Firstly, an application can use the system call instruction directly without calling of Windows API (Application Programming Interface–contained in various system DLLs), in this way bypassing entirely the hooks installed by AntiViruses at User Mode level.
Secondly, at Kernel level, harmless code can be passed to the AntiVirus through the hooks functions for analysis and as soon as it bypass the security checks, the system switch the execution to the malicious code, which is never subject for analysis. This is made because of Multi-Threading technology of processors and the main role is played by the processor scheduler. That’s it, if another thread of application is scheduled onto the processor, immediately after the hook function security checks, it might change the original argument–string stored in User Mode portion of address space and passed to the hook function as “the original” without the hook handler to detect any modification because is too far advanced in the checks routine. The pointer for instructions can not be modified because it is copied to the kernel stack, but can be modified the region of memory at which pointer refers to that one beeing accesible from within User Mode. When the processor scheduler switch back the
-3D EQSecure Professional Edition 4.2 -avast! Internet Security 5.0.462 -AVG Internet Security 9.0.791 -Avira Premium Security Suite 10.0.0.536 -BitDefender Total Security 2010 13.0.20.347 -Blink Professional 4.6.1 -CA Internet Security Suite Plus 2010 6.0.0.272 -Comodo Internet Security Free 4.0.138377.779 -DefenseWall Personal Firewall 3.00 -Dr.Web Security Space Pro 6.0.0.03100 -ESET Smart Security 4.2.35.3 -F-Secure Internet Security 2010 10.00 build 246 -G DATA TotalCare 2010 -Kaspersky Internet Security 2010 9.0.0.736 -KingSoft Personal Firewall 9 Plus 2009.05.07.70 -Malware Defender 2.6.0 -McAfee Total Protection 2010 10.0.580 -Norman Security Suite PRO 8.0 -Norton Internet Security 2010 17.5.0.127 -Online Armor Premium 4.0.0.35 -Online Solutions Security Suite 1.5.14905.0 -Outpost Security Suite Pro 6.7.3.3063.452.0726 -Outpost Security Suite Pro 7.0.3330.505.1221 BETA VERSION -Panda Internet Security 2010 15.01.00 -PC Tools Firewall Plus 6.0.0.88 -PrivateFirewall 7.0.20.37-Security Shield 2010 13.0.16.313 -Sophos Endpoint Security and Control 9.0.5 -ThreatFire 4.7.0.17 -Trend Micro Internet Security Pro 2010 17.50.1647.0000 -Vba32 Personal 3.12.12.4 -VIPRE Antivirus Premium 4.0.3272 -VirusBuster Internet Security Suite 3.2 -Webroot Internet Security Essentials 6.1.0.145 -ZoneAlarm Extreme Security 9.1.507.000
As I said always, thinking you are 100% protected by an AntiVirus is the worst conception you can have, you can better prevent computer infections only by knowledge. A lot of developers or researchers even from the Security software companies recognise that in our days, the traditional methods for defending against the new threats simply does not work or has a lot of vulnerabilities.