Files with the .CER extension contain certificates, which are security files for webpages distributed by an external Certificate Authority, such as VeriSign or Thwate.

CER files allow the web browser to verify the authenticity of the page. It is installed onto the server that houses the webpage. CER files can be viewed by clicking the lock icon in the web browser, which can usually be found at the border of the secure webpage window. The icon’s exact placement depends on the browser, though – in Internet Explorer, it can be found in the lower right corner, while in Safari, it is seen in the upper right corner of the browser window.