Since remote users don’t have a problem you can test/verify if your computer is the issue by using your CELL PHONE wifi network rather than local eth/wifi. If the injection stops, then it may indicate the issue is on your local router, not your computer.
There are many exploits for older routers. They are harder for hackers to exploit, but unpatched routers with default services still running often have esoteric security holes.
I’ve used Malewarebytes to scan my computers in the past but have never found anything important; Martha says that’s a good thing.