Security researchers have discovered one of the most subtle and sophisticated examples of Windows rootkit software known to date.
The AutoRun-NOX worm extends the standard VXer trick of using software vulnerabilities to infect systems, by including functionality that allows the worm to exploit Windows security bugs to hook into parts of the Windows system that operate below the radar of anti-virus packages.
"Most malware with rootkit functionality will tamper with the Windows kernel and attempt to execute code in kernel mode," net security firm F-Secure reports. "Typically, a special driver is used to do this... AutoRun.nox is different — it uses a vulnerability to do the job. For malware, it's rather unique to see such a technique being used."
The worm uses a long-standing Windows vulnerability, patched by Microsoft in April 2007, involving a GDI privilege elevation flaw. If the attack using the vulnerability fails, the worm falls back to plan B - using the more common (but less elegant) driver method.
A blog posting by F-secure containing screenshots and a detailed technical run-down of the worm's modus operandi can be found here. ®