Saturday, February 10, 2007

Bookmarklets are NOT secure

Jungsonn wrote a post entitled "Defeating Phishers" where he wrote about how one could distribute risk across two servers, and essentially have one site where XSS vulnerabilities are unimportant, and one which would need to be audited heavily. He also recommended using Bookmarklets because "Bookmarklets are actually pretty secure things, no software or website can access them.".

Personally I disagree with both those statements. Firstly, if you find an XSS hole in the main domain, you can easily make the page say that they've changed their practices, sure it would be a little odd, but the amount of user education required for this attack to be impractical would be enough to solve the whole phishing issue, not just this one.

But more importantly I want to debunk the myth that Bookmarklets are secure. Leaving aside the fact that trojans and similar can easily alter them because they have access to the file system, they are still insecure; they are as insecure as the page they are clicked on is untrustworthy.

For example, lets take the Bookmarklet Jungsonn posted:
javascript:QX=document.getSelection();if(!QX){void(QX=prompt('Type your firstname',''))};if(QX)document.location=''

It would seem fairly secure, except for the fact that with the very allowing Javascript engines, we can stop this from working, here's how:
function changeHandler(x, y, z) {
if (z == '') {
return '';
} else {
return z;
}'location', changeHandler);

If the bookmarklet is clicked on a page with that on it - say a phishing page at or a legitimate page on the domain if it has an XSS hole in it, then we can easily send the user to a phishing page, even though the value is hard coded in the bookmarklet.

Of course, the bookmarklet can try to detect and remove such things, but its a technological battle that will be fought on a bookmarklet by bookmarklet basis; which is essentially where security generally fails - custom code. Of course the other scenario is that we find a secure method of redirecting users, but even if we do, we're not going to be able to get everyone to use it; so I'd rather not recommend bookmarklets as security; just tell users to create a simple bookmark to the site.

Or we could try to educate users to only click on the bookmarklet from a blank page, but thats another area where security generally fails - user education.

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