Archive for April, 2008
Monday, April 14th, 2008
Now, normally when I find an XSS vulnerability on a popular domain I just report it to the appropriate security team and move on, but this one is interesting…
By taking advantage of the content-type returned by spreadsheets.google.com (and a caching flaw on the part of Google), I was able to pull off a full blown XSS against the google.com domain. For those of you who don’t understand what this means, allow me to elaborate. When Google sets their cookie, it is valid for all of their sub domains. So, when you log into gmail (mail.google.com), your gmail cookie is actually valid for code.google.com, docs.google.com, spreadsheets.google.com…and so on. If someone (like me) finds an XSS vulnerability in any one of these sub domains, I’ll be able to hijack your session and access any google service as if I were you.
So, in this instance, I have an XSS on spreadsheets.google.com. With this single XSS, I can read your Gmail, backdoor your source code (code.google.com), steal all your Google Docs, and basically do whatever I want on Google as if I were you! Google’s use of “document.domain=” also make things a little easier to jump from one domain to the next, but that’s another story…
This particular XSS takes advantage of how Internet Explorer determines the content type of the HTTP response being returned by the server. Most would think that explicitly setting the content-type to something that isn’t supposed to be rendered by the browser would easily solve this issue, but it does not. IE isn’t the only browser that will ignore the content-type header in certain circumstances, Firefox, Opera, and Safari will ignore the content-type header as well (in certain circumstances). Security professionals and more importantly developers need to understand the nuances of how the popular web browsers handle various content-type headers, otherwise they may put their web application at risk of XSS. The most comprehensive paper I’ve seen on the subject was written by Blake Frantz of Leviathan. The paper can be found here. It’s a “MUST HAVE” reference for web app security pros. Read it, understand it, protect yourself appropriately or expect others to exploit appropriately…
In this issue, Google set the content-type header for a response which I controlled the content to text/plain. If I can inject what looks like HTML into the first few bytes of the response, I’ll be able to “trick” Internet Explorer into rendering the content as HTML. Luckily for me, I was able to do just that.
I created a spreadsheet on spreadsheets.google.com and for the first cell (A1) I put the following content: “<HTML><body><script>alert(document.cookie)</script></body></HTML>”
I then saved the spreadsheet and generated a link for the spreadsheet to be served as a CSV.
When this option is selected, the contents of the spreadsheet are displayed inline (the content-disposition header was not explicitly set to “attachment”), IE ignores the content-type header, sniffs the content-type from the response, then proceeds to render the response as if it were HTML. At this point, I control the entire HTML being rendered under an xxx.google.com domain.
To be fair, Google included a subtle defense to protect against content-type sniffing (padding the response), but those protection measures failed (with a little prodding by me). The issue is fixed, but if you try to reproduce this issue, you’ll see their defense in play. It a solid defense which shows they understand the nuances of content-type sniffing.
I’ll provide some tips on taking ownership of untrusted content and serving it from your server in a later post, but for now take a look at the paper written by Blake Frantz. I’m sure it will open some eyes…
Sunday, April 13th, 2008
RSA is officially over! It was a great experience and I’ll talk about a few of the talks that really captured me in later posts. I do want to thank Jeremiah Grossman for throwing the WASC get together, the BAYSEC crew, McAfee (their party was awesome), iSEC (their party was AWESOME), Thirsty Bear, everyone at the W, and everyone that came to the Breaking and Securing Web Applications talk!
There were tons of people trying to get some answers to their web appsec questions after the talk, if you weren’t able to talk to me after the session or during the conference, please don’t hesitate to shoot me an email.
I’ll be at toorcon next week, if you’re in the Seattle area, look me up…
Friday, April 4th, 2008
Taking ownership of someone else’s content is always a tricky deal. Nate McFeters and I spoke about some of the issues related to taking “ownership” of someone else’s content last year at Defcon, but we continue to see more and more places willingly accepting third party content and happily serving it from their domain. I came across an interesting cross domain issue based on content ownership that involved Google. Google has fixed the issue, but I thought the issue was interesting so I’ll share the details… but before I do… I wanted to mention the efforts put forth by the Google Security Team (GST). Fixing this issue was not trivial… it involved significant changes as to how content was served from Google servers. Needless to say, the GST moved quickly and the issue was fixed in an amazingly expedient and effective manner… KUDOS to the GST!
On to the issue:
I discovered that users could upload arbitrary files to the code.google.com domain by attaching a file to the “issues” portion of a project. The uploaded file is then served from the code.google.com domain. Normally, these types of attacks would make use of the Flash cross domain policy file and the System.security.loadPolicyFile() API, however due to the unique path of each project, the cross domain capabilities of Flash are very limited in this instance as policy files loaded via loadPolicyFile() are “limited to locations at or below its own level in the server’s hierarchy”.
Flash isn’t the only option here though. Java has a different security policy and uploading a Java class file to the code.google.com domain gives me access to the entire domain, as opposed to only certain folders and sub folders.
Sounds pretty straight forward huh? Well, I ran into some issues as the JVM encodes certain characters in its requests for class files made via the CODE attribute within APPLET tags. After poking around a bit, I realized that requests made via the ARCHIVE would be sent as is, without the encoding of special characters. With this newfound knowledge in hand, I created a JAR file with my class file within it and uploaded it to code.google.com.
Now, the CODE attribute is a required attribute within the APPLET tag, so I specified name of the class file I placed within the JAR file. When the APPLET tag is rendered, the JVM first downloads the JAR file specified in the ARCHIVE attribute, the JVM then makes the request for the class file specified in the CODE attribute. In this instance, the request for the class file specified in the CODE attribute will fail as the class file is not on the code.google.com server (even if it was, we wouldn’t be able to reach it as requests made via the CODE attribute are encoded). The failure to locate the class file causes the JVM to begin searching alternate locations for the requested class file and the JVM will eventually load a class file with the same name located inside of the JAR file…
Once the class file is loaded, the JVM will fire the init() method and Java’s Same Origin policy allows me to use the applet to communicate with the domain that served the applet class file (as opposed to the domain that hosts the HTML calling the APPLET tag). Here’s a screenshot of the PoC page I was hosting on XS-Sniper.com.
I don’t think there is a tool on the market today that even attempts to detect something like this and I’ve met many “security professionals” that have no idea that vulnerabilities like this even exist. This isn’t the first time I’ve come across a cross domain hole based on content ownership. I’m expecting we’ll see a lot more of these types of vulnerabilities in the future as cross domain capabilities becomes more prevalent in client side technologies and as content providers become more and more comfortable in taking ownership of others content.