An information disclosure vulnerability exists in Outlook when a message is opened. This vulnerability could potentially result in the disclosure of sensitive information to a malicious site.
To exploit the vulnerability, an attacker would have to send a malicious email to a user and convince the user to open the email. A connection to a remote SMB server could then be automatically initiated, enabling the attacker to brute-force attack the corresponding NTLM challenge and response in order to disclose the corresponding hash password. Depending on the URL contained in the malicious email, Outlook could fall back to initating a web request to a remote server, disclosing the the external IP of the user's system.
The security update addresses the vulnerability by correcting how Outlook processes embedded URLs.