Friday, July 15, 2016

Abusing XSS Filter: One ^ leads to XSS(CVE-2016-3212)

In the past, I talked about XSS attacks exploiting IE XSS filter in CODE BLUE, which is an information security conference in Japan. A similar bug is fixed in June patch as CVE-2016-3212.
So, in this post, I would like to explain details of this bug.

As described in my slides, applying the XSS filter rules to an irrelevant context, we can do XSS attacks using the filter behavior replacing the . with the # even if the page does not have an XSS bug.

To prevent such attacks, Microsoft changed the filter behavior by December 2015 patch. After this patch, the ^ is used as the replacement character of the . instead of the #. Indeed, this can prevent attacks above. But it brought another nightmare. After several months, I confirmed an XSS using this behavior in Google's domain, and I got $3133.7 as rewards through Google VRP.

Google sets X-XSS-Protection: 1;mode=block header in almost their services. But not all. So, I checked carefully some pages which have no mode=block. As a result, I discovered that the vulnerable page exists in Javadoc on

I put the approximate copy:

This page becomes vulnerable to XSS when one . is replaced with the ^ by the XSS filter.
Can you find where it is?

The answer is the . of the yellow part:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "">
<!-- NewPage -->
<html lang="en">
<script type="text/javascript">
    targetPage = "" +;
    if (targetPage != "" && targetPage != "undefined")
targetPage = targetPage.substring(1);
if (targetPage.indexOf(":") != -1 || (targetPage != "" && !validURL(targetPage)))
        targetPage = "undefined";
    function validURL(url) {
        try {
            url = decodeURIComponent(url);
        catch (error) {
            return false;
        var pos = url.indexOf(".html");
        if (pos == -1 || pos != url.length - 5)
            return false;
        var allowNumber = false;
        var allowSep = false;
        var seenDot = false;
        for (var i = 0; i < url.length - 5; i++) {
            var ch = url.charAt(i);
            if ('a' <= ch && ch <= 'z' ||
                    'A' <= ch && ch <= 'Z' ||
                    ch == '$' ||
                    ch == '_' ||
                    ch.charCodeAt(0) > 127) {
                allowNumber = true;
                allowSep = true;
            } else if ('0' <= ch && ch <= '9'
                    || ch == '-') {
                if (!allowNumber)
                     return false;
            } else if (ch == '/' || ch == '.') {
                if (!allowSep)
                    return false;
                allowNumber = false;
                allowSep = false;
                if (ch == '.')
                     seenDot = true;
                if (ch == '/' && seenDot)
                     return false;
            } else {
                return false;
        return true;
    function loadFrames() {
        if (targetPage != "" && targetPage != "undefined")
             top.classFrame.location = top.targetPage;
<frameset cols="20%,80%" title="Documentation frame" onload="top.loadFrames()">
<frameset rows="30%,70%" title="Left frames" onload="top.loadFrames()">
<frame src="/" name="packageListFrame" title="All Packages">
<frame src="/" name="packageFrame" title="All classes and interfaces (except non-static nested types)">
<frame src="/" name="classFrame" title="Package, class and interface descriptions" scrolling="yes">
<div>JavaScript is disabled on your browser.</div>
<h2>Frame Alert</h2>
<p>This document is designed to be viewed using the frames feature. If you see this message, you are using a non-frame-capable web client. Link to <a href="overview-summary.html">Non-frame version</a>.</p>
In the <script>, it checks whether the given string via is safe.
For example, the following unsafe URL is blocked:

Then, what will happen when the . of the yellow part is replaced with the ^?

Let's actually try it. If you put the following strings in the target URL, the page content is forcibly matched to XSS filter rules, and we can replace the aimed . with the ^:

You can reproduce this bug from the following URL using IE/Edge which does not have June 2016 patch:"++++++++++++.i+++=

Also I put the replaced page for you who already applied the patch. You can confirm same behavior:

A crucial difference from # and ^, the # is not the operator in JavaScript, but the ^ is the operator. For example, if the a.b; is in the page and it is replaced with # and ^, a#b; is the syntax error but a^b; is valid syntax. It brings an XSS bug.

After June 2016 patch, when the XSS filter replaces the ., the mode=block behavior is enforced even if the page does not have X-XSS-Protection header.

I was surprised and disgusted when the ^ is displayed but anyway it has finally calmed down!

Also, in the recent patch(July 2016), it seems that Microsoft killed almost possibilities of XSS attacks exploiting XSS filter. I will write this details in next post :)