Member Avatar for priyamtheone

Which is the most reliable free antivirus for USB flash drives that resides and runs from the device itself and doesn't need to be installed on the computer?

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Short answer: a bootable "rescue" environment on USB is the most reliable free option for scanning a machine without installing anything on the host OS. rightly separated portable-in‑OS scanners (useful as a second opinion) from bootable scanners (which run outside the possibly‑compromised OS). Autorun-style approaches are both widely blocked by Windows updates and unsafe given firmware/USB‑level attacks like BadUSB, so they should not be relied on. Microsoft Security Advisory 967940. (learn.microsoft.com) BadUSB background (why firmware attacks matter). (wired.com)

Practical, free choices that meet "runs from the device and needs no install" are bootable rescue ISOs and Microsoft’s offline tool. Microsoft Defender Offline can create bootable media and run outside Windows. Microsoft Defender Offline info. (learn.microsoft.com) Major vendors also publish rescue ISOs (for example, Kaspersky Rescue Disk) with instructions to write the ISO to USB and update signatures after boot. Kaspersky Rescue Disk guidance. (support.kaspersky.com) Note that some older rescue tools may be discontinued or no longer updated; check the vendor support page before relying on an image. (support-eol.eset.com)

Portable scanners (what described) like the ClamAV/ClamWin family are useful as a quick/manual scan from USB but are not on‑access and run inside the host OS — they cannot reliably find kernel/rootkit/firmware threats. ClamWin notes (no real‑time protection). (clamwin.com)

Quick workflow to follow (keeps risk low):

  • Download the rescue ISO or offline tool from the vendor site and verify the download if the vendor publishes a checksum.
  • Create USB media on a known‑clean PC (tools like Rufus are commonly recommended by vendors).
  • Boot the target PC from the USB, update definitions inside the rescue environment if network access is available, then run a full scan.
  • Treat unknown or shared USB devices as untrusted (firmware attacks exist); use read‑only / hardware write‑protected media when available.

This complements the earlier replies: accept ’s conclusion (bootable is best), treat ’s autorun idea as obsolete on modern Windows, and use / suggestions as second opinions or rescue alternatives while preferring a vendor‑supported rescue USB for the strongest, installation‑free scanning.

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Member Avatar for rproffitt

I can think of two types of apps that could answer your question.

  1. Portable versions of an antivirus. These are only used to scan once the OS has booted. I'm going to call these trash since a rootkit can easliy evade this.
  2. Bootable Antivirus on USB. Now we're talking. Remember that no antivirus today is 100% effective and again we are talking SCAN ONLY as to be effective when the OS is running, we must install this to the OS.

So bootable AntiVirus (AV) scanners? Here's a list: https://www.geckoandfly.com/19582/free-usb-antivirus-fix-unbootable-windows-unmountable-boot-volume/
All these can be put onto USB or CD/DVD.

commented: what he said :-) +15
Member Avatar for jessica_22

autorun antivirus is considered to be the most reliable.
but, it needs to be installed on PC

commented: That was helpful. NOT -4
Member Avatar for happygeek

autorun antivirus is considered to be the most reliable but, it needs to be installed on PC

A non-answer that doesn't even attempt to address the actual question asked by the OP. The actual answer, which does address the question, has already been given by rproffitt...

Member Avatar for rubberman

Check out WinClam - a free AV. There is also a Unix/MacOSX/Linux version available (called ClamAV). I use it all the time, and have never had a problem with it not detecting malware on my systems.

Member Avatar for Leif_1

Avira PC Cleaner does the work for me, just a simple yet effective and user friendly antivirus.

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