Users who access Google's Gmail or the Facebook social-networking site over Wi-Fi could put their accounts at risk of being hijacked, according to research from Errata Security, a computer security company.

It's not just those sites but any rich Web applications that exchange account information with users, including blogging sites such as Blogspot or even software-as-a-service offerings such as Salesforce.com, that could pose a risk for users, wrote Errata's Robert Graham, CEO, and David Maynor, chief technology officer, in a paper.

Most Web sites use encryption when passwords are entered, but because of the expense, the rest of the information exchanged between a browser and a Web site is not encrypted, they wrote in a paper presented at the Black Hat 2007 security conference in Las Vegas this week.

Using a packet sniffer, which can pick up data transferred between a wireless router and a computer, it's possible to collect cookie information while a user is accessing one of those sites over Wi-Fi.

Cookies consist of bits of data sent to a browser by a Web site to remember certain information about users, such as when they last logged in. Included in the cookie can be a "session identifier," which is another bit of unique information generated when people log into their accounts.

By collecting cookie information and the session identifier with the packer sniffer and importing it into another Web browser, the hacker can get inside a person's account. The attacker may not, however, be able to change a person's password, since many Web 2.0 applications require a second log-in to change account information.

Nonetheless, it could allow a hacker to create blog postings, read e-mail or do other malicious activity. Meanwhile, the victim is directed to a version of the Web page they intended to visit, which Errata calls "sidejacking."

There is a remedy, however. "The consequence of this is that users should never use a Wi-Fi hotspot unless they are using VPN (virtual private networking) or SSL (secure sockets layer) to access their accounts," they wrote.