Beware the perils of free WiFi.

A security company's recent investigation of the security of free WiFi connections at public venues indicates those joining unknown networks to avoid telecom fees may be paying an unexpectedly high price.

More than 11 percent of the 68,000 plus WiFi networks at major public places - including airports, railway stations, scenic spots and shopping malls in Beijing, Shanghai and Guangzhou - were unsafe, said RainRaid, an independent information security consultancy based in Shanghai.