Safari adds links on any page I had strange issue with Safari on Macbook today.  It add on any pages I had visited links to other sites.  It looks like an advertising.

For example, if I had the page with next text:

".... answers for you Apple question ..." 

Safari changed page so "Apple" was hyperlink to other site with an advertising of Apple production.  I can see it on any page and with different words.

I have not install any Safari extensions last time and the only thing that looks suspicious is the new Apple update which I have installed today.

I have restarted Safari and now all looks fine, but I just want to know what it had been and what I can do now to find the reason this strange behavior and to dispose future risk to security.
I think what Rob is getting at and what I think might be happening is that you have some malware (virus, etc) on your Mac (yes they do exist) that could be adding things to web pages.

It could be as simple as a browser extension or plug-in with some hijacking code in it, or something more system wide.

you could try resetting Safari (from the Safari menu select Reset Safari), turning off extensions (Safari menu -> preferences -> extensions. then turn them all off) or going into private browsing mode (Safari menu -> private Browsing). 

Optionally you could try another browser, Chrome or Firefox would be my choices. If they do the same thing then it may be system-wide and time for a virus checker or a visit with your favorite Mac repair shop (Apple store or local reseller) to determine what is going on.

Also some routers and some ISPs inject ads and other stuff into web pages. Try it on a different WiFi (Starbucks, etc...) and see if the issue goes away