What is a DNS leak and why should I care?
When using an anonymity or privacy service, it is extremely important that all traffic originating from your computer is routed through the anonymity network. If any traffic leaks outside of the secure connection to the network, any adversary monitoring your traffic will be able to log your activity.
DNS or the domain name system is used to translate domain names such as www.privacyinternational.org into numerical IP addresses e.g. 188.8.131.52 which are required to route packets of data on the Internet. Whenever your computer needs to contact a server on the Internet, such as when you enter a URL into your browser, your computer contacts a DNS server and requests the IP address. Most Internet service providers assign their customers a DNS server which they control and use for logging and recording your Internet activities.
Under certain conditions, even when connected to the anonymity network, the operating system will continue to use its default DNS servers instead of the anonymous DNS servers assigned to your computer by the anonymity network. DNS leaks are a major privacy threat since the anonymity network may be providing a false sense of security while private data is leaking.
via DNS leak test.
Lenovo consumer personal computers employing the pre-installed Superfish Visual Discovery software contain a critical vulnerability through a compromised root CA certificate. Exploitation of this vulnerability could allow a remote attacker to read all encrypted web browser traffic (HTTPS), successfully impersonate (spoof) any website, or perform other attacks on the affected system.
US-CERT recommends users and administrators review Vulnerability Note VU#529496 and US-CERT Alert TA15-051A for additional information and mitigation details.
P.S.: you can disable this by typing “about:config” as an url then change the “browser.safebrowsing.enabled” parameter.