Reported on Secunia - Microsoft Internet Explorer FTP Credentials Exposure
A weakness has been reported in Microsoft Internet Explorer, which may expose FTP usernames and passwords.
When Internet Explorer is used to access an FTP site, the username and password is stored in the file (e.g. an HTML file) when viewing and then saving (using "File" -> "SaveAs") it. This may lead to exposure of the username and password if a user e.g. uploads the saved file or makes it otherwise accessible to other people.
The weakness is reported in Internet Explorer version 6 and 7 (when using the "Open FTP Site in Windows Explorer"). Other versions may also be affected.
Solution:
Do not make files (e.g. HTML files) that were opened and then saved via an FTP session in Internet Explorer available to others.
Source: http://secunia.com/advisories/26427/