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For some time now, I've configured the anti-virus software on the systems on my network to only scan files on write, not on read, on-top of a regular full scan overnight.

I base this on the assumption that for malware to exist on the drive, it must have been written there at some point and so scanning again on every read is a waste of resources.

Note that this policy I do not apply to removable drives.

Is there anything significant I'm overlooking with this approach? I'm not looking for opinions, rather ways that such protection could be circumvented which I haven't considered.

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Normally most antiviruses and enterprises I have seen set the antivirus to atleast do a Scan on Read with Scan on Write disabled.

The logic here is that (Atleast what I feel):

  1. You can have an removable thumb drive added to the system that can contain malware.
  2. You may add a hard drive to the system / machine which may already have data written to the drive.
  3. Someone can add data to a laptop / machine in an offline mode using Live CD etc and it will be executed once OS boots.
  4. One can safeguard against malware in media like CD/DVD with Scan on Read only.
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The bootable media point is moot - if someone has physical access, they can add malware to their heart's content unless the drive is encrypted - which would prevent files from being added via removable media anyway. The others are good points (though Graham implies scan on read is enabled for removable media). – Bob Nov 23 '12 at 15:42
@Bob I agree partially. You may not know that the media has malware. If you do not enable Scan on Read, and if autorun is enabled, you may unknowingly infect the PC. – Ganesh R. Nov 23 '12 at 15:45
Uh, sorry, typo. I meant encryption would prevent files from being added via bootable media (live CD, etc.), not removable. – Bob Nov 23 '12 at 16:10

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