Google Apps hit by session-stealing attack

by Matthew Broersma

April 16, 2008 —

A security researcher has uncovered a serious flaw in Google
Spreadsheets, which could give an attacker access to all of a user's Google
services.

While the bug, a cross-site scripting (XSS) flaw, has now been fixed by Google,
it is an indication of the perils that can accompany the growing popularity
of Software as a Service (SaaS), according to researcher Billy Rios, who uncovered
the problem.

Because of the way Google structures its authentication processes, a single
XSS attack can deliver access to all of a user's Google services and documents,
Rios said.

"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," he said in a blog post.

The exploit relied on the way Internet Explorer determines the content type
of server responses, ignoring the content-type header in certain circumstances.
Browsers such as Firefox, Opera and Safari can be made to share the same behavior,
Rios said.

"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," he wrote.

To carry out the attack, Rios injected HTML into the first cell of a table,
along with Javascript designed to display the user's cookie. IE then rendered
the content as HTML, allowing the cookie to be viewed.

The attack could be delivered via a link to the specially formed spreadsheet,
Rios said.

"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)," he wrote.

Rios recently publicized a vulnerability (also now fixed) in Google Code allowing
the theft of passwords.

Google Apps began as a set of hosted services, but Google this month has begun
rolling out offline access to them, beginning with the word processor, Google
Docs.

Over the next three weeks or so, Google will turn on the feature for all word
processor users, giving them the ability to view and edit documents offline.
During the same time period, Google Docs' spreadsheet will gain offline ability
for viewing, but not editing documents.

Google Docs' third component, an application to make slide presentations, will
remain for now without offline access. However, Google has plans to extend the
offline access to it and to other hosted services in the Google Apps suite,
of which Docs is part. Apps also includes Gmail,
Calendar,
Talk and others.

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