A new Trojan horse is so good at hiding itself that some security researchers claim a new chapter has begun in their battle against malicious-code authors. The new pest, dubbed "Rustock" by Symantec and "Mailbot.AZ" by F-Secure, <A HREF="http://news.zdnet.com/2100-1009_22-6095762.html?tag=zdnn.alert">uses "rootkit" techniques</A> crafted to avoid the detection technology used by security software, Symantec and F-Secure said in recent analyses.