system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How and why is Flash Player apparently such a security risk to users?

Everything I google and read keeps bringing up that it is full of security risks and holes, but nothing explains why. How does using this thing put my computer or information at risk?
gpt
Secure languages provide separation of code (the bit that does things) and data (the bits which have things done). They provide things like bounds checking (is the data being written bigger than the space you are writing to? is the address you are writing to part of the variable you think it is, or is it part of something else?) and strict type checking (is it the same fundamental type of thing?).

Flash doesn't have these things. And, it appears to depend on being 'mutable' -being able to construct new code, and then point to it and say "run this" which is a hugely flexible thing to do, but very hard to do securely. 

Flash is written to be 'helpful' so its written to provide services which look like 'display this picture' but the same logic can be doing things in the background, the user isn't always able to know. So, it can do things you don't know are happening.

Flash is significantly less constrained than other things, in what it can do to your computer. It runs inside some tools which are not 'sandboxed' -that is are not confined to a kind of virtualized substate, where they cannot write outside of areas which are tightly controlled.

Flash can access your network. So it can set up to download extra code, including cracking code, which exploits other weaknesses in your host, and then allows it to be abused by other non-flash systems.

If you combine all of this, you have something which is very compelling for some people to use,  because its so flexible, but that flexibility is the root cause of its massive insecurity. It should be removed, and people should move to using more secure frameworks to support 'actionscript' tasks like the sandboxed javascript environments built into the browser. 

There is very little flash does which couldn't be done by something else.