Juha-Matti Laurio has reported a weakness in Outlook Express, which may disclose email addresses in "BCC:" fields to other recipients.

The weakness is caused due to an error when sending multipart messages. This may allow recipients in the "To:" and "CC:" fields to see the email addresses of recipients in the "BCC:" field.

In Outlook Express, this requires users to open the email message in a editor. In Lotus Notes and certain web-based email applications, the "BCC:" recipients are displayed directly in the message body without having to open it in an editor.

Successful exploitation requires that the user sending the email has configured Outlook Express to break apart emails larger than a size specified with the "Break apart messages larger than" setting (not enabled by default) for the used email account.

Solution
An update is available via the Microsoft Windows Update web site.
Further details available to Secunia VIM customers

Provided and/or discovered by
Juha-Matti Laurio

Changelog
Further details available to Secunia VIM customers

Original Advisory
Microsoft Knowledge Base Article - 843555:
http://support.microsoft.com/kb/843555

Deep Links
Links available to Secunia VIM customers