Rootkit writers have started exploiting a loophole that lets them write malware able to bypass the PatchGuard driver signing protection built into 64-bit versions of Windows, Kaspersky Lab has reported.A product of the BlackHole Exploit Kit, a hugely successful kit for building malware to hit specific software vulnerabilities, the first element of the attack on a system is straightforward enough, using a downloader to hit the system through two common Java and Adobe Reader software flaws. On 64-bit Windows systems open to these exploits, this calls a 64-bit rootkit, Rootkit.Win64.Necurs.a., which executes the 'bcdedit.exe -set TESTSIGNING ON command, normally a programming command for trying out drivers during development.The loophole abused by the malware writers is that this stops Windows' Patchguard from objecting to the unsigned and insecure nature of the driver (in this case a rootkit driver) being loaded.The power of the technique is double-edged, however. Once loaded, the rootkit is able to block the correct loading of antivirus software that might detect and remove it, but this is also a giveaway. Security programs that do not work correctly could be taken to infer the presence of something unusual.Windows PatchGuard, formally known as Kernel Patch Protection (KPP), is a design feature of 64-bit versions of Windows, including XP, Vista, Windows 7, and Windows Server, designed to stop malware undermining the operating system at the highest level of privilege.As an aside, Kaspersky reports that the malware also attempts to download Hoax.OSX.Defma.f, a recent and well-publicised fake antivirus program targeting Mac OS X users, which can't run on Windows."It appears that the developers of the latest rogue AV program for MacOS are actively distributing it via intermediaries, who don't really understand what it is they are supposed to install on users' computers," said Kaspersky researcher, Vyacheslav Zakorzhevsky, in his blog covering the malware.This underlines the ominous way that Macs are now seen as just another platform to be targeted where possible using multi-platform malware.