Certificate errors on websites occur when a Web browser cannot verify the authenticity of the website's digital security certificate when attempting to establish an encrypted Secure Sockets Layer connection. This error can be caused by issues with the Web browser, an expired security certificate or malicious attempts to intercept communications.
Web browsers rely on digitally signed security certificates to ensure that websites they establish secure connections to are genuine. This helps prevent Internet fraud, such as criminals setting up a website identical to a major Internet retailer and collecting personal information and credit card numbers. If the Web browser cannot verify the security certificate that the website offers, it informs the user via a certificate error.
Many issues cause certification errors to occur with otherwise legitimate server connections. In some cases, the website may present a certificate that is legitimate but is not signed by a certification authority known to the Web browser, while in other cases the certificate may be expired or not yet validated by the certification authority. Some websites present certificates that are not validated by an external authority, which may be cause for concern if the site handles sensitive information, such as payment data. Errors with the SSL implementation of the Web browser or server can also cause certificate errors.