Even if your anti-virus software is capable of detecting a particular virus, it might not succeed, at least, not in all circumstances. It depends on what it is configured to scan, and that is a trade-off between speed and security. Many types of file cannot harbour an active virus, such as JPEG or GIF images, or plain, ASCII text. Opening a file, and examining the contents to determine the type is time-consuming. It is much faster to simply rely on the filename, if a file has a .jpg extension, then it is never going to get executed, and therefore there is no point in scanning it.
Thus, anti-virus software usually has a list of file extensions that are scanned. The default list is usually reasonable, but sometimes it will not reflect the changing circumstances in the real world. We have seen this recently, with one anti-virus software missing some instances of W32/Klez.H@mm because it did not scan .bat files by default. There will usually also be an option for "scan all files". Our recommendation is:
- If possible, select "scan all files" - this is the safest, but many users will find the speed reduction intolerable.
- Set a minimum of these extensions to be scanned: no extension 386 3GR ACM ADD ASD ASP AX BAT BIN BOO CHM CMD CNV COM CPL CSC DBX DLL DMD DO? DRV EML EXE FLT FON FOT HLP HT? HTA HTML HTT I13 IFS INF INI JS JSE LNK MDB MOD MPD MPP MPT MSG MSO NWS OBD OBT OCX OV? PCI PDF PDR PIF PL POT PP? PRC PWZ RTF SCR SH SHB SHS SRC SWE SYS TD0 TLB TSP TT6 VB? VWP VXD WBK WBT WIZ WML WPC WSC WSH XL?
- If an outbreak occurs, scan all files on affected systems.