Dropbox disables old shared document links to prevent unintended access

Stored files are vulnerable to cut-and paste clumsiness or to links embedded in shared documents

Dropbox has disabled old shared document links in a bid to prevent its users' files from being accessed by unintended recipients.

Before the change, anyone in possession of a Dropbox shared document link could access the file it pointed to, but the documents were inaccessible to those without the link, a form of security through obscurity. That poses no problems when users know who has the link, but there are a number of unexpected ways that information can fall into other hands, prompting Dropbox to disable links shared before May 5.

Links created from now on will be protected as Dropbox has patched its systems, the company said in a blog post Monday. Contacted for this article, the company declined to say how it had resolved the issue.

The problem was highlighted by Dropbox competitor IntraLinks on its Collaborista blog. The posting also implied that the problem affects another online storage service, Box.net, but that company did not immediately respond to a request for comment.

IntraLinks, like many other companies, books search ads using competitors' brands as keywords. The reports it receives about its advertising campaigns indicate the searches that prompt the ads to appear. Some of those searches were for Dropbox shared links to documents containing personal information, including completed tax returns or mortgage applications, perhaps intended to be shared with a spouse or a financial adviser.

How those links came to be used as search terms will be no mystery to anyone who has ever inadvertently pasted a URL into the search box of their browser instead of the address box. Alongside the search results for that URL, the search engine also returns ads.

Another way in which URLs could reach unintended recipients is through links contained in the shared documents, according to security expert Graham Cluley.

If someone clicks on a link then their browser passes the target webserver both the page requested and the address of the page containing the link, which in this case would be the shared document link itself. That link would then be visible to anyone with access to the web server's log files including, at a minimum, the webmaster, and possibly (in the case of poorly configured web servers), anyone performing a web search for terms found in the log file itself.

Dropbox recommended that users of its Dropbox for Business service turn on a setting that restricts access to shared documents to team members. For users of its consumer service, it recommended that anyone needing to share a document for which it had disabled the link simply create a new sharing link.

Box.net made no comment on the issue, and did not respond to questions. However, Cluley pointed out that the service already offers a way for users of the service to protect their shared documents, by turning on a setting to "restrict shared links to collaborators only." However, the setting, in both consumer and business versions of the service, is turned off by default, he said.

Peter Sayer covers open source software, European intellectual property legislation and general technology breaking news for IDG News Service. Send comments and news tips to Peter at peter_sayer@idg.com.

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