ELI5: What is this major security flaw in the microprocessors inside nearly all of the world’s computers?

Below is a great 'Explain Like I'm 5' post by u/WarioZero I discovered last night on Reddit

At it's heart, your computer works just like a library. It's constantly reading and moving information just like students read and move books. And just like any good library, your computer has a friendly librarian: Ms. Kernel.

Whenever you go to check out a book, you give Ms. Kernel the title and she goes to fetch it for you.

However, this library may contain some pretty secret stuff, so Ms. Kernel always checks to make sure you've got permission to read the book you're asking for.

Back in the old days, Ms. Kernel had to do all the work herself, and as you might imagine, it was painfully slow. However, recently we've gotten better at designing computers that can do multiple operations at once, meaning that Ms. Kernel now has a bunch of assistants helping her out. Yay!

This is great for the library, because now it can handle more people than ever before. However, it also creates a weakness that was only just discovered.

Here's how it works...

You, a mischievous ne'er do well, want to read Suzie's diary, which she keeps in the library. However, Suzie hasn't shared her diary with you, so Ms. Kernel won't let you check it out. So you decide to do something rather clever...

Rather than asking for the diary directly, you ask Ms. Kernel to do two things...

1. Fetch Suzie's diary
2. Fetch a book where the first word in the title is the first word in Suzie's diary

Now, back in the old days this wouldn't have been a problem. The first thing Ms. Kernel would have done would be to ask Suzie if you could read her diary. When Suzie said no, Ms. Kernel would have stopped.

However the assistants make things more complicated.

To save time, Ms. Kernel asks one assistant to ask Suzie for permission while the other goes to find the two books you asked for.

When the first assistant tells Ms. Kernel that Suzie said no, the second one gives her the books which Ms. Kernel sets on her desk.

Ms. Kernel tells you that you can't have the books. However, because they're sitting on the desk, you can read the titles. The first one's called "Suzie's Diary" and the second one's called "The Cat in the Hat".

Because you asked for a book that has a title that begins with the same word as the first word in Suzie's diary, you know that the first word in the diary must be "The".

If you wanted to, you could repeat this process for every word in the diary, until you could read the whole thing.

The problem with this exploit is that it works in every library that has assistant librarians, which covers just about every modern processor out there.

The only real fix is to force the assistants to run the check before fetching the books, which will slow down the library as a whole.

The biggest issue is that the parallel processing (assistants) is leaving the cache (desk) in a different state than they found it, even though the permission check failed.

Thankfully, there isn't any known malware that exploits this bug, but the safest thing to do is to update your devices as soon as a fix is released, to prevent then from being affected in the future.

The performance issue shouldn't be a major problem for most personal devices (small library, few assistants) but will likely cause a performance decreases on larger machines (i.e. university supercomputers).