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I've been checking the error 404 (or "Page not found") logs for my website and there's a bunch of entries for the /js/comm.js script with the user agent set to Mozilla/5.0 (Windows NT 10.0; WOW64; rv:48.0) Gecko/20100101 Firefox/48.0.

What is that .js script that Firefox is looking for?

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A .js file is a javascript file which is generally used for adding rich client side behavior on your website. There could be a number of reasons why it is showing up in your 404 error log which all relate to the file not actually existing on your server. It could be that on one or more pages of your site this javascript file actually has been added to the HTML and so the browser is trying to download it, however given that all the requests for it seem to be coming from a single user agent string (able to be spoofed so not 100% reliable) it could be someone running a crawler or something similar trying to find if you have a file named comm.js on your site and if so to download it.

A quick check on Google identifies a potential candidate for a file named comm.js which relates to cross protocol js based communications which hasn't been updated or patched for the last 3 years and it is possible that someone may be looking for potential sites with it as they may have identified a bug with it that can be exploited.

But basically this is nothing to worry about. It is very common to have any number of 404 errors in your logs either from URL's which once existed and no longer do, typo's in links from other sites to your own, etc. The fact that you don't have such a file on your server means all that is happening is that a relatively inconsiquential amount of bandwidth is being used to return an error message to the client requesting this path and nothing more. If the requests become too frequent and start to concern you with their bandwidth consumption or server load you can discuss with your host blocking the connection at the firewall or do so yourself if you have a firewall you are able to control to block connections from a particular address.

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