Several flaws exist in Microsoft Internet Explorer that could allow an attacker to masquerade as a legitimate web site if the attacker can compromise the validity of certain DNS information. These problems are different from the problems reported in CERT Advisory CA-2000-05 and CERT Advisory CA-2000-08, but they have a similar impact. Digital certificates are small documents used to authenticate and encrypt information transmitted over the Internet. One very common use of digital certificates is to secure electronic commerce transactions through SSL (Secure Socket Layer). The kind of certificates used in e-commerce transactions are called X.509 certificates. The X.509 certificates help a web browser and the user ensure that sensitive information transmitted over the Internet is readable only by the intended recipient. This requires verifying the recipient's identity and encrypting data so that only the recipient can decrypt it. The "padlock" icon used by Internet Explorer (as well as Netscape and other browsers) is an indication that an SSL-secured transaction has been established to someone. It does not necessarily indicate to whom the connection has been established. Internet Explorer (and other browsers) take steps to warn users when DNS-based information conflicts with the strongly authenticated information contained in the X.509 certificates used in SSL transactions. These warnings are supplemental information to help users decide if they're connecting to whom they think they are connecting. These steps and warnings are designed to protect against attacks on the DNS information.