Several Trojan horses that target mobile phones have been discovered since the start of the year, but the threat level remains low. Symantec, which sells software to protect mobile devices, has since the beginning of the month identified nine new Trojan horses that target the Symbian operating system. The pests crash phones, attempt to install other malicious software or try to wirelessly transmit personal data to other gadgets, according to Symantec. Symbian OS is the most popular operating system for smart phones, including those sold by market leader Nokia.