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Antivirus scanner cannot read/scan some files

Problem

Your anti-virus program reports that some files could not be scanned, usually when conducting a manual scan of the entire hard disk.

Solution

This is quite normal. In Windows there are always a number of files that are kept open by the operating system and are not available to be scanned.

Some examples of files that cannot be scanned are:

  • The contents of C:\Windows\System32\Config or C:\Winnt\System32\Config — these are the registry and event log files.

  • Pagefile.sys or Win386.swp — these are swap files used by Windows.

  • Various instances of Index.dat — these are index files used by Internet Explorer.

  • Various .LOG files — these are log files created by various applications and are often kept open.

  • Various .MDF and .LDF files — these are SQL Server database files.

  • Various .CAB files — these are cabinet files which are compressed collections of files and usually cannot be scanned because they need to be opened as a sequence and not individually.

BUT: while the inability to scan certain files is normal, it is also true that certain type of malicious files cannot be scanned because they are designed to be inaccessible. Normally when a program executes, the original files (.exe or .dll) are closed and can therefore be scanned even when the program is running. So any files such as .exe or .dll which cannot be scanned can be an indicator of malicious intent since it is not normal to keep the file locked.

Therefore there is a value in regularly examining scan logs for files of this type.

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