Google Toolbar allows spoofing the information presented in the dialog which is being displayed when adding a new Google Toolbar button. This can allow an attacker to convince the users that his button comes from a trusted domain. This button can then be used to download malicious files or conduct phishing attacks (e.g. show a login form of a bank).
Affected versions Google Toolbar 5 beta for Internet Explorer Google Toolbar 4 for Internet Explorer * Google Toolbar 4 for Firefox (partially)
Technical details: http://aviv.raffon.net/2007/12/18/GoogleToolbarDialogSpoofingVulnerability.a spx