This thingy. You should probably read it.
TL;DR there's some sort of phishing virus going around steam profile pages that can infect any profile and the people who view those profiles in their browsers- even yours.
How to avoid:
1. Don't click on any non steam urls on steam profiles
2. Wait until around 7 tonight when steam goes offline for maintenance. Hopefully Valve will fix it then.
I also believe this happened last year, and they fixed it then.
Also, phishing has been a thing in steam for a while now. If someone randomly adds you and gives you a URL, DONT OPEN IT. The urls look very similar to steam profile urls, but a letter or two is changed and leads you to a phishing site
You do realize that valve already like did a whole message thing on the forums about it, to warn everyone not to click on any links. I mean, it's obvious that it's a scam/phishing links.