When you enter a website address or when you click a link on a web page, Chrome checks the URL against a database of known bad websites and if it is found, then it will prevent you from accessing the site.
Google says that it checks billions of URLs a day looking for bad websites that could potentially cause problems in various ways. For example a site may have malware or spyware hidden in software downloads. You might even be able to go to a website and it is not until you click a download link that Chrome will block the action and prevent the download.
A site might be used for phishing and it could pretend to be something else, such as the login page for your bank, PayPal, eBay, Apple ID, Microsoft ID, Google account, or some other online service. Phishing sites try to trick you into revealing your user name and password for various services.
Sometimes good websites go bad. They get hacked and malicious code is inserted into the site. It isn’t the site owner that is at fault and it could be the victim of an attack by hackers.
The sort of thing you might see is this:
It says that the website it is going to contains harmful programs. There is a Details link at the bottom and clicking it displays this:
It reports that Google Safe Browsing found harmful programs on the website. This warning is nearly always right, so you might want to go elsewhere by clicking a bookmark or typing another URL into the address box.
Sometimes, although quite rarely, it can be wrong. It is like an anti virus program giving a false positive. If a website owner discovers the problem, they might have fixed it, but Google has not updated its information. Google clearly thinks there is a problem for some reason, but if you are absolutely sure that the website is OK, you can click the visit this site link. Think carefully before doing this though.
There is a tool at Google Transparency Report that enables you to check whether a website is on Google’s no-go list of bad websites. Go to Safe Browsing Site Status and enter the URL of the website you want to check.
Almost instantly, Google responds with some information about the site and in this case it says that it is partially dangerous, and it gives some information as to why it thinks this.
There are some other interesting features in the Google Transparency Report and there is an overview and a malware dashboard. The heat map on the malware dashboard shows the amount of malware found on websites on a global map. Mouse over the map to see the details.