December 9th, 2014
Disabling SSL 3.0 fallback
We previously published Security Advisory 3009008 advising that Microsoft will disable SSL 3.0 by default in Internet Explorer and across all Microsoft online services over the coming months. To continue to help protect customers, we are taking the interim step to provide the option to disable SSL 3.0 fallback in Internet Explorer 11 for Protected Mode sites, which is the default for Internet sites and Restricted sites. This change is currently off by default, and we plan to turn it on by default in Internet Explorer 11 on February 10, 2015.
What is SSL 3.0 fallback?
The POODLE SSL 3.0 vulnerability exposed a weakness not only in the SSL 3.0 protocol, but in the way that browsers negotiate an HTTPS connection with web servers. By interfering with the connection between the target client and server, a man-in-the-middle can force a downgrade from TLS 1.0 or newer, more secure protocols, to the SSL 3.0 protocol.
At a high level, commonly when a Web browser connects to an HTTPS Web site, it will first try to do so by using the highest-available encryption protocol. If this connection fails during the handshake, the browser will fall back and retry the connection with a lower encryption protocol, eventually falling to SSL 3.0. The vast majority of the time, a fallback from TLS 1.0 to SSL 3.0 is the result of an innocent error, but this is indistinguishable from a man-in-the-middle attack.
When will Internet Explorer block SSL 3.0 fallback?
The December 2014 Internet Explorer Cumulative Update (KB3008923), released today, allows users to opt-in and block SSL 3.0 fallback in Internet Explorer 11. Enterprise customers are able to configure this behavior via Group Policy, and this behavior will also be configurable via registry or using an easy, one-click Fix it solution. Details on how to configure this behavior can be found in KB3013210.
From February 10, 2015, Internet Explorer 11 will prevent insecure fallback to SSL 3.0 for Protected Mode sites.
How can I test if my server will be impacted?
Please review your web server settings and technical documentation, as many servers even if they support TLS 1.0, fallback to SSL 3.0. There are a number of third-party tests available that may help.
Disabling SSL 3.0 in your browser will allow you to see which sites do not support TLS and need to be updated. We encourage users to use the workarounds and easy, one-click Fix it provided in Security Advisory 3009008 to disable SSL 3.0 in your browser.
- Microsoft Security Bulletin MS14-080 – This critical security update resolves fourteen privately reported vulnerabilities in Internet Explorer. For more information see the full bulletin.
- Security Update for Flash Player (3008925) – This security update for Adobe Flash Player in Internet Explorer 10 and 11 on supported editions of Windows 8, Windows 8.1 and Windows Server 2012 and Windows Server 2012 R2 is also available. The details of the vulnerabilities are documented in Adobe security bulletin APSB14-27. This update addresses the vulnerabilities in Adobe Flash Player by updating the affected Adobe Flash binaries contained within Internet Explorer 10 and Internet Explorer 11. For more information, see the advisory.
Most customers have automatic updating enabled and will not need to take any action because these updates will be downloaded and installed automatically. Customers who have automatic updating disabled need to check for updates and install this update manually.
— Alec Oot, Program Manager, Internet Explorer