Logging on to a free Wi-Fi hot spot is a definite perk of the wireless age–unless it gets you slapped with a fine or spills your secrets.

Just connecting to a wireless access point leaks “all kinds of information that an attacker can use,” said David Maynor, Errata Security’s chief technology officer, in a presentation at the Black Hat conference in Washington, D.C. “If the government was taking this information from you, people would be up in arms. Yet you’re leaking this voluntarily using your laptop at the airport.”

With the right technology, sniffers and snoops can find out your username and password, and pinpoint where else you’ve connected. Even Macs aren’t immune.