Just when you thought it was safe to resume Tweeting as normal in your corner Starbucks, think again. A London-based software developer has developed a hijacking tool that compromises Twitter accounts on open, Wi-Fi networks and then ominously posts a warning to the victim.

Jonty Warieing’s tool, named “Idiocy,” searches for users logged onto Twitter over insecure, public Wi-Fi networks. Once found, it hijacks their session to post a tweet informing them that they are vulnerable to attack and then links to a website explaining what happened.

The tool follows in the footsteps of the “Firesheep” vulnerability to which we earlier alerted users, but is specifically aimed at Twitter and not other social networking sites like Facebook and Flickr.

Warieing appears to have released the tool with the same purpose as the creators of Firesheep, who claimed they took the step to expose the dangers of accessing the Web from public Wi-Fi spots.

“Idiocy is designed to warn people of the risks they’re taking on public Wi-Fi networks,” Wareing blogged. “Run Idiocy on your laptop and make people aware.”

Firesheep has been downloaded more than 300,000 times since its release. It enables users to easily download a Firefox add-on through which they can target anyone on the same wireless network.

Firesheep’s creators, Eric Butler and Ian Gallagher, wrote in their own blog post that the problem is not just open Wi-Fi, but the failure of websites to support application encryption, or SSL.

Users, they said, can stop gate the issue with virtual private networks or extensions like Firefox’s HTTPS-Everywhere, but none of these is a “silver bullet.” The only real solution is properly implemented SSL/HTTPS.