Microsoft Internet Explorer 4.0 through 6.0 could allow local users to differentiate between alphanumeric and non-alphanumeric characters used in a password by pressing certain control keys that jump between non-alphanumeric characters, which makes it easier to conduct a brute-force password guessing attack.

Internet Explorer 4.x and 5.x does not properly re-validate an SSL certificate if the user establishes a new SSL session with the same server during the same Internet Explorer session, aka one of two different "SSL Certificate Validation" vulnerabilities.

Internet Explorer 4.x and 5.x does not properly verify all contents of an SSL certificate if a connection is made to the server via an image or a frame, aka one of two different "SSL Certificate Validation" vulnerabilities.

Internet Explorer 5.0 and 5.01 allows remote attackers to bypass the cross frame security policy and read files via the external.NavigateAndFind function.

A Microsoft ActiveX control allows a remote attacker to execute a malicious cabinet file via an attachment and an embedded script in an HTML mail, aka the "Active Setup Control" vulnerability.

By default, Internet Explorer 5.0 and other versions enables the "Navigate sub-frames across different domains" option, which allows frame spoofing.