Amit, this is very interesting solution and it will probably work in most cases. However, if the attacker is able to upload PDF documents, he/she can craft one that will produce the desired result as soon as it gets opend by the user. This can be achieved by setting the PDF file to redirect. David K. from michaeldawe.org had a POC somewhere. This attack vector is limited due to the fact that the attacker needs to upload malicious PDF files, but it is still very dangerous. Think about email attachments. I haven't tested this idea myself, though. On 1/3/07, Amit Klein <aksecurity@xxxxxxxxx> wrote:
Amit Klein wrote: > pdp (architect) wrote: >> I will be very quick and just point to links where you can read about >> this issue. >> >> It seams that PDF documents can execute JavaScript code for no >> apparent reason by using the following template: >> >> >> http://path/to/pdf/file.pdf#whatever_name_you_want=javascript:your_code_here >> >> >> You must understand that the attacker doesn't need to have write >> access to the specified PDF document. In order to get an XSS vector >> working you need to have a PDF file hosted on the target and that's >> all about it. The rest is just a matter of your abilities and desires. >> > Amazing, and kudos to Sven Vetsch who found this. > Oops, seems that the credit went to the wrong guy. Actual credit go to Stefano Di Paola, with contributions from Giorgio Fedon (IE Dos, UXSS Analysis) and Elia Florio (Poc and Code Execution analysis). BTW, one way to mitigate this is for the server, upon receiving a request to file.pdf (without a valid query+cookie pair, see below), to redirect it to file.pdf?token_query=X (where X is an unpredictable, high entropy string) accompanied with a Set-Cookie header for a cookie named say "token_cookie" with value X and short expiration period (e.g. 10 seconds). The browser will then respond with a request to file.pdf?token_query=X with Cookie token_cookie=X. At this point, if the server receives token_query==token_cookie, the server knows this is a legit request (i.e. without fragment), and it can safely serve the PDF resource. I suppose this can be implemented easily as an Apache module/ISAPI/NSAPI filters. -Amit ---------------------------------------------------------------------------- The Web Security Mailing List: http://www.webappsec.org/lists/websecurity/ The Web Security Mailing List Archives: http://www.webappsec.org/lists/websecurity/archive/ http://www.webappsec.org/rss/websecurity.rss [RSS Feed]
-- pdp (architect) | petko d. petkov http://www.gnucitizen.org
