After questions from a security researcher, Dropbox says that it opens links in uploaded documents in conjunction with the preview feature. Dropbox also is trying to address DDoS concerns.

Mathew J. Schwartz, Contributor

September 16, 2013

3 Min Read

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Why has Dropbox been following links in uploaded documents?

That question surfaced Thursday after a security researcher found that links inside documents that he uploaded to the file-sharing service had been accessed. "I had the opportunity recently to beta-test, a Web app that generates documents that can 'buzz home,'" wrote Daniel McCauley Thursday on WNC InfoSec Blog. "This is done by a unique embedded GET request that is initiated when the generated document has been opened."

The first "phone home" operation occurred just 10 minutes after McCauley uploaded a Zip file that contained a "sting" .doc file generated by HoneyDocs. The link-accessing activity traced to an IP address that appeared to be an Amazon EC-2 instance in Seattle, which listed "LibreOffice" in the HTTP user-agent header. "All in all, I made three attempts to upload embedded documents and all appeared to be opened from different Amazon instances," McCauley said, noting that the Dropbox infrastructure is also built to use Amazon S3 buckets.

It's well-known that Dropbox generates a checksum of uploaded files to allow the service to identify duplicates, so it only needs to store a single copy. But why would it be touching links inside those documents? "Several use cases came to mind, but I was most interested in seeing if my cloud storage services were manipulating my files in a way that I may not have been aware of," McCauley said.

[ Is coverage of the NSA's reach into private data overblown? Read The NSA And Your Cloud Data: Navigating The Noise. ]

Dropbox, however, quickly dismissed any security concerns, saying it was simply generating previews of uploaded documents. "Dropbox allows people to open and preview files from their browser," a company spokeswoman said Monday via email. She noted that the external resource loading behavior that McCauley observed relates to backend processes that automatically create these document previews, making it easier for users to view docs within their Dropbox.

McCauley likewise got that message. "Dropbox views/opens certain file types in order to convert them to a compatible format so they are easily accessible via Web browser for its users," he said in a follow-up post. "This makes sense and is common practice for many cloud storage services to provide the convenience of browser access while not needing any additional software to open these documents."

Even so, news of the link-checking feature quickly spawned related discussions about how the functionality might have information security or privacy repercussions if abused by attackers. "Could Dropbox perhaps let me disable this feature?" asked "Helium" on the Hackers News Site. "I almost never use the Web interface so I wouldn't miss it and I prefer that my documents are not opened after being synched."

Marcus Carey, principal developer of -- which offers -- said the Dropbox previews might be abused, perhaps to create a distributed denial-of-service (DDoS) attack against Dropbox.

Dropbox downplayed that possibility, but has promised to explore alternative approaches. "We do use LibreOffice to render previews of Office documents for viewing in a browser, and have permitted external resource loading to make those previews as accurate as possible," said Dropbox security team lead Andrew Bortz on Friday on Hacker News. "While this could theoretically be used for DDoS, we haven't seen any such behavior. However, just to be extra cautious we've temporarily disabled external resource loading while we explore alternatives."

About the Author(s)

Mathew J. Schwartz


Mathew Schwartz served as the InformationWeek information security reporter from 2010 until mid-2014.

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