Short URLs, big problems

Short URL services are becoming increasingly popular among social networks, especially on Twitter. When you have to limit your message to just 140 characters, every character becomes important, and posting links to searches on Google or news websites can rapidly fill an entire Twitter message.

Of course, for every problem there is a solution, so what URL shortening services like TinyURL, Is.gd or Bit.ly are doing is to offer for free short URLs that redirect to the longer ones. Everything might seem great until the moment you start thinking about security, and several problems come to my mind.

Social engineering is made easier. The user doesn’t really see the URL of the page he’s going to, but just the shortened version, which usually doesn’t offer any clue of where the destination page is hosted. An attacker can say he’s linking to “nice pictures with bunnies”, but instead sending the user to a website hosting malicious content.

The reliability is questionable. In order to get to the final destination, it’s not only necessary for the destination’s server to be reachable, but also for the short URL service to be up and running. Reliability problems with TinyURL were what made Twitter to switch to Bit.ly recently.

Trust can be a problem. The user wants to only click on safe link, so now he does not only have to trust the person who sends him a link, but also an intermediate player: the URL shortening service.

Security concerns are being raised by these URL shortening services, and I am very glad to see the media also starting to notice them and raise the security awareness level throughout their readers: AP recently posted an article about short URL services that also touches on the security problems.