now, I have found a workaround to also bypass IE10 protections and run the script:
It is based on the following simple fact:
Flash URLDecode feature that can be used to bypass possible protections and obfuscate the attack:
If you need to send the vectors to a server behind a firewall (flashvars can be sent after the “#” character to be hidden from the server) or to bypass client-side Anti-XSS protections, this method can be very useful.
Flash discards invalid URL encoded values completely:
A) It discards 2 characters if you have an invalid hex character ([^0-9a-fA-F]) immediately after the percentage character. Example: “%X” or “%=”
B) It discards 3 characters if you have a valid hex character after the percentage character followed by an invalid hex character. Example: “%AX” or “%A&”
Note1: During this test, I have observed that sometimes values greater than 127 in ASCII will be converted to a question mark (“?”) character. This happens in URL redirection cases.
Note2: Encoded BOM characters (“%EF%BB%BF”) can also replace the space characters. Example: “alert(1)” can be rewritten as “alert%EF%BB%BF(1)” (http://en.wikipedia.org/wiki/Byte_order_mark)
Exploits can even be more deceptive if you use the following vectors: “%#” or “%A#”. It will not send your complete vector to the server because of the “#” character.
New equivalent queries:
As you can see in these examples, a flash based XSS attack can be obfuscated very well!
NoScript was bypassed initially by using this trick but it has been patched since version 126.96.36.199 (http://noscript.net/changelog). Thanks to Giorgio Maone (@ma1).
I will try to post more related materials in regards with Flash security. I may divulge some 0days here…, who knows?