Be careful the next time you turn on your Bluetooth-enabled phone: You could unknowingly be opening the door to a nasty intruder who could steal confidential information such as your address book or even use your phone to make expensive calls.Security experts in the U.K. have discovered serious flaws in some Bluetooth-enabled phones, prompting one supplier of the vulnerable phones, Nokia, to recommend precautionary measures."We have developed a tool that allows us to connect to a number of Bluetooth-enabled phones and download all sorts of confidential information, such as address books, calendars and other attachments without going through the normal pairing, or handshaking, process between devices," said Adam Laurie, technical director and co-founder of A.L. Digital in London. "In fact, we have been able to obtain this confidential data without giving users any indication whatsoever that an intrusion is taking place."A.L. Digital has discovered security flaws in four Nokia phone models: 6310, 6310(i), 8910 and 8910(i).Janne Ahlberg, manager of technology platforms at Nokia, confirmed on Wednesday that these models are susceptible to potential attacks. Users of these phones in public places, he recommended, should either switch their phone to the "non-discoverable" or hidden mode, making them invisible to others, or turn off the Bluetooth functionality completely. Users should also check that their Bluetooth "pairings," or approved connections with trusted partners, are correct.The U.K. security company detected similar flaws in phones manufactured by Sony Ericsson Mobile Communications. The Sony Ericsson models include the R520, T68i, T610 and Z1010.Sony was unavailable for immediate comment.Bluetooth technology allows users to swap data between mobile phones, PDAs, notebook computers and a string of other devices within a few meters of each other. It is becoming a standard feature of many high-end devices.Until now, the only known Bluetooth security shortcoming has been "bluejacking," an increasingly popular means of exchanging short three- or four-word messages in the display area designated for the name of the initiating device, according to Laurie. The process, essentially, allows communication to take place without pairing, which requires partners to exchange a\u00a0personal identification number to establish a connection.But Laurie said he and his colleagues at A.L. Digital have uncovered not one but two new security flaws. He referred to the one as "bluesnarf" and the other as a "backdoor" attack."Bluesnarf is a tool I've written that allows you to bypass the pairing process to connect to a Bluetooth-enabled phone and, essentially, break into the device to steal or manipulate data," he said.The backdoor attack, according to Laurie, involves establishing a trust relationship through the pairing mechanism but later making the pairing information invisible on the target's register of paired devices to enable an anonymous connection. The process requires participating users to first create a personal identification number and then enter this number in each device, in order to initiate a connection, he said.The problem arises, Laurie said, when one of the "trusted" persons decides to use the backdoor hacking method to hide the identification data and gain unauthorized access to that person's device. "Unless you happen to be staring at your phone and see a little icon appear indicating a connection, you won't know that anyone has gained access to your phone," he said.Nokia said that it is not aware of any attacks against Bluetooth-enabled phones and believes it is "highly unlikely" that these phones will become broadly exposed to security attacks."From a security viewpoint, Bluetooth is actually very strong," Ahlberg said. "There were just some implementation flaws that made these security flaws possible in a couple of models."Additional information about the security flaws detected by A.L. Digital is available\u00a0here.