This security update resolves a privately reported vulnerability in Microsoft Silverlight. The vulnerability could allow information disclosure if an attacker hosts a website that contains a specially crafted Silverlight application that is designed to exploit this vulnerability and then convinces a user to view the website. The attacker could also take advantage of compromised websites and websites that accept or host user-provided content or advertisements. Such websites could contain specially crafted content that could exploit this vulnerability. In all cases, however, an attacker would have no way to force users to visit a website. Instead, an attacker would have to convince users to visit a website, typically by getting them to click a link in an email message or in an Instant Messenger message that takes them to the attacker’s website. It could also be possible to display specially crafted web content by using banner advertisements or by using other methods to deliver web content to affected systems.
This security update is rated Important for Microsoft Silverlight 5 and Microsoft Silverlight 5 Developer Runtime when installed on Mac and all supported releases of Microsoft Windows. For more information, see the subsection, Affected and Non-Affected Software, in this section.
The security update addresses this vulnerability by correcting how Microsoft Silverlight checks memory pointers when accessing certain Silverlight elements. For more information about the vulnerability, see the Frequently Asked Questions (FAQ) subsection for the specific vulnerability entry under the next section, Vulnerability Information.
Note This security update upgrades previous versions of Silverlight to Silverlight version 5.1.20913.0, which is the first version of Silverlight 5 that is not affected by the vulnerability.
Recommendation. Most customers have automatic updating enabled and will not need to take any action because this security update will be downloaded and installed automatically. Customers who have not enabled automatic updating need to check for updates and install this update manually. For information about specific configuration options in automatic updating, see Microsoft Knowledge Base Article 294871.
For administrators and enterprise installations, or end users who want to install this security update manually, Microsoft recommends that customers apply the update at the earliest opportunity using update management software, or by checking for updates using the Microsoft Update service.