Does anyone have the complete download URL for the above-captioned security update with download authenticity verified?
I am having a bitch of a time accessing such and downloading the standalone update.

It would look something like this:
http:// supportdownload.apple.com/download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/Mac_OS_X/downloads/091-25503.20171206.164cee1f-52a7-4829-8f49-810b78a12a2b/SecUpd2017-005ElCapitan.dmg

Try the links on this Google search page. BTW, can you get it via Software Update?

Been there, done that.
Software Update is not available to me given finite time constraints vis-à-vis dialup access. The update is 863MB.

I managed to figure out how to manipulate the URL to be able to download on a Windows/IE platform. Problem solved.

For the record: https://support.apple.com/kb/DL1955?locale=en_US

Indeed it is. But that wasn't my problem.
I have to use public access computers for large downloads (usually anything over a couple MB in size). Recently the IT folk who manage these have found it difficult to deal with so-called enterprise versions of Internet Explorer (IE) and Google Chrome, especially the former. (Firefox apparently has no enterprise version and so is not an option.)
What I come up against is going to, say, the above URL via IE, only to find that the download button is disabled. I then go to Google Chrome where that isn't the case (where attempts to download only do so to the public access computer which doesn't allow retrieval). I discovered that by right-clicking on the download button and pasting the URL to the clipboard I can then paste same into the address window in IE and thereby download the content of the URL. A major PITA but doable.
So, done and done.

You might try a download manager, they used to be quit popular back in the dialup and early internet days. I haven't seen any of them around in the last several years, but Fiplab has very recently been pushing their Download Shuttle for Mac. Back in the day a good download manager could make a surprising difference in downloading large files over a 64Kb dialup connection.

Thanks for the suggestion. But it's generally less trouble just popping into a local library branch to do my 'major' downloading, as well as to grab a few movies and books.