May 27, 2017

Gone In Six Characters: Short URLs Considered Harmful for Cloud Services

[This is a guest post by Vitaly Shmatikov, professor at Cornell Tech and once upon a time my adviser at the University of Texas at Austin. — Arvind Narayanan.]

TL;DR: short URLs produced by,, and similar services are so short that they can be scanned by brute force.  Our scan discovered a large number of Microsoft OneDrive accounts with private documents.  Many of these accounts are unlocked and allow anyone to inject malware that will be automatically downloaded to users’ devices.  We also discovered many driving directions that reveal sensitive information for identifiable individuals, including their visits to specialized medical facilities, prisons, and adult establishments.

URL shorteners such as and perform a straightforward task: they turn long URLs into short ones, consisting of a domain name followed by a 5-, 6-, or 7-character token.  This simple convenience feature turns out to have an unintended consequence.  The tokens are so short that the entire set of URLs can be scanned by brute force.  The actual, long URLs are thus effectively public and can be discovered by anyone with a little patience and a few machines at her disposal.

Today, we are releasing our study, 18 months in the making, of what URL shortening means for the security and privacy of cloud services.  We did not perform a comprehensive scan of all short URLs (as our analysis shows, such a scan would have been within the capabilities of a more powerful adversary), but we sampled enough to discover interesting information and draw important conclusions.  Our study focused on two cloud services that directly integrate URL shortening: Microsoft OneDrive cloud storage (formerly known as SkyDrive) and Google Maps.  In both cases, whenever a user wants to share a link to a document, folder, or map with another user, the service offers to generate a short URL – which, as we show, unintentionally makes the original URL public.
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