“It’s extremely common for Websites to protect your password by encrypting the initial login, but surprisingly uncommon for Websites to encrypt everything else,” he wrote. “This leaves the cookie (and the user) vulnerable. HTTP session hijacking (sometimes called “sidejacking”) is when an attacker gets a hold of a user’s cookie, allowing them to do anything the user can do on a particular Website. On an open wireless network, cookies are basically shouted through the air, making these attacks extremely easy.”
With Firesheep installed, users can target anyone on the same open wireless network visiting non-encrypted sites such as Twitter and Facebook and take over the person’s account.