Download Virus Checker by James Fray

"Download Virus Checker" adds additional security level to your browsing experience by checking all download links against 68 anti-virus solutions. The extension needs zero configuration. Once a new download request is submitted, the download link is sent to VirusTotal web service and will be checked for viruses. Once the report is ready, extension evaluates the response and notify you if the link is a threat. If there are more than pre-defined positive responses to the link, a warning window will appear to notify you about the possible threat of this download. You can either decide to allow or deny this download.

The extension is built on top of VirusTotal web service; Wikipedia:
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VirusTotal aggregates many antivirus products and online scan engines to check for viruses that the user's own antivirus may have missed, or to verify against any false positives. Files up to 128 MB can be uploaded to the website or sent via email. Anti-virus software vendors can receive copies of files that were flagged by other scans but passed by their own engine, to help improve their software and, by extension, VirusTotal's own capability. Users can also scan suspect URLs and search through the VirusTotal dataset. VirusTotal for dynamic analysis of malware use Cuckoo sandbox. VirusTotal was selected by PC World as one of the best 100 products of 2007.
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For FAQs please visit:
http://add0n.com/virus-checker.html

For bug reports please use:
https://github.com/james-fray/download-virus-check

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Please note that the FAQs page of the add-on has an embedded Google Analytics script. You may review terms of service here; https://www.google.com/analytics/terms