A serious flaw in Google Spreadsheets could give an attacker access to all of a user's Google services, according to security researcher Billy Rios.
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), Rios said.
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 behaviour, 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.
The attack could be delivered via a link to the specially formed spreadsheet, Rios said.
"To be fair, Google included a subtle defence to protect against content-type sniffing (padding the response), but those protection measures failed (with a little prodding by me)," he wrote.
Rios recently publicised 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.